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IC 
 
 9004 
 
 Bureau of Mines Information Circular/1985 
 
 The Bureau of Mines Noise-Control 
 Research Program— A 10-Year Review 
 
 By William W. Aljoe, Thomas G. Bobick, 
 
 Gerald W. Redmond, and Roy C. Bartholomae 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
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Information Circular 9004 
 
 The Bureau of Mines Noise-Control 
 Research Program— A 10-Year Review 
 
 By William W. Aljoe, Thomas G. Bobick, 
 
 Gerald W. Redmond, and Roy C. Bartholomae 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
 Donald Paul Model, Secretary 
 
 BUREAU OF MINES 
 Robert C. Norton, Director 
 

 Library of Congress Cataloging in Publication Data; 
 
 The Bureau of Mines noise-control research program. 
 
 (Information circular / United States Department of the Interior, Bu- 
 reau of Mines ; 9004) 
 
 Bibliography: p. 83-85. 
 
 Supt. of Docs, no.: I 28.27:9004. 
 
 I. Mineral industries— Noise control. I. Aljoe, William VV. II. 
 United States, Bureau of Mines. III. Series: Information circular 
 (United States. Bureau of Mines) ; 9004. 
 
 -^Wf^tSOM — -fTD893.6.M5] 622s [622'. 8] 84-600226 
 
a CONTENTS 
 
 C^ Page 
 
 ^ Abstract 1 
 
 ^ Introduction 1 
 
 Part 1. — Government Involvement In mining noise control 2 
 
 The mining noise problem. 2 
 
 K) Federal noise regulations pertaining to mining 2 
 
 ^ The Bureau of Mines approach to mining noise control 5 
 
 ^ Part 2. — Overview of the Bureau's noise-control research 6 
 
 ^ Underground coal mining 7 
 
 ^s^ Coal extraction methods 8 
 
 .> Haulage methods 10 
 
 Roof support methods 11 
 
 Underground hardrock mining 12 
 
 Hardrock extraction methods 12 
 
 Haulage methods 15 
 
 Roof support methods 16 
 
 Surface mining 16 
 
 Preparation and processing plants 17 
 
 Part 3. — Results of selected research programs 19 
 
 Underground coal mining 19 
 
 Coal cutting 19 
 
 Chain conveyors 29 
 
 Mantrip vehicles 34 
 
 Stoper drills 36 
 
 Underground hardrock mining 41 
 
 Jumbo-mounted percussion drills 42 
 
 Load-haul-dump machines 48 
 
 Surface mining 53 
 
 Bulldozers 55 
 
 Front-end loaders 58 
 
 Preparation and process ing plants 61 
 
 Coal preparation plants 62 
 
 Taconlte processing plants 67 
 
 Nonmetallic processing plants 72 
 
 Additional research on screen-noise abatement 73 
 
 Use of hearing protectors in the mining environment 74 
 
 Limitations of hearing protector effectiveness 75 
 
 Assessing earmuf f attenuation 76 
 
 Hearing protector interference with required acoustical cues 77 
 
 Part 4. — Future Bureau noise-control efforts 77 
 
 Facilities and equipment 78 
 
 Research programs 79 
 
 Coal cutting 79 
 
 Conveying 79 
 
 '^ Percussion drilling. 79 
 
 '\^ Hearing protectors 80 
 
 ^ Technology transfer 80 
 
 Summary 80 
 
 References 83 
 
 (\ 
 
11 
 
 ILLUSTRATIONS 
 
 Page 
 
 1 . Hearing thresholds of retired miners 2 
 
 2 . Hearing loss among miners 3 
 
 3. Noise levels and operating times of underground coal mining machines 7 
 
 4 . Noise sources on continuous mining machines 8 
 
 5. Noise sources on typical coal mine stoper drill 12 
 
 6. Major components of a jumbo drill rig 13 
 
 7. Noise sources on jumbo-mounted drills 14 
 
 8. Noise sources of mobile, diesel-powered surface mining equipment 17 
 
 9 . Grinding mills in taconite processing plant 18 
 
 10. Linear cutting apparatus 20 
 
 11. Coal-cutting force versus time 21 
 
 12. Coal-cutting force (power spectral density) versus frequency 22 
 
 13. Continuous miner in reverberation room 25 
 
 14. Cutting sequence of auger-type continuous miner 26 
 
 15. Standard auger-miner cutting head 27 
 
 16. Reduced-noise auger-miner cutting head 27 
 
 1 7 . Fabrication drawing of reduced-noise auger 28 
 
 18. Noise-producing components of a continuous miner chain conveyor 29 
 
 19. Noise-control treatments on conveyor decks and sidewalls 30 
 
 20. Noise-control treatments on conveyor idler (tail) roller and takeup plate. 31 
 
 21. Mantrip vehicle used in retrofit noise-control program 35 
 
 22. Mantrip noise sources and transmission paths 35 
 
 23. Resilient wheels to isolate mantrip from wheel-rail noise 36 
 
 24. Redesigned suspension system of factory-quieted mantrip 37 
 
 25. Motor enclosure of factory-quieted mantrip 37 
 
 26. Wraparound jacket-type muffler for stoper drill 38 
 
 27. Damping collar for stoper drill steel 39 
 
 28. Stoper with retrofit noise-control treatments in operation 39 
 
 29. Internal components of standard stoper drill with rifle-bar rotation 39 
 
 30. Internal components of redesigned "quiet" stoper drill 40 
 
 3 1 . Redesigned "quiet" stoper on f eedleg 41 
 
 32. Drilling controls of redesigned "quiet" stoper drill 42 
 
 33. Jumbo drill within retrofit muffler enclosure (cover open) 43 
 
 34. Jumbo drill within retrofit muffler enclosure (cover closed) 43 
 
 35. Schematic views of retrofit muffler enclosure for jumbo drill 44 
 
 36. Retrofit shroud tube for controlling jumbo drill-steel noise 44 
 
 37. Jumbo drill with retrofit noise-control treatments in underground zinc 
 
 mine 45 
 
 38. Redesigned, noise-controlled jumbo drill at start of hole 47 
 
 39. Redesigned, noise-controlled jumbo drill at completion of hole 47 
 
 40. Noise sources on typical diesel-powered LHD vehicle 49 
 
 41. Sound-absorbing foam lining within LHD transmission compartment 49 
 
 42. Retrofit acoustical enclosure for LHD engine 50 
 
 43. Retrofit noise-control treatments on LHD engine cooling fan 51 
 
 44. Exhaust muffler on redesigned, noise-controlled LHD vehicle 52 
 
 45. LHD operator-compartment noise-control treatments 53 
 
 46. Vibration-isolation mount for LHD transmission 53 
 
 47. Caterpillar D-9G bulldozer (ROPS-FOPS only) with retrofit noise-control 
 
 treatments 54 
 
 48. Noise-control treatments installed on Caterpillar D-9G bulldozer (ROPS- 
 
 FOPS only) 54 
 
ILLUSTRATIONS—Continued 
 
 iii 
 
 Page 
 
 49. Step-by-step noise reduction of Caterpillar D-9G bulldozer (ROPS-FOPS 
 
 only) 55 
 
 50. Noise-control treatments on cab-equipped Caterpillar D-9G bulldozer 57 
 
 51. Step-by-step noise reduction of cab-equipped Caterpillar D-9G bulldozer... 58 
 
 52. Noise-control treatments installed on International Harvester TD-25C 
 
 bulldozer 59 
 
 53. Step-by-step noise reduction of International Harvester TD-25C bulldozer.. 59 
 
 54. Noise-control treatments installed on Caterpillar 988 front-end loader.... 60 
 
 55. Step-by-step noise reduction of Caterpillar 988 front-end loader 61 
 
 56. Noise-control treatments installed on International Harvester H-400 B 
 
 front-end loader 61 
 
 57. Step-by-step noise reduction of International Harvester H-400 B front-end 
 
 loader 62 
 
 58. Flow chart of Georgetown coal preparation plant 62 
 
 59. Curtain around screening area in Georgetown coal preparation plant 63 
 
 60. Schematic of enclosed gallery-type walkway in preparation plant 69 
 
 61. Secondary crusher area in taconite processing plant 70 
 
 62. Taconite grinding mill and noise barrier 71 
 
 63. Noise-control treatments for rapper on taconite fines screen 72 
 
 64. Flow chart of screening simulation algorithm 74 
 
 65. Noise pathways to ear protected by hearing protective device 75 
 
 66. Quieted versus unquieted noise levels of underground mining equipment 81 
 
 67. Quieted versus unquieted noise levels of preparation and processing plant 
 
 equipment 82 
 
 TABLES 
 
 1. Typical noise levels of various sounds , 3 
 
 2. Allowable noise exposure time per day 4 
 
 3. Illustrative calculation of worker NEI 4 
 
 4. "Noise offenders" in surface mining operations 16 
 
 5. Results of continuous miner cutting-noise tests 24 
 
 6. Summary of aboveground chain conveyor noise-control tests 32 
 
 7 . Summary of underground chain conveyor noise-control tests 33 
 
 8. Results of retrofit jumbo drill noise-control tests 45 
 
 9 . Breakdown of noise sources on unmodified LHD vehicle 48 
 
 10. Results of underground tests of redesigned, noise-controlled LHD 53 
 
 11. Breakdown of bulldozers used in U.S. surface mines, 1977, by model 55 
 
 12. Summary of bulldozer retrofit noise-control treatment results 56 
 
 13. Summary of treatments and costs for bulldozer noise-control treatments.... 56 
 
 14. Summary of front-end loader retrofit noise-control treatment results 58 
 
 15. Summary of treatments and costs and for front-end loader noise-control 
 
 packages 60 
 
 16. Short-term and long-term effectiveness of noise-control treatments at 
 
 Georgetown coal preparation plant 64 
 
 17. Noise-control treatments used at Georgetown coal preparation plant 65 
 
 18. Noise-control alternatives for new coal preparation equipment 68 
 
 19. Noise-control treatments installed in three nonmetallic processing plants. 73 
 

 UNIT OF MEASURE ABBREVIATIONS 
 
 USED IN THIS 
 
 REPORT 
 
 dB 
 
 decibel 
 
 lb/ft2 
 
 pound per square foot 
 
 dBA 
 
 decibel, A-weighted 
 
 kHz 
 
 kilohertz 
 
 dB/Hz 
 
 decibel-second per cycle 
 
 mi/h 
 
 mile per hour 
 
 ft 
 
 foot 
 
 min 
 
 minute 
 
 ff Ibf 
 
 foot pound (force) 
 
 ym 
 
 micrometer 
 
 ff Ibf/min 
 
 foot pound (force) per minute 
 
 ms 
 
 millisecond 
 
 ga 
 
 gauge 
 
 pet 
 
 percent 
 
 h 
 
 hour 
 
 re.v/niin 
 
 revolution per minute 
 
 h/d 
 
 hour per day 
 
 s 
 
 second 
 
 Hz 
 
 hertz 
 
 ton/h 
 
 ton per hour 
 
 in 
 
 inch 
 
 yd3 
 
 cubic yard 
 
 in/s 
 
 inch per second 
 
 yr 
 
 year 
 
 Ibf 
 
 pound (force) 
 
 
 
THE BUREAU OF MINES NOISE-CONTROL RESEARCH PROGRAM-A 10-YEAR REVIEW 
 
 By William W. Aljoe, ^ Thomas G. Bobick, ^ Gerald W. Redmond, and Roy C. Bartholomae"^ 
 
 ABSTRACT 
 
 This report summarizes the Bureau of Mines noise-control research 
 program from 1972 to 1982, Each segment of the mining industry — under- 
 ground coal, underground hardrock, surface mining, and processing 
 plants — has different noise-control problems because of vast differ- 
 ences in working procedures, equipment, and workplace design. The Bu- 
 reau has identified the most serious noise problems in each segment and 
 has developed strategies for attacking these problems. 
 
 This publication points out the need for noise control in the mining 
 industry, discusses Federal regulations governing worker exposure to 
 noise, and describes the Bureau's overall approach to mining noise- 
 control research. It traces the history of noise overexposure in each 
 segment of the mining industry and discusses the major noise sources. 
 It provides detailed information on noise-control research efforts in 
 the Bureau's major areas of emphasis, including the results of these 
 efforts. Finally, the report discusses the Bureau's future role in 
 research on mining noise control, emphasizing the need to expend more 
 effort on long term in-house investigations into the noise problems 
 that have been identified in past programs as the most serious ones, 
 
 INTRODUCTION 
 
 Noise exposures of workers in the mining industry often exceed those 
 specified under Federal noise regulations. Because of the severity and 
 diversity of this problem, the Bureau of Mines has conducted a wide 
 variety of noise-control research efforts. This report summarizes the 
 Bureau's noise-control research program during the 1972-82 period, giv- 
 ing an overall picture of the Bureau's activities in this area and its 
 major accomplishments. Further details on the projects described in 
 this publication can be found in the reports included in the reference 
 list or by contacting the authors of this report at the Bureau's Pitts- 
 burgh Research Center, 
 
 ^Mining engineer, 
 ^Industrial hygienist. 
 ^Supervisory electrical engineer. 
 Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA, 
 
PART 1. — GOVERNMENT INVOLVEMENT IN MINING NOISE CONTROL 
 
 THE MINING NOISE PROBLEM 
 
 Noise is often regarded merely as a 
 nuisance; however, it is also a wide- 
 spread occupational health problem. In 
 the mining industry, the noise problem is 
 especially serious because overexposure 
 can cause permanent hearing loss. The 
 extent and severity of the noise problem 
 in mining was revealed in a 1976 study by 
 the National Institute for Occupational 
 Safety and Health (NIOSH) (25).^ This 
 study found that underground coal miners 
 had measurably worse hearing than the na- 
 tional average population; for example, 
 the median hearing threshold of retired 
 miners was 20 dB greater than that of the 
 general population (fig. 1). Figure 2 
 shows that, at age 60, over 70 pet of all 
 miners had a hearing loss greater than 25 
 dB , and about 28 pet had a hearing loss 
 
 '^Underlined numbers in parentheses re- 
 fer to items in the list of references at 
 the end of this report. 
 
 greater than 40 dB, Miners experience 
 greater hearing impairment than most 
 other industrial workers because of their 
 work-related overexposure to noise. 
 Because this hearing loss occurs gradual- 
 ly over many years, the individual is not 
 aware of it until he or she notices dif- 
 ficulties in coimnuni eating with other 
 people or an inability to hear safety 
 signals in the workplace, 
 
 FEDERAL NOISE REGULATIONS 
 PERTAINING TO MINING 
 
 The Federal Government regulates the 
 noise exposure of miners under the Fed- 
 eral Mine Safety and Health Act of 1977 
 (Public Law 95-164). This act, which 
 supersedes the previous Federal Coal Mine 
 Health and Safety Act of 1969 (Public Law 
 91-173) , covers surface and underground 
 operations of metal and nonmetal mines as 
 well as coal mines. The Code of Federal 
 Regulations (CFR) defines permissible 
 noise exposures for all mine workers in 
 
 
 
 J L 
 
 4 5 
 
 FIGURE 
 
 1 1 1 
 
 _ Ages 
 
 1 
 
 
 65 to 74 
 
 ^ 
 
 ^ 
 
 — / 
 
 
 ^^( 
 
 _ J 
 
 ^O" 
 
 _ 
 
 / / 
 — 1 1 
 / / 
 / / 
 / / 
 
 
 - 
 
 / / 
 / / 
 
 ir^ p' 
 
 
 - 
 
 
 
 
 '■-o' 
 
 
 
 1 1 1 
 
 1 
 
 
 6— --0 
 
 / KEY 
 
 General 
 
 population 
 
 Retired - 
 
 miners 
 
 I I I 
 
 I 234501 2345 
 FREQUENCY, kHz 
 
 Hearing thresholds of retired miners. 
 
Q. 
 
 {/f 
 (/) 
 O 
 
 _J 
 
 o 
 z 
 a: 
 
 X 
 X 
 
 I- 
 
 DC 
 LlI 
 
 80 
 
 60- 
 
 40 
 
 20 
 
 Hearing loss >25 dB 
 
 
 
 20 30 40 50 
 
 MINERS' AGE, yr 
 
 FIGURE 2. - Hearing loss among miners. 
 
 terms of noise dose, which includes both 
 the level and duration of the noise. ^ 
 
 ^Mining noise regulations are contained 
 in the following sections of 30 CFR: 
 55.5-50 (surface metal and nonmetal 
 mines), 56.5-50 (sand, gravel, and 
 crushed stone), 57.5-50 (underground met- 
 al and nonmetal), 70.500 through 70.511 
 (underground coal), and 71.800 through 
 71.805 (surface coal). 
 
 Noise level is measured in deci- 
 bels,^ using a sound level meter, and the 
 A-weighted decibel (dBA) is the unit of 
 measure used in the regulations. The 
 A-weighting scale takes into account the 
 fact that the human ear is more respon- 
 sive to high-frequency sounds (1,000 to 
 5,000 Hz) than to low-frequency sounds 
 (below 1,000 Hz). Table 1 shows the typ- 
 ical A-weighted noise levels of everyday 
 events. 
 
 Table 2 gives the relationship between 
 noise level and allowable exposure time 
 per day for mining operations. Exposure 
 
 ^The decibel is a dimensionless rela- 
 tionship between two quantities, de- 
 fined as 20 times the logarithm of 
 the ratio of a measured quantity to a 
 reference quantity. When measuring 
 noise, the sound pressure is the quan- 
 tity of interest, and the sound pres- 
 sure level (SPL) is defined as 
 measured sound pressure 
 reference sound pressure 
 The standard reference sound pressure 
 used in the United States is 4.32 x 10"^ 
 Ib/ft^. Further discussion of the SPL 
 can be found in reference 18 or in any 
 standard textbook on noise control. 
 
 20 log,o 
 
 -^ dB. 
 
 TABLE 1, - Typical noise levels of various sounds, A-weighted decibels 
 
 Noise source 
 
 Noise level 
 
 Subjective e 
 
 140 
 
 Deafening. 
 
 '130 
 
 Do. 
 
 120 
 
 Intolerable. 
 
 110 
 
 Do. 
 
 105 
 
 Very loud. 
 
 100 
 
 Do. 
 
 90 
 
 Do. 
 
 80 
 
 Loud, 
 
 70 
 
 Do. 
 
 60 
 
 Moderate. 
 
 50 
 
 qvilet. 
 
 40 
 
 Faint. 
 
 30 
 
 Do. 
 
 20 
 
 Very faint. 
 
 10 
 
 Do. 
 
 Jet engine 
 
 Pneumatic chipping 
 
 Underground pneumatic percussion drill, , 
 Automatic punch press; hand grinding..,. 
 Bulldozer (at operator's position)...... 
 
 Auto horn (at 10 ft) 
 
 Construction site in urban area 
 
 Busy street; school cafeteria 
 
 Loud radio; stenographic room 
 
 Restaurant; department store 
 
 Average office; quiet conversation 
 
 Residential area at night 
 
 Quiet residence (inside) 
 
 Background in TV studio 
 
 Rustle of leaves 
 
 Threshold of pain. 
 
TABLE 2. - Allowable noise exposure 
 time per day 
 
 90. 
 
 No 
 
 ise level, 
 dBA 
 
 Allowable exposure 
 time , h 
 
 8 
 
 c>?. 
 
 
 
 
 6 
 
 95. 
 P7. 
 
 
 
 
 4 
 3 
 
 100. 
 
 
 
 
 2 
 
 102. 
 
 
 
 
 1.5 
 
 lOS. 
 
 
 
 
 1 
 
 107, 
 
 
 
 
 .75 
 
 110. 
 
 
 
 
 .5 
 
 115. 
 
 
 
 
 <.25 
 
 to noise levels less than 90 dBA is not 
 regulated. It is evident from table 2 
 that the higher the noise level, the 
 shorter the allowable exposure time is. 
 At 90 dBA, the maximum allowable exposure 
 time is 8 h/d; every 5-dBA increase in 
 noise level reduces the maximum allowable 
 exposure time by half. Exposure to con- 
 tinuous noise levels higher than 115 dBA 
 is not permitted by law. 
 
 In practice, compliance with noise reg- 
 ulations is determined by measuring each 
 worker's noise exposure index (NEI) , 
 which includes both the noise level and 
 the exposure time at each level. NEI is 
 defined as 
 
 Ci C2 
 
 — L + _^ + • • • » 
 
 Ti T2 ^ 
 
 where C^, C2 , etc., are the actual times 
 the worker was exposed to each noise lev- 
 el above 90 dBA, and T] , T2 , etc., are 
 
 the allowable exposure times (table 2) 
 for the noise levels to which the worker 
 was exposed. A worker is overexposed, or 
 out of compliance, when the NEI value ex- 
 ceeds 1. NEI values are often expressed 
 as a percent of the allowable noise dose, 
 i.e., actual NEI value x 100 pet. For 
 example, table 3 shows that during an 8-h 
 shift, a worker is exposed to noise for 
 2 h at 89 dBA, 2 h at 90 dBA, 2 h at 95 
 dBA, and 2 h at 100 dBA. The NEI for 
 this worker is 
 
 2 2 2 
 
 I" + -| + ^ = 0.25 + 0.5 + 
 
 1.0 
 
 = 1.75 X 100 pet = 175 pet. 
 
 Thus, the worker is out of compliance 
 with Federal noise regulations (because 
 the NEI exceeds 100 pet). 
 
 The NEI measures only the worker's ex- 
 posure to continuous noise; it does not 
 include "impulse" or "impact" noise, 
 which takes place over too short a time 
 to be included in NEI calculations. Im- 
 pulse or impact noise is defined as a 
 sound that (1) reaches its peak level 
 within 35 ms after initiation and (2) de- 
 creases to at least 20 dB below its peak 
 level within 500 ms after reaching the 
 peak level (535 ms after initiation) 
 (24). If the impulses recur at intervals 
 less than 1 s apart, the noise is classi- 
 fied as continuous. All of the Bureau's 
 noise-control research efforts have dealt 
 with continuous noise because this is by 
 far the most prevalent type of mining 
 noise. 
 
 TABLE 3. - Illustrative calculation of worker NEI 
 during 8-h shift 
 
 Noise level. 
 
 dBA 
 
 Allowable 
 
 exposure time 
 
 (T), h 
 
 Actual 
 exposure time 
 (C), h 
 
 Dosage 
 [(C/T) X 100], 
 pet 
 
 89 
 
 Unlimited 
 
 8 
 
 4 
 
 2 
 
 NAp 
 
 2 
 2 
 2 
 2 
 
 NAp 
 
 
 
 90 
 
 25 
 
 95 
 
 50 
 
 100 
 
 100 
 
 Total 
 
 1175 
 
 NAp Not applicable, 
 
 NEI. 
 
THE BUREAU OF MINES APPROACH 
 TO MINING NOISE CONTROL 
 
 Because noise affects large numbers of 
 workers in all segments of the mining in- 
 dustry the Bureau has become involved in 
 many different aspects of noise control. 
 The objective of the Bureau's noise- 
 control research efforts is to investi- 
 gate techniques that would, if adopted, 
 help bring the mining industry into com- 
 pliance with Federal noise regulations. 
 
 Because the mining noise problem is so 
 widespread, the Bureau must define those 
 areas in which it can appropriately be 
 involved. Three criteria must be met be- 
 fore the Bureau will investigate a spe- 
 cific noise problem: (1) The problem 
 must result in serious worker overexpo- 
 sure and risk of hearing loss; (2) it 
 must affect a large number of mine work- 
 ers; and (3) the mining industry itself 
 must be incapable of solving the prob- 
 lem because of insufficient financial re- 
 sources or technical expertise. For ex- 
 ample, the Bureau is conducting long-term 
 basic studies into all the major noise- 
 generating mechanisms of the coal-cutting 
 process; this requires a level of effort 
 and technical expertise that equipment 
 manufacturers cannot provide. Converse- 
 ly , the Bureau is not heavily involved in 
 acoustical booth technology because these 
 structures are commercially available and 
 can be improved through efforts by the 
 private sector. However, the Bureau has 
 become involved in specific areas where 
 acoustical booth technology is especially 
 difficult to apply, such as in portable 
 mineral processing plants. 
 
 An important aspect of the Bureau's ap- 
 proach to noise control is its emphasis 
 on engineering controls for the major 
 noise sources in the mining workplace. 
 Although Federal noise regulations spec- 
 ify the use of both engineering and ad- 
 ministrative (e.g., job switching) con- 
 trols as the primary means of addressing 
 mining noise problems , Bureau research 
 has shown that engineering controls can 
 often be more effective. For example, 
 job switching during any given day would 
 be nearly impossible in many mines 
 
 because of long, nonproductive travel 
 times between noisy and quiet workplaces. 
 In addition, serious safety hazards could 
 result if inexperienced workers were 
 switched into noisy jobs that require 
 high levels of skill. Finally, if an 
 engineering control can be applied suc- 
 cessfully to the noise source, admini- 
 strative controls will be unnecessary 
 and the solution will generally be more 
 permanent. 
 
 At first glance, personal hearing pro- 
 tectors (earplugs, earmuffs, etc.) seem 
 to be a relatively cheap, simple solution 
 to almost any noise problem. However, 
 Federal regulations state that hearing 
 protectors should be considered as only a 
 secondary noise-control treatment in sit- 
 uations where engineering or administra- 
 tive controls cannot be used. Reasons 
 for this include the following: (1) Ear- 
 plugs and earmuffs do not usually provide 
 the same degree of protection in the min- 
 ing workplace as they do in the labora- 
 tory or in other types of workplaces; 
 (2) miners often refuse to wear hearing 
 protectors because they are uncomfort- 
 able, annoying, or cumbersome (especially 
 earmuffs); and (3) hearing protectors can 
 prevent miners from hearing warning sig- 
 nals such as "roof talk" (the noise that 
 often precedes a roof fall) or backup 
 alarms on moving equipment. Although the 
 Bureau is still conducting research into 
 the potential effectiveness of hearing 
 protectors in the underground mining 
 environment, they are viewed as only a 
 temporary solution to a problem that 
 requires the use of engineering controls. 
 
 Unfortunately, the mining environment 
 often prohibits the use of standard engi- 
 neering noise-control techniques common 
 to other industrial workplaces. In most 
 cases (especially underground) , the min- 
 ing workplace is very confined, and it 
 changes constantly because it is con- 
 trolled primarily by local geology rather 
 than by engineering design. Physical 
 conditions are usually quite severe 
 (e.g., wet floors and sloughing roof and 
 ribs), thus restricting the use of stan- 
 dard acoustical materials such as sound- 
 absorbing foam panels. Another reason 
 
treating the mining workplace would be 
 impractical is that many miles of mine 
 tunnels would have to be treated. Sur- 
 rounding the noise source with an acous- 
 tical enclosure is another standard en- 
 gineering noise-control technique that 
 cannot usually be employed because the 
 most severe mining noise sources are of- 
 ten large mobile machines such as contin- 
 uous miners and jumbo drills. 
 
 Engineering noise controls in mining 
 can be as simple as adding sound-absorb- 
 ing material to the inside of a bulldozer 
 operator's cab, or as complex and demand- 
 ing as redesigning the inner workings of 
 percussion drills to produce less noise 
 while maintaining the same drilling 
 power. The two basic engineering noise- 
 control strategies are (1) to install 
 noise-control treatments on existing min- 
 ing equipment (the retrofit approach) and 
 (2) to design inherently quieter mining 
 equipment. 
 
 The retrofit approach is usually 
 preferred if a straightforward noise- 
 control technique can be employed, and it 
 is especially desirable in the capital- 
 intensive mining industry. If a retrofit 
 noise-control treatment is successful, it 
 can often allow timely and relatively 
 inexpensive compliance with Federal noise 
 regulations. An acoustical cab for a 
 bulldozer operator is one example of a 
 successful application of the retrofit 
 approach. However, even in this simple 
 case, careful application of the noise- 
 control treatments is essential for 
 optimum noise reduction. Unfortunately, 
 the retrofit approach is not appropri- 
 ate for many existing pieces of mining 
 equipment. 
 
 In some cases , engineering noise con- 
 trols can be incorporated into the ma- 
 chine design without affecting overall 
 performance. For example, the manufac- 
 turer of an underground load-haul-dump 
 (LHD) vehicle can easily install vibra- 
 tion-isolation mountings (a noise-reduc- 
 ing feature) on the engine and trans- 
 mission of a newly built machine. Even 
 though this treatment requires only minor 
 dimensional changes in the original vehi- 
 cle design, it could not be accomplished 
 through the retrofit approach. The rede- 
 sign approach is especially advantageous 
 when the manufacturer has already decided 
 to change its equipment for production 
 puropses; noise-control technology would 
 then constitute only a fraction of the 
 redesign cost. 
 
 Cost-effectiveness is of primary impor- 
 tance in any approach to mining noise 
 control. The Bureau considers an engi- 
 neering noise-control technique to be 
 cost-effective if it (1) does not reduce 
 production or productivity, (2) costs 
 relatively little to install, (3) does 
 not increase long-term maintenance costs, 
 and (4) does not introduce new hazards 
 into the mining environment. The total 
 amount of noise reduction is also impor- 
 tant to the cost-effectiveness of any 
 noise-control program. That is, a low- 
 cost "quick-fix" approach may not be 
 the best way to treat a noisy machine 
 if a more expensive long-term research 
 and development program can result in a 
 much quieter, more efficient machine 
 design. The Bureau has consistently 
 sought the most cost-effective engineer- 
 ing approaches to mining noise problems 
 and has therefore used both retrofit and 
 redesign techniques in its research. 
 
 PART 2. —OVERVIEW OF THE BUREAU'S NOISE-CONTROL RESEARCH 
 
 Historically, the amount of mechaniza- 
 tion in mines has determined the extent 
 of worker overexposure to noise. This 
 part of the report reviews the history of 
 mechanization in the four major segments 
 of the mining industry (underground coal, 
 underground hardrock, surface mining, and 
 mineral processing) and identifies the 
 
 most serious "noise offenders" in each 
 segment. The Bureau research programs 
 designed to control noise from these 
 "noise offenders" are described briefly 
 in the following sections; part 3 dis- 
 cusses in detail some of the more suc- 
 cessful Bureau research programs. 
 
UNDERGROUND COAL MINING 
 
 When the first commercial U.S. under- 
 ground coal mines were developed in the 
 18th century, the three major unit opera- 
 tions — extraction, haulage, and roof sup- 
 port — were performed manually. The coal 
 was cut from the solid bed using hand 
 tools such as picks and bars, then shov- 
 eled into baskets, boxes, carts, or 
 wheelbarrows and dragged by workers or 
 animals to the outside or to the foot of 
 a shaft. Roof supports were emplaced 
 with hand tools. Obviously, none of 
 these manual extraction, hauling, and 
 roof support methods generated much 
 noise. 
 
 longwall shearer, and the air-powered 
 "stoper" roof drill — became possible. 
 These three machines commonly produce 
 noise levels that result in violations of 
 Federal noise regulations (5), 
 
 Figure 3 shows the noise levels and 
 operating times of most of the noise- 
 producing machines in today's underground 
 coal mines. This illustration shows that 
 at least one noise offender is present in 
 each major unit operation (extraction, 
 haulage, and roof support). Therefore, 
 the Bureau has conducted research pro- 
 grams in all three of these areas . 
 
 Underground coal mining operations 
 started to become mechanized in the late 
 1800' s with the development of punching 
 machines and chain-type cutters to under- 
 mine the coal seam before blasting. Oth- 
 er early mining machines were coal and 
 rock drills, electric and compressed-air 
 locomotives, and gathering-arm-type load- 
 ing machines. Still, very few workers 
 were overexposed to noise because few 
 mining operations were mechanized. 
 
 Full mechanization, and the noise asso- 
 ciated with it, emerged in the 1920' s 
 when gathering-arm-type loading machines 
 became more common. Rubber-tired and 
 crawler-mounted machines for cutting 
 and hauling coal away from the face re- 
 duced the number of workers underground, 
 but those who remained were exposed to 
 increasing amounts of machine-generated 
 noise. The desire for increased coal 
 production led to the development of 
 larger, faster-moving machines with 
 greater mechanical power; unfortunately, 
 the noise associated with these machines 
 also increased tremendously. 
 
 The final development contributing to 
 increased noise in the coal mining envi- 
 ronment came just after World War II, 
 when the introduction of the tungsten 
 carbide cutting bit enabled coal mining 
 machinery to cut both rock and coal. 
 With the tungsten carbide cutting bit, 
 the three noisiest machines in present- 
 day coal mines — the continuous miner, the 
 
 I25| r 
 
 CD 
 T3 
 
 > 
 
 LlI 
 
 UJ 
 
 to 
 
 40 80 120 160 
 
 OPERATING TIME, mm 
 
 200 
 
 FIGURE 3o - Noise levels and operating times of 
 underground coal mining machines. 
 
Coal Extraction Methods 
 
 Continuous Mining 
 
 Figure 4 Illustrates five different 
 noise-producing mechanisms on continuous 
 mining machines: 
 
 1. Face-radiated noise - When the bits 
 on the rotating cutting drums strike and 
 break the coal or rock, noise radiates 
 from the coal or rock face, 
 
 2. Cutting head noise - Cutting bit 
 vibration is transferred to the drums, 
 the boom holding the drums, and the rest 
 of the machine structure. These vibrat- 
 ing components often generate a very com- 
 plex noise pattern. 
 
 3. Gathering head noise - After the 
 coal or rock is cut from the face, it 
 falls onto a pan called the gather- 
 ing head, beneath the cutting drums. 
 
 Rotating arms or discs on this pan force 
 the broken material into the mouth of a 
 conveyor that carries it to the rear of 
 the miner. Gathering head noise results 
 from various impacts involving the ro- 
 tating arms or discs, the pan, and the 
 broken material. 
 
 4. Conveyor noise - The conveyor con- 
 sists of a continuously moving chain to 
 which a series of flights are attached. 
 The chain and flights move at the bottom 
 of a metal trough. Scraping and impacts 
 involving the chain, flights, trough, and 
 broken material cause a great deal of 
 noise. 
 
 5. Motor and drive-train noise - The 
 motors and drive train supplying power to 
 the cutting drums, gathering head, con- 
 veyor, and other machine components gen- 
 erate noise both internally (gear noise) 
 and externally (vibration Imparted to ma- 
 chine structures). 
 
 Motor and 
 drive train noise 
 
 I 
 
 FIGURE 4. - Noise sources on continuous mining machines. 
 
Laboratory research sponsored by the 
 Bureau (see part 3) has indicated that 
 face-radiated noise could be reduced, 
 without harming production, if the bits 
 on the miner cutting head moved at a 
 slower speed while penetrating deeper in- 
 to the coal face (J^, 3), Unfortunately, 
 present continuous miner designs are not 
 adaptable to the Bureau's slow, deep- 
 cutting technique. More research is 
 needed to evaluate the potential noise 
 reduction that can be obtained using 
 slow, deep-cutting miners. 
 
 Cutting head noise can be reduced by 
 preventing the transfer of cutting bit 
 vibration to the cutting drums and to 
 other machine components. The Bureau is 
 presently investigating the merits of 
 completely redesigning the cutting drum 
 of the miner to reduce vibration trans- 
 fer. Preliminary laboratory studies have 
 shown a potential for significant noise 
 reduction, but full-scale tests with a 
 continuous miner equipped with an "iso- 
 lated cutting drum" are needed to verify 
 the laboratory predictions. It appears 
 that this approach (isolating the cutting 
 drum) may be applicable to a wide vari- 
 ety of coal- and rock-cutting machines 
 because vibrational energy is absorbed 
 close to its source — the interface be- 
 tween the cutting bits and the coal or 
 rock. 
 
 Cutting head noise can also be reduced 
 by limiting the vibration of the cutting 
 drums and other structural components of 
 the continuous miner. In one Bureau 
 project (27) , the noise radiated from the 
 cutting heads of a low-coal auger-type 
 miner was reduced by about 10 dBA by en- 
 larging the auger core, stiffening the 
 helix surrounding the core, and damping 
 its vibration with sand. (This project 
 is discussed in detail in part 3.) Mine 
 operators can easily modify their augers 
 in the same manner with the aid of de- 
 tailed fabrication instructions provided 
 by the Bureau (28). This reduced-noise 
 auger can also be obtained from the auger 
 manufacturer. 
 
 When the continuous miner is cutting 
 and loading coal, the gathering head is 
 
 usually less noisy than the cutting head 
 or the conveyor. Although the impact of 
 the rotating gathering arms on the pan 
 generates significant noise when the 
 miner is "running empty," this is not one 
 of its major operating modes. Therefore, 
 the Bureau has directed most of its ef- 
 forts toward controlling cutting noise 
 and conveyor noise rather than gathering 
 head noise. Although disc-type gathering 
 devices may be quieter than gathering 
 arms, because they eliminate arm-to-pan 
 impacts, no data exist to verify their 
 potential for reducing of gathering head 
 noise. 
 
 Conveyor noise can be reduced by 
 (1) smoothing discontinuities between 
 various sections of the trough, (2) iso- 
 lating the chain and flights from the 
 trough, and (3) structurally damping the 
 large metal panels of the trough. A re- 
 cent Bureau project ( 16 ) showed that con- 
 veyor-generated noise could be reduced 
 by about 10.5 dBA through these tech- 
 niques. However, this work was done in 
 an aboveground test facility, using only 
 the conveyor portion of the continuous 
 miner (no cutting heads, gathering arms, 
 drive train and motors, etc.). (For more 
 details, see part 3.) Further testing on 
 operating continuous miners is necessary 
 to determine the overall noise reduction 
 and durability of conveyor noise-control 
 treatments. 
 
 Motor and drive 
 ally not as loud 
 or noise, but it 
 components of the 
 orate. This noi 
 negligible if sys 
 tors and shorter, 
 were used. Bureau 
 directed toward 
 source. 
 
 train noise is usu- 
 as cutting and convey- 
 tends to increase as 
 drive system deteri- 
 se would probably be 
 terns with smaller mo- 
 simpler drive trains 
 research has not been 
 control of this noise 
 
 Conventional Mining 
 
 Although the number of coal mines using 
 the conventional mining system has de- 
 creased since the advent of the continu- 
 ous miner, many smaller mines continue to 
 use this system. In conventional mining, 
 coal is blasted rather than cut from the 
 
10 
 
 face, and mechanized coal extraction 
 procedures include (1) undercutting the 
 face with chain-type cutting machines, 
 
 (2) drilling holes for explosives with 
 machine-mounted coal drills, and 
 
 (3) loading the coal into shuttle cars 
 with gathering-arm-type loading machines. 
 
 As indicated in figure 3, the loading 
 machine (or loader) is the noisiest ma- 
 chine used in conventional mines. Since 
 loader noise is due primarily to conveyor 
 noise and gathering head noise, control 
 of loader noise includes the same 
 conveyor-quieting techniques as have been 
 described for continuous miners. In 
 fact, controlling loader noise is some- 
 what simpler than controlling miner noise 
 because there is no coal-cutting noise. 
 
 The Bureau has not conducted noise-con- 
 trol research directed specifically to- 
 ward conventional coal mining equipment, 
 except for research on chain conveyors. 
 The two major reasons for this are 
 (1) the decreasing number of conventional 
 mines as compared to continuous and long- 
 wall mines and (2) the relatively low 
 noise levels of coal cutters and face 
 drills compared with the noise levels of 
 continuous miners and longwall shearers 
 (fig. 3). 
 
 Longwall Mining 
 
 Although longwall mining systems were 
 almost nonexistent in the United States 
 prior to 1965, they now produce about 
 10 pet of all domestic coal mined un- 
 derground. A continued increase in the 
 use of these systems is expected in the 
 future; therefore, the Bureau is now 
 attempting to identify and develop the 
 fundamentals that will be necessary to 
 control all major noise sources in long- 
 wall mining. Figure 3 shows that the 
 longwall shearer operator is often ex- 
 posed to more noise than the continuous 
 miner operator. Approaches to longwall 
 noise-control have been somewhat similar 
 to those for continuous miners because 
 both mining systems utilize rotary cut- 
 ting heads and chain conveyors. Part 3 
 describes the basic coal-cutting and 
 
 chain conveyor research applicable to 
 both continuous and longwall mining sys- 
 tems. However, design differences be- 
 tween the actual hardware in the two sys- 
 tems have necessitated separate Bureau 
 efforts in both areas (42-43) , 
 
 Haulage Methods 
 
 Because more coal miners work in the 
 face area than in any other single loca- 
 tion, the Bureau has concentrated on 
 reducing the noise produced by face 
 equipment. However, underground coal- 
 haulage equipment also generates substan- 
 tial noise, and the Bureau has addressed 
 some of the problems in this area. 
 
 Face Haulage 
 
 Continuous miners and loading machines 
 load coal into shuttle cars or some type 
 of continuous face-haulage system. Shut- 
 tle car noise is usually not as loud as 
 continuous miner and loader noise (fig. 
 3) , so the shuttle car operator is ex- 
 posed to the most noise while loading 
 coal. Although operators may also exper- 
 ience noise levels slightly above 90 dBA 
 while unloading the shuttle car, they 
 usually spend more time tramming than 
 loading and unloading combined. Since 
 tramming is a relatively quiet procedure 
 (less than 90 dBA) , overexposure of shut- 
 tle car operators is minimal. 
 
 Conveyors are undoubtedly the greatest 
 source of face-haulage noise. Continuous 
 coal-haulage systems in the face area 
 usually contain one or more conveyors, or 
 "bridges," mounted on a self-propelled 
 carrier vehicle. The bridge carrier op- 
 erator either rides on this vehicle or 
 walks and/or crawls beside it while load- 
 ing coal. Conveyor noise-control treat- 
 ments developed for continuous miners 
 could also be used to benefit bridge car- 
 rier operators. 
 
 Secondary and Main Mine Haulage 
 
 Shuttle cars and continuous face-haul- 
 age systems usually transfer coal to 
 belt conveyors or locomotive-drawn rail 
 
11 
 
 cars that haul the coal out of the mine. 
 Electrically driven motors are the pri- 
 mary noise sources in belt-conveyor sys- 
 tems , but they do not usually contribute 
 to worker overexposure. Likewise, the 
 Mine Safety and Health Administration 
 (MSHA) has not identified a serious over- 
 exposure problem among operators of elec- 
 trically powered haulage locomotives. 
 
 Personnel Haulage 
 
 In the early days of coal mining, when 
 coal was mined and transported by hand or 
 animals, men usually walked or crawled to 
 the face areas. When electrical power 
 was introduced into the mines , personnel- 
 haulage vehicles were developed. Typical 
 of these is the rail-mounted "mantrip" or 
 "portal bus" that carries workers from 
 the mine portal to the face areas. 
 
 Because mantrips operate for only 15 to 
 45 min per shift, they are not a primary 
 cause of noise overexposure; however, 
 they commonly produce noise levels rang- 
 ing from 90 to 95 dBA and can contribute 
 significantly to the total daily noise 
 dose experienced by face workers. Man- 
 trips generate noise primarily through 
 (1) interaction between the wheels and 
 rails and (2) operation of the electric 
 motor and drive train. 
 
 Bureau research programs aimed at con- 
 trolling mantrip noise have been quite 
 successful. (See part 3.) Noise levels 
 have been reduced to below 90 dBA on two 
 different mantrip vehicles. First, a 
 mantrip was equipped with retrofit noise- 
 control treatments on its wheels, passen- 
 ger compartments, and motor enclosure 
 (17) . Similar noise-control techniques 
 were then used by a mantrip manufacturer 
 to produce an "inherently quieter" vehi- 
 cle at its factory (15) . 
 
 Roof Support Methods 
 
 Wooden posts, crossbars, and cribs were 
 the original forms of coal mine roof sup- 
 port and were usually emplaced by hand. 
 Roof bolts, the most common means of roof 
 support in present-day coal mines, were 
 
 first used extensively in U.S. coal mines 
 in the mid-1940' s. Holes must be drilled 
 into the mine roof to install roof bolts, 
 and two basic methods are used to drill 
 roof bolt holes — handheld pneumatic per- 
 cussion ("s toper") drills and machine- 
 mounted electric rotary drills. 
 
 Stoper drills are by far the loudest 
 machines used in coal mines today, com- 
 monly producing noise levels of about 120 
 dBA (fig. 3). Because Federal regula- 
 tions do not allow continuous noise lev- 
 els above 115 dBA, any continuous opera- 
 tion of a stoper drill would cause the 
 operator to be overexposed. For this 
 reason, the reduction of stoper drill 
 noise was one of the Bureau's first high- 
 priority areas of noise-control research. 
 Part 3 includes a detailed description 
 of the Bureau's stoper noise-control 
 program. 
 
 The noise-generating mechanisms of 
 stoper drills are basically the same as 
 those of jumbo-mounted pneumatic percus- 
 sion drills used in hardrock mines. (The 
 operating mode of jumbo drills is dis- 
 cussed in some detail under the heading 
 "Hardrock Extraction Methods" in the next 
 section.) Although stopers are smaller 
 and lighter than jumbo drills, the three 
 major noise sources are the same — drill- 
 body vibration, drill-steel vibration, 
 and air-exhaust noise. Figure 5 shows a 
 typical stoper drill and the location of 
 these noise sources. 
 
 Two basic approaches to stoper noise 
 control were investigated — retrofit tech- 
 niques and complete redesign of the 
 stoper drill. Although the redesign ap- 
 proach was more difficult and expensive 
 than the retrofit approach, it resulted 
 in a quieter, more efficient drilling 
 machine. However, the retrofit treat- 
 ments were also effective because they 
 reduced noise substantially and did not 
 require modification of the drill itself. 
 The retrofit treatments reduced noise 
 levels at the operator's position ranged 
 from 102 to 106 dBA, reflecting an 11- to 
 14-dBA reduction versus noise levels of 
 untreated stopers (38). The redesigned 
 
12 
 
 Driil steel 
 
 FIGURE 5c. = Noise sources on typical coal 
 mine stoper drill. 
 
 about 90 to 95 dBA; this occurs while 
 drilling the roof and tightening the 
 bolt. However, these two operations com- 
 prise only 55 pet of the time spent 
 by the bolter in the face area and only 
 20 pet of an 8-h shift. Based on these 
 data, MSHA estimated that only 5 pet of 
 all rotary roof bolter operators would 
 be out of compliance with Federal noise 
 regulations (_5 ) . For this reason, noise- 
 control programs for rotary roof bolters 
 have not been initiated by the Bureau. 
 
 UNDERGROUND HARDROCK MINING 
 
 Extraction, haulage, and roof support 
 are unit operations that are common to 
 both coal and hardrock mines. However, 
 underground hardrock mining systems and 
 equipment are quite different from those 
 used in coal mines for the following rea- 
 sons: (1) In most cases, explosives must 
 be used for hardrock extraction because 
 the rock is much too hard for continuous- 
 type mining and cutting machines. (Ex- 
 ceptions are "soft" ores such as salt, 
 potash, and trona.) (2) Hardrock ore 
 bodies are more irregular than coal seams 
 and require more complex mine layouts . 
 (3) Diesel-powered equipment is more 
 prevalent in hardrock mines than in coal 
 mines. 
 
 Bureau studies in the mid-1970 's ( 26 ) 
 identified the major noise sources in un- 
 derground hardrock mines , and research 
 since that time has included efforts to 
 control these sources. Because under- 
 ground hardrock mines use such a wide 
 variety of equipment types , Bureau re- 
 search has addressed only the most seri- 
 ous noise offenders in these mines. 
 
 Hardrock Extraction Methods 
 
 ("quiet") stoper reduced operator noise 
 levels to about 98 dBA, reflecting a 22- 
 dBA reduction in noise versus that of 
 standard drills (13). A smaller, lighter 
 version of the "quiet stoper" produced 
 noise levels of 102 to 105 dBA and was 
 more readily accepted by drill operators. 
 
 Figure 3 shows that rotary roof-bolting 
 machines can produce noise levels from 
 
 Since the 16th century, hardrock ores 
 have been freed from the earth by blast- 
 ing. For more efficient blasting, holes 
 are drilled into the rock mass, and 
 explosives are placed in the holes. Al- 
 though the types of explosives and drill- 
 ing techniques used have changed sub- 
 stantially through the years, the basic 
 principles of drilling and blasting have 
 not. Increased noise associated with 
 
13 
 
 underground ore extraction is due primar- 
 ily to the increased power of modern 
 drilling machines. 
 
 Early miners found that the most effec- 
 tive way to bore a hole in rock was to 
 place a hard, pointed object against the 
 rock surface and strike the object with a 
 hammer. At first this was done manually, 
 but attempts were made throughout the 
 1800' s to mechanize the process. The 
 first mechanically powered rock drills 
 were steam-driven; but they weighed sev- 
 eral thousand pounds and were very cum- 
 bersome, so they were unacceptable com- 
 mercially. Compressed air proved to be a 
 better power source; in 1861, the Mont 
 Cenis Tunnel in the French Alps marked 
 the first successful commercial applica- 
 tion of pneumatic rock drills. Improve- 
 ments in pneumatic rock drill design 
 continued throughout the late 1800' s, in- 
 cluding John George Leyner's development 
 of a self-rotated striking tool (drill 
 steel) with a hollow core to allow the 
 passage of air or water for flushing the 
 hole. 
 
 Metallurgical improvements in the 20th 
 century have permitted the development of 
 high-strength rock drill components that 
 impart tremendous amounts of energy to 
 the rock face. Modern drills commonly 
 deliver 30 blows per second to the face 
 
 at about 200 ft'lbf per blow — approxi- 
 mately 360,000 ft»lbf/min. 
 
 Often, percussion drills are most ef- 
 fective when mounted on a self-propelled 
 vehicle called a jumbo. Figure 6 shows 
 the major components of a typical jumbo 
 drill system. The jumbo vehicle supports 
 from one to three hydraulically powered 
 booms that position the drill against the 
 rock face. In operation, the drifter 
 (drill body) generates a series of impul- 
 sive blows upon the steel by means of 
 a rapidly oscillating piston, which is 
 driven by either a pneumatic or hydraulic 
 power source. With each blow, a stress 
 wave moves from the drifter through the 
 steel and bit into the rock, which shat- 
 ters under the tungsten carbide cutting 
 edges of the bit. After each blow, the 
 steel and bit rotate slightly to bring 
 the cutting edges of the bit into contact 
 with fresh rock surface. The feed, a 
 chain- or screw-drive mechanism within a 
 channel, supports the drifter and moves 
 it forward as drilling progresses; and 
 this forces the bit against the rock. 
 The centralizer wraps around the drill 
 steel and keeps it from wandering side- 
 ways at the start of drilling. The chips 
 and dust produced during drilling are 
 flushed from the hole by either com- 
 pressed air or water that flows through 
 the center of the steel and out through 
 
 Jumbo 
 
 FIGURE 6. - Major components of a jumbo drill rig. 
 
14 
 
 ports In the bit. The drill operator 
 stands on a platform on the jumbo vehi- 
 cle, just behind the booms. 
 
 Figure 7 shows the noise-generating 
 mechanisms of jumbo-mounted pneumatic 
 percussion drills. These mechanisms can 
 be placed in two broad categories: ex- 
 haust noise and vibration of mechanical 
 components. Exhaust noise, drifter-body 
 vibration, and drill-steel vibration out- 
 weigh the other noise sources shown 
 in figure 7; therefore, the Bureau has 
 focused its efforts on ways to control 
 these sources. 
 
 A straightforward approach for con- 
 trolling exhaust noise is to channel it 
 away from the operator through ductwork. 
 A second approach is to attach a baffle- 
 type muffler directly to the exhaust 
 port(s) of the drifter. A third is to 
 place the drifter inside an acoustical 
 enclosure. These approaches are effec- 
 tive in reducing noise, but all share a 
 common problem: freezing. The rapidly 
 expanding exhaust air cools quickly, and 
 the moisture in the air condenses on the 
 inside surfaces of the ductwork, muffler, 
 
 or enclosure; after a short time, perhaps 
 only a few minutes, ice begins to accumu- 
 late, inhibiting the flow of exhaust air 
 and causing the drill to stall. The Bu- 
 reau presently has an in-house test set- 
 up designed to investigate the muffler- 
 freezing problem in more detail. Also, 
 as described in part 3, ice-inhibiting 
 enclosures for two different jumbo drills 
 were developed under two recent Bureau 
 contracts. The enclosures reduced both 
 the exhaust noise and the noise generated 
 by vibration of the drifter body. 
 
 The vibrating drill steel often gener- 
 ates more noise than any other component 
 of the jumbo rig. To address this prob- 
 lem, the Bureau is investigating several 
 different drill-steel noise-control tech- 
 niques. The two contracts mentioned 
 above explored the "shroud-tube" concept, 
 whereby a tube-shaped acoustical enclo- 
 sure completely surrounded the drill 
 steel. One contract (11) produced a 
 form-fitting shroud-tube that closely 
 surrounded the drill steel, like a 
 sheath. The tube diameter was slightly 
 smaller than the cutting bit diameter, 
 allowing the tube to enter the hole as 
 
 Bit-rock impact 
 
 Piston-striking bar impact 
 
 Leakage air noise 
 
 Air motor noise 
 
 Drifter-body noise 
 
 ^^ J"" Exhaust noise 
 
 FIGURE 7. - Noise sources on jumbo-mounted drills. 
 
15 
 
 drilling proceeded. The Bureau is now 
 testing this "in-the-hole" shroud-tube 
 design to determine its long-term dura- 
 bility and acoustical effectiveness. A 
 large-diameter springlike shroud tube was 
 developed under the second contract and 
 is now being field-tested by the con- 
 tractor (14). This "spring shroud" is 
 located between the drifter body and the 
 front centralizer (fig. 7) and is fully 
 extended before drilling begins. As the 
 hole is drilled, the drifter body moves 
 closer to the front centralizer, and the 
 spring shroud collapses around itself. 
 Unlike the form-fitting shroud tube de- 
 scribed above, the spring shroud does not 
 enter the drill hole; it covers only the 
 portion of the drill steel that is out- 
 side the hole. Noise generated by the 
 drill steel that is within the hole is 
 attenuated by the rock mass. 
 
 Handheld hardrock drills are also used 
 for underground ore production, espe- 
 cially in tight quarters where jumbo- 
 mounted drills cannot fit. Handheld 
 drills are smaller and less powerful than 
 jumbo-mounted drills, but the operating 
 principles are the same except for the 
 feed mechanism. Instead of a chain- or 
 screw-type feed, handheld hardrock drills 
 utilize air-powered cylinders called 
 feedlegs, which are similar to the feed- 
 legs used on coal mine stoper drills 
 (fig. 5). Handheld hardrock drills are 
 similar in design and construction to 
 coal mine stopers, but must deliver more 
 energy per blow to the bit because they 
 drill in harder rock. The same basic 
 noise-control treatments are required for 
 both types of handheld drills; however, 
 adapting coal mine stoper noise controls 
 to hardrock drills is not a simple task 
 because of the higher energy require- 
 ments. A prototype quiet handheld hard- 
 rock drill is now being developed under 
 Bureau contract (8). 
 
 Haulage Methods 
 
 As in coal mines , man- or animal-pow- 
 ered carts were the first step toward 
 mechanization of underground ore haulage 
 in hardrock mines. Although air- and 
 
 steam-powered loading and hauling vehi- 
 cles were used in the 1800' s, they were 
 gradually replaced with diesel-powered 
 machines. The diesel engine provides a 
 safe, efficient, compact, and highly mo- 
 bile power source and has been applied in 
 the last 50 yr to many different types of 
 underground hardrock mining equipment. 
 It is by far the most popular source of 
 power for today's underground hardrock 
 haulage machines. 
 
 Recent Bureau studies revealed that a 
 wide variety of diesel-powered loading 
 and hauling machines are now used in 
 underground hardrock mines ( 26 ) . Most 
 of these machines generate noise lev- 
 els above 95 dBA at the operator's posi- 
 tion, and operate long enough to causev 
 overexposure. The noisiest machines in 
 terms of operator overexposure are LHD 
 machines, followed by ore trucks, ram 
 haulers, tractor-trailer units, front-end 
 loaders, skid-steer loaders, and shuttle 
 cars. All of these machines share the 
 following noise sources: (1) engine air- 
 borne noise (emanating directly from the 
 engine or block structure); (2) engine 
 structureborne noise (engine vibration 
 radiated through the structure of the ma- 
 chine); (3) auxiliary-component noise — 
 airborne and structureborne noise from 
 the transmission, drive train, and 
 hydraulic system; (4) noise generated 
 by the cooling system fan; (5) exhaust 
 noise — airborne noise from the outlet and 
 structural radiation from the exhaust 
 piping (or shell); and (6) air-intake 
 noise — airborne noise from the intake 
 and structural radiation from the intake 
 piping. 
 
 The Bureau directed its initial noise- 
 control research efforts toward LHD vehi- 
 cles because many LHD operators are over- 
 exposed to noise and because the LHD is 
 one of the most common machines in under- 
 ground hardrock mines. Also, compared 
 with other machines , the LHD is more dif- 
 ficult to treat for noise control; its 
 structure is more complex, it places 
 greater visual demands on the operator, 
 and it is not designed to accommodate an 
 acoustical cab (e.g., for underground ore 
 
16 
 
 trucks and surface haulage equipment) . 
 It was apparent that if the noise-control 
 treatments developed for LHD's were suc- 
 cessful, they could be modified and ap- 
 plied to other diesel-powered machines. 
 As described in part 3, the Bureau is 
 presently involved in both retrofit ( 22 ) 
 and "factory-integration" ( 41 ) approaches 
 to LHD noise control. 
 
 Roof Support Methods 
 
 Roof support in underground hardrock 
 mining followed about the same evolution- 
 ary path as in coal mining — wooden posts 
 and timbers, installed manually, were 
 gradually replaced by mechanized roof- 
 bolting systems. Handheld and jumbo- 
 mounted percussion drills are used to 
 drill the holes for roof bolts; in addi- 
 tion, diesel-powered machines (shotcrete 
 machines, roof bolters and scalers, tran- 
 sit mixers and placers, etc.) are some- 
 times used in the roof support systems of 
 underground hardrock mines. Because the 
 diesel engine of the roof support machine 
 is the primary noise-generating mecha- 
 nism, the results of the Bureau's LHD 
 noise-control program could be applied. 
 However, the Bureau has not directly ad- 
 dressed the control of noise produced by 
 roof support machines (other than percus- 
 sion drills) because they do not contrib- 
 ute to widespread worker overexposure. 
 
 SURFACE MINING 
 
 Surface mining, like underground min- 
 ing, involves excessive levels of equip- 
 ment-generated noise. One of the first 
 powered surface mining machines was a 
 steam-driven "spoon dredge" developed in 
 England in 1796. The need for faster 
 railroad construction in the mid-1800' s 
 was responsible for the development of 
 today's large mining shovels. Steam pow- 
 er gave way to internal-combustion and 
 diesel engines, and finally to electric 
 power shortly after 1900. 
 
 Ironically, the largest and most pow- 
 erful surface mining machines — shovels 
 and draglines — are not responsible for 
 
 widespread overexposure to noise because 
 (1) they are comparatively few in number, 
 and (2) they often have factory-installed 
 noise-controlled operator cabs. Instead, 
 mobile diesel-powered machines such as 
 bulldozers and front-end loaders have 
 been identified by the Bureau as the pri- 
 mary noise offenders in surface min- 
 ing operations (table 4) (9^, 40) . Even 
 though these machines have about the same 
 noise sources (fig. 8) as their under- 
 ground counterparts, the LHD vehicles, 
 the Bureau's approach to noise control 
 has been much different. Since under- 
 ground equipment must operate in a very 
 confined environment, it is very diffi- 
 cult to design and install an acoustical 
 cab that does not interfere prohibitively 
 with operator movement and vision. How- 
 ever, this is not true of surface min- 
 ing equipment , so for this equipment the 
 acoustical cab approach was pursued as 
 the most cost-effective means of reducing 
 operator overexposure. 
 
 TABLE 4. - "Noise offenders" in surface 
 mining operations 
 
 Equipment type 
 
 Bulldozers 
 
 Front-end loaders .... 
 
 Haulage trucks 
 
 Draglines 
 
 Scrapers 
 
 Overburden drills .... 
 
 Highway trucks 
 
 All others 
 
 Pet of total over- 
 exposed operators 
 
 48 
 15.5 
 
 8.5 
 
 8 
 
 5.5 
 
 2 
 .5 
 12 
 
 Although several types and models of 
 surface mining equipment now have 
 factory-installed acoustical cabs, many 
 older bulldozers and front-end loaders 
 do not. Therefore, the Bureau developed 
 acoustical cab retrofit treatments for 
 two popular models of each of these two 
 machines (^J) • Although each model of 
 bulldozer and front-end loader requires a 
 slightly different retrofit package, the 
 same basic procedures can be used on all 
 models. Part 3 describes the results of 
 these retrofit treatments. 
 
17 
 
 Cab 
 
 A 
 
 nz3 
 
 I 
 
 Exhaust 
 
 Engine 
 
 iZ ii 
 
 Fan 
 
 Main frame 
 
 A 
 
 Final drive 
 
 T 
 t 
 
 A 
 
 A 
 
 Transmission 
 
 KEY 
 Airborne noise 
 
 D Structureborne 
 noise 
 
 FIGURE 8. • Noise sources of mobile, diesel-powered surface mining equipment. 
 
 PREPARATION AND PROCESSING PLANTS 
 
 Almost all substances extracted from 
 mines must undergo some form of process- 
 ing to become a usable product. Mineral 
 processing predates recorded history; "De 
 Re Metallica," written in 1556 by Geor- 
 gius Agricola, reveals the existence of 
 equipment and techniques such as picking 
 tables, smelting furnaces, sieves, crush- 
 ers, and other items, all still used in 
 one form or another. The noise associ- 
 ated with coal and mineral processing 
 plants became much greater as more power- 
 ful energy sources, especially electric 
 power, replaced wind and water as the 
 primary driving forces behind mineral 
 processing equipment. 
 
 All preparation plants contain equip- 
 ment that performs one of three primary 
 functions: crushing (size reduction). 
 
 screening (size separation) , and dewater- 
 ing. In addition, many plants contain 
 equipment that separates valuable con- 
 stituents (coal or ore) from waste mate- 
 rial through differences in their den- 
 sities, physical properties, chemical 
 properties, and/or magnetic properties. 
 These machines come in a wide variety of 
 shapes and sizes, and the flow of mate- 
 rial through each preparation plant is 
 different. Figure 9 shows the overall 
 layout of the grinding mill area in a 
 typical taconite (iron ore) processing 
 plant. Each piece of equipment generates 
 noise; furthermore, the mere transfer 
 of material through the plant generates 
 noise as the result of impacts between 
 stationary components — chutes, bins, hop- 
 pers, etc. — and the moving material. 
 Fluid movement, such as air flowing 
 through a size-separation or dewatering 
 device, also generates noise. 
 
18 
 
 FIGURE 9. •= Grinding mills in taconite processing plant. 
 
 Noise exposures of preparation plant 
 workers depend mostly on their proxi- 
 mity to noise-producing equipment. Sta- 
 tionary workers have specific job sta- 
 tions that may or may not be close to a 
 loud noise source. The noise exposures 
 of stationary workers are relatively 
 easy to measure compared to those of mo- 
 bile workers — mechanics, samplers, clean- 
 up workers, supervisors, etc. — who move 
 throughout the plant during their normal 
 working day. (Note the walkways in fig- 
 ure 9.) Consequently, the approach for 
 reducing worker overexposure may be dif- 
 ferent for each type of worker. Acousti- 
 cal booths and/or engineering noise con- 
 trols are often better for stationary 
 workers, while hearing protectors and/or 
 administrative controls may be better for 
 mobile workers. 
 
 Regardless of the nature of worker 
 overexposure, however, engineering noise 
 control of preparation plant equipment, 
 if feasible, is the best long-term solu- 
 tion to the problem. The Bureau has 
 therefore identified the most serious 
 noise offenders in coal and mineral pro- 
 cessing plants and has investigated means 
 to control them. Noise generated by 
 crushing devices can be controlled effec- 
 tively by surrounding either the operator 
 or the entire crusher with an acoustical 
 barrier. Screening noise can be con- 
 trolled by enclosing the area of the 
 plant containing the screens or by using 
 screen decks made of nonmetallic, energy- 
 absorbing materials. Chutes and bins can 
 be lined with rubber or other energy- 
 absorbing materials to reduce the sever- 
 ity of impacts caused by moving material; 
 
 1 
 
19 
 
 damping materials can also be added to 
 reduce the vibration of chute and bin 
 panels. An acoustical operator booth can 
 be the most cost-effective solution, es- 
 pecially in small, portable mineral pro- 
 cessing plants. 
 
 Part 3 describes the Bureau's prepara- 
 tion plant noise-control programs in more 
 detail. A related study dealing with 
 the screening efficiency of nonmetallic 
 screen decks is also described in part 3. 
 
 PART 3. —RESULTS OF SELECTED RESEARCH PROGRAMS 
 
 This part of the report describes in 
 detail the results of selected Bureau 
 noise-control research projects. As in 
 part 2, noise-control projects for under- 
 ground coal mining are discussed first, 
 followed by projects involving under- 
 ground hardrock mining, surface mining, 
 and preparation and processing plants. 
 The final section of part 3 discusses 
 the results of recent Bureau research 
 into the potential effectiveness of per- 
 sonal hearing protectors in the mining 
 environment. 
 
 UNDERGROUND COAL MINING 
 
 Part 2 reviewed the Bureau's under- 
 ground coal mining noise-control research 
 programs in terms of the three unit oper- 
 ations involved — extraction, haulage, and 
 roof support. Here, the results of four 
 specific Bureau noise-control research 
 programs are discussed in detail, again 
 in terms of unit operations: (1) coal 
 cutting (extraction), (2) chain convey- 
 ors (material haulage), (3) mantrip vehi- 
 cles (personnel haulage), and (4) stoper 
 drills (roof support). Although several 
 other projects dealing with underground 
 coal mining noise have been conducted by 
 the Bureau, the results of these four 
 programs appear to have the most poten- 
 tial for impact on the industry. 
 
 Coal Cutting 
 
 Two major projects concerned with 
 coal-cutting noise have been completed 
 thus far; one was a series of extensive 
 laboratory studies into the mechanics 
 of the coal-cutting process, and the 
 other was the development of reduced- 
 noise cutting heads for auger-type con- 
 tinuous miners. The results of these 
 programs are now being used in cur- 
 rent Bureau research into the reduction 
 
 of coal-cutting noise and will contin- 
 ue to influence the direction of this 
 research. 
 
 Laboratory Studies (J_, 3) 
 
 Early Bureau experiments revealed that 
 an in-depth scientific understanding of 
 the coal-cutting process was needed 
 to devise an effective method for con- 
 trolling coal-cutting noise. The three 
 components of coal-cutting noise are 
 (1) fracture noise, which is produced by 
 the movement of air to fill the voids re- 
 sulting from the formation of cracks in 
 the coal; (2) face radiation, the vibra- 
 tory response of the coal face itself; 
 and (3) cutting head vibration, which re- 
 sults from the transfer of cutting forces 
 from the bits to the rotating head assem- 
 bly. The three basic goals of the labo- 
 ratory studies were to (1) identify the 
 dominant component of coal-cutting noise 
 (fracture, face radiation, or cutting 
 head vibration); (2) determine the magni- 
 tude of coal- and shale-cutting forces; 
 and (3) show how the coal- and shale- 
 cutting noise level can change due to ma- 
 chine operating parameters such as depth 
 of cut, cutting speed, bit style, etc. 
 
 Noise of Coal Fracture and Face 
 Radiation 
 
 One of the most important tools used 
 in the laboratory studies was a gravity- 
 powered "linear cutting apparatus" (LCA). 
 The three major components of the LCA 
 (fig. 10) were a sliding carriage capable 
 of holding a small sample block of coal, 
 a vertical guide mast on which the sample 
 carriage was mounted, and an instrumented 
 cutting bit on a pedestal. To operate 
 the LCA, the coal sample carriage was 
 raised above the instrumented bit and re- 
 leased; the falling coal sample then 
 
20 
 
 •~-^,' 
 
 
 m'" 
 
 
 '^. 
 
 \^ 
 
 m^ 
 
 1 
 
 
 \ 
 
 "%l 
 
 -if 
 
 ■«rf^ 
 
 1 
 
 FIGURE 10. - Linear cutting apparatus. 
 
21 
 
 struck the bit and continued downward, 
 resulting in a single, linear cut on one 
 face of the sample. The instrumented bit 
 measured the coal-cutting forces, a coun- 
 terbalance system regulated the accelera- 
 tion of the coal-sample carriage (i.e., 
 the "cutting speed"), and the bit-mount- 
 ing pedestal could be adjusted to control 
 the depth of cut. 
 
 as independent variables (i.e., they 
 were controlled by the experimenters), 
 five dependent variables were measured: 
 cutting force, sound pressure level, 
 coal-sample surface acceleration, pro- 
 duction rate, and specific energy. The 
 results of the LCA tests are described 
 below in terms of these dependent 
 variables. 
 
 The entire LCA was enclosed in an an- 
 echoic chamber — virtually no noise was 
 reflected from the walls, floors, or 
 ceiling. The anechoic chamber was en- 
 closed in an underground building with 
 thick concrete walls to prevent outside 
 noises from entering the chamber. The 
 hollow interiors of the carriage-guide 
 mast and the bit-mounting pedestal were 
 filled with sand to reduce the LCA's 
 "self-noise" during cutting. Coal-cut- 
 ting noise could be measured quite accu- 
 rately with this test setup because it 
 was the only significant noise within the 
 enclosure. 
 
 LCA tests showed how coal fracture 
 and face-radiation noise was affected 
 by eight different cutting parameters: 
 cutting speed, depth of cut, overburden 
 pressure, cut spacing, bit type, cutting 
 angle, bedding plane orientation, and 
 coal type. Using these eight parameters 
 
 Cutting Force 
 
 One of the most important findings of 
 the LCA tests was that coal resists the 
 advance of a cutting bit in a manner 
 analagous to the action of a spring. The 
 cutting force of the bit increases until 
 the tensile stress in the coal initiates 
 a localized brittle fracture. This frac- 
 ture propagates out from the point of 
 force application for a small distance. 
 The bit continues to advance through 
 fractured coal, meeting relatively little 
 resistance until it again contacts un- 
 fractured coal, when the process is 
 repeated. 
 
 Figure 11 is a graph of coal-cutting 
 force versus time; it clearly shows the 
 impulsive nature of the coal-cutting pro- 
 cess. Note that the initial impact force 
 is no higher nor longer lasting than the 
 subsequent fracture events. The peak 
 
 ,000 
 
 £ 500 
 
 o 
 
 u. O 
 
 500 
 
 Peak force = 633 lb 
 
 Mean cutting force = 309 I b 
 
 
 
 25 50 75 100 125 
 
 TIME,ms 
 
 FIGURE llo - Coal-cutting force versus time. 
 
 50 
 
 175 
 
 200 
 
22 
 
 2,000 4,000 10,000 
 
 200 400 1,000 
 
 FREQUENCY, Hz 
 
 FIGURE 12. - Coal-cutting force (power spectral density) versus frequency. 
 
 I 
 
 force initiating coal fracture and the 
 number of fracture events occurring with- 
 in a given length of cut were found to be 
 independent of cutting speed. However, 
 the type of coal, depth of cut, and bit 
 configuration significantly affected the 
 cutting force. 
 
 Figure 12 is a graph of coal-cutting 
 force (power spectral density^) versus 
 frequency, taken from the force-time 
 history of figure 11. The cutting force 
 
 'In figure 12, power spectral density 
 is expressed as the force level (in deci- 
 bels) divided by the frequency bandwidth 
 (in hertz) in which the force is mea- 
 sured. To obtain the force level, the 
 measured force is compared to a reference 
 force of 1 X 10"-' lb. (The decibel is 
 defined in footnote 6.) 
 
 is relatively flat (fig. 12) until the 
 "cutoff frequency," the point after which 
 it declines at a rate that is inversely 
 proportional to the square of the fre- 
 quency. The cutoff frequency is pri- 
 marily a function of the coal type and 
 cutting speed — the faster the cut, the 
 higher the cutoff frequency. Knowledge 
 of this force-versus-f requency behavior 
 played a very important role in the sub- 
 sequent design of noise-control treat- 
 ments for coal-cutting machinery. 
 
 In addition, the relationship between 
 cutting speed and cutoff frequency could 
 be exploited in terms of reducing the 
 noise of existing cutting machinery. 
 Since the A-weighting scale deemphasizes 
 the importance of low-frequency noise, 
 slower cuts would be "less noisy" than 
 faster cuts with respect to human 
 
23 
 
 hearing, because the forces initiating 
 the noise would be predominantly low- 
 frequency forces. 
 
 Sound Pressure Level 
 
 With a cutting speed of 96 in/s and a 
 cut depth of 1.0 in, the sound pressure 
 level (SPL) ranged from 89 to 102 dB, 
 depending on coal type and bit type. The 
 SPL increased with increasing cutting 
 speed and depth of cut, and the noise 
 spectra shifted to higher frequencies as 
 cutting speed increased. 
 
 Coal Sample Surface Acceleration 
 
 The root-mean-square (RMS) values of 
 face acceleration were proportional to 
 the SPL's and were affected by the in- 
 dependent variables in the same manner 
 as the SPL's. This proportionality sug- 
 gested that sample face vibration rather 
 than fracture noise was the dominant 
 noise source during the LCA tests. 
 
 Production Rate 
 
 The average production rate was mea- 
 sured by dividing the total weight of 
 coal produced from a linear cut by the 
 time needed to make the cut. The pro- 
 duction rate was found to be directly 
 proportional to the cutting speed; it in- 
 creased slightly with increasing overbur- 
 den pressure but was unaffected by the 
 angle of bit attack. The production rate 
 varied with the depth of cut according to 
 the relationship 
 
 Production rate = Depth", 
 
 where n = an experimentally determined 
 exponent. The value of n ranged from 
 1.29 to 1.99, depending on the coal and 
 bit type. 
 
 was measured as the ratio of power con- 
 sumed (energy per unit time) to the pro- 
 duction rate (mass per unit time). The 
 most important variable affecting spe- 
 cific energy was the depth of cut; spe- 
 cific energy decreased as the depth of 
 cut increased. In terms of noise con- 
 trol, this finding was important because 
 it showed that for equal production 
 rates , deep cuts would be quieter than 
 shallow cuts. 
 
 Coal type, overburden pressure, bed- 
 ding-plane orientation, and bit type also 
 affected the specific energy of the cut- 
 ting process. (But somewhat surprising- 
 ly, cutting speed did not.) Of these 
 variables , only the bit type can be 
 changed by the mine operator to achieve 
 quieter, more efficient cutting. Howev- 
 er, the "most efficient" cutting bit in 
 terms of specific energy (as measured in 
 the LCA tests) may not last as long as a 
 "less efficient" bit when used on operat- 
 ing continuous miners or longwall shear- 
 ers. More rigorous cutting tests are 
 needed to confirm the mine-worthiness of 
 the bits found to be "most efficient" 
 when used on the LCA. 
 
 To summarize, the results of the LCA 
 tests suggested that the noise produced 
 by coal fracture and face radiation could 
 be reduced by increasing the cut depth, 
 decreasing the cutting speed, and choos- 
 ing the most efficient cutting bit. The 
 Bureau has also found that deeper, slow- 
 er, more efficient coal cutting reduces 
 the amount of respirable dust generated 
 (31) . Therefore, if these characteris- 
 tics could be incorporated into future 
 designs for coal-cutting machinery, mul- 
 tiple health and safety benefits would 
 result. 
 
 Noise Caused by Cutting Drum Vibration 
 
 Specific Energy 
 
 The specific energy of a cut is the 
 single best indicator of cutting effi- 
 ciency; it is defined as the cutting en- 
 ergy per unit of coal extracted. In the 
 LCA tests, the specific energy of a cut 
 
 As previously stated, noise generated 
 by coal cutting has three components: 
 fracture noise, face-radiation noise, and 
 vibration of the cutting head of the min- 
 ing machine. The LCA tests only investi- 
 gated the first two components; there- 
 fore, further laboratory tests were 
 
24 
 
 required to determine the contribution of 
 cutting head vibration to coal-cutting 
 noise. These tests consisted of operat- 
 ing a continuous mining machine in a 
 large reverberation chamber — a building 
 whose predictable internal sound-reflect- 
 ing characteristics made it possible to 
 accurately measure the sound power gener- 
 ated by noise sources within. A large 
 block of synthetic coal was placed in the 
 reverberation chamber to serve as a cut- 
 ting medium. The synthetic coal material 
 was a concrete-and-coal mixture that ac- 
 curately simulated the acoustic effi- 
 ciency (ability to radiate sound power 
 from mechanical cutting power) of real 
 coal. Figure 13 shows the continuous 
 miner and synthetic coal test setup. 
 
 In order to determine the contribution 
 of cutting head vibration to the overall 
 noise level, the noise generated by the 
 continuous miner while idling was first 
 compared to the overall noise generated 
 while cutting. The conveyor of the miner 
 was not operated during these tests be- 
 cause the presence of conveyor noise 
 would have made cutting noise more diffi- 
 cult to measure. Idling noise (pumps on, 
 cutting drums spinning in air) was found 
 to be 81.5 dBA at the operator's posi- 
 tion. As shown in table 5, the overall 
 noise level was measured in two cutting 
 modes — sumplng (advancing forward into 
 the seam) and shearing (cutting downward 
 at an 8-ln sump depth) . Noise levels in 
 these two modes (94.8 and 97.0 dBA, re- 
 spectively) were substantially higher 
 than in the idling mode. 
 
 The surface of the miner's cutting 
 drum was then wrapped with an acoustical 
 
 TABLE 5. - Results of continuous miner 
 cutting-noise tests 
 
 (Overall sound levels, dBA) 
 
 
 Cutting mode 
 
 
 Sumplng 
 
 Shearing 
 (8-in 
 depth) 
 
 Untreated drum 
 
 Treated drum 
 
 94.8 
 91.2 
 
 97.0 
 92.6 
 
 Noise reduction, . 
 
 3.6 
 
 4.4 
 
 material (vinyl-backed foam) that greatly 
 reduced its ability to radiate noise. 
 The differences between the noise levels 
 measured while cutting with the treated 
 and untreated drums (table 5) showed that 
 cutting head vibration contributed an 
 additional 3.6 to 4.4 dBA to the overall 
 noise level. This suggested that cutting 
 head vibration was Indeed a significant 
 contributor to overall cutting noise. 
 Due to the logarithmic nature of the dec- 
 ibel scale, a 3-dB increase in noise lev- 
 el is approximately equal to a doubling 
 of the sound power within the reverberant 
 environment, and a 6-dB increase in noise 
 level represents a fourfold increase in 
 sound power. Since the noise levels of 
 the untreated drum tests were 3.6 to 4.4 
 dBA greater than those of the treated 
 drum tests, the contribution of cutting 
 head vibration to the overall noise level 
 was at least as great, and often greater, 
 than the sum of coal-fracture noise, 
 face-radiation noise, and the idling 
 noise of the miner. 
 
 The acoustical treatment used on the 
 cutting drum in these experiments may 
 not have completely eliminated the noise 
 generated by cutting head vibration. 
 This suggests that cutting head vibration 
 may be even more important than implied 
 above. Unfortunately, a much sturdier 
 acoustical treatment than the vinyl- 
 backed foam wrapping would be required 
 for underground use; but to date, no 
 mlneworthy acoustical treatments for cut- 
 ting drums have been developed. A 3.6- 
 to 4,4-dBA noise reduction through cut- 
 ting drum treatments appears to be a rea- 
 sonable goal; however, if the cutting 
 drums themselves were redesigned, greater 
 noise reductions would be possible. 
 
 The results of the cutting head vibra- 
 tion tests suggest that coal-cutting 
 noise can be reduced substantially if 
 coal-cutting forces are not allowed to 
 excite the cutting drum. The concept 
 presently being investigated by the Bu- 
 reau is the "isolated cutting drum," a 
 technique that Involves the complete re- 
 design of the drum and a significant 
 change in the bit-mounting scheme. The 
 LCA tests provided valuable information 
 
25 
 
 FIGURE 13. - Continuous miner in reverberation room (top) and closeup showing cutting 
 head and synthetic coal (bottom). 
 
26 
 
 on how coal-cutting forces are generated, 
 how large they are, and how they are 
 transferred from the bit block to the 
 standard cutting drum. This infomiation 
 was needed to select appropriate energy- 
 absorbing materials for the new "iso- 
 lated" drums now being designed for both 
 continuous miners ( 42 ) and longwall 
 shearing machines ( 43 ) . 
 
 Reduced-Noise-Auger-Miner 
 Cutting Head (26-27) 
 
 Auger-type continuous miners are de- 
 signed to extract coal from thin seams, 
 approximately 26 to 50 in in height. 
 Figure 14 shows how auger miners advance 
 into the coal face with a sweeping mo- 
 tion. The two rotating augers at the 
 front of the miner cut the coal and move 
 it to the chain conveyor at the center of 
 the machine. The conveyor carries the 
 coal to the rear of the machine and dumps 
 it onto a bridge conveyor system. The 
 bridge conveyor connects with a panel 
 conveyor (panline) , which removes the 
 coal from the face area. 
 
 The anchor jacks (fig. 14) of most old- 
 er auger miners are emplaced manually 
 by "jacksetters." Newer auger miners 
 do not require jacksetters, but other 
 
 support workers (timbermen and/or cleanup 
 workers) are still needed in the imme- 
 diate face area, inby the operator. Be- 
 cause of their close proximity to the 
 cutting heads, these workers are exposed 
 to more noise than most other underground 
 coal miners. Noise levels in a typical 
 auger-mining section are 106 to 108 dBA 
 at the jacksetter position and 102 dBA 
 at the operator position. Table 2 shows 
 that the allowable operating time at 
 these levels is only 45 to 90 min per 
 shift. The standard auger cutting heads 
 are by far the dominant noise sources for 
 the jacksetters, timbermen, and cleanup 
 workers; whereas cutting noise and con- 
 veyor noise are approximately equal at 
 the operator position. 
 
 The laboratory studies described in 
 the previous section greatly aided in the 
 design of a reduced-noise auger cutting 
 head. Since coal-cutting forces are pre- 
 dominantly low-frequency forces (fig, 
 12) , the auger design was based on reduc- 
 tion of its vibratory response to low- 
 frequency excitation forces. The cutoff 
 frequency for the standard auger cutting 
 head was found to be approximately 100 to 
 200 Hz while cutting the synthetic coal 
 seam shown in figure 13, 
 
 Anchor 
 jack 
 
 Miner pivots on extended right 
 pivot jack as it swings to right, 
 making cut /. Retracted left 
 pivot jack swings forward toward 
 cut 2 pivot point. 
 
 Pivoting on extended left pivot 
 jack, miner swings to left through 
 cut 2. Retracted right pivot jack 
 advances toward cut 3 pivot point. 
 
 Again pivoting on extended right 
 pivot jack, miner swings right, 
 making cut 3. Retracted left 
 pivot jack moves ahead toward 
 zw\4 pivot point. 
 
 FIGURE 14. - Cutting sequence of auger-type continuous miner. 
 
27 
 
 The standard auger-miner cutting head 
 (fig. 15) consists of a central cylindri- 
 cal core surrounded by two helixes. The 
 cutting helix contains all the cutting 
 bits; the bitless, or conveying helix 
 helps transport cut coal to the conveyor 
 of the miner. These helixes vibrate vio- 
 lently, like high-powered speakers, when 
 excited by coal-cutting forces. Since 
 they are the primary noise sources on the 
 cutting head, the design effort was di- 
 rected at reducing helix vibration. 
 
 The mass, stiffness, and damping char- 
 acteristics of the helix control its 
 vibrational response to coal-cutting 
 forces. The first natural vibration fre- 
 quency of the standard helix was found to 
 be about 200 Hz. Below this frequency, 
 the helix's mass and stiffness control 
 its vibrational response; above 200 Hz, 
 the amount of damping at its resonant 
 frequencies becomes more important. 
 These structural response characteristics 
 indicated that the reduced-noise auger 
 design would have to (1) increase the 
 first natural frequency of the helix and 
 (2) increase the amount of damping at its 
 resonant frequencies. 
 
 Several methods were used to stiffen 
 and damp the auger helixes; however, the 
 design most suitable for in-mine use was 
 
 found to be the sand-filled auger shown 
 in figure 16. First, the conveying helix 
 shown in figure 15 was removed. Exten- 
 sive in-mine tests of the reduced-noise 
 augers showed that the single remaining 
 helix was sufficient for both cutting and 
 conveying. Although this change alone 
 did not drastically reduce cutting noise, 
 it simplified the auger substantially and 
 made the subsequent stiffening and damp- 
 ing treatments easier to install. 
 
 Next, the helix was stiffened by en- 
 larging the auger's core size and adding 
 a "conical helix stiffener." The larger 
 core helped reduce helix vibration by 
 decreasing the helix's height. The coni- 
 cal helix stiffener was a second helix 
 that was leaned against and welded solid- 
 ly to the main helix to act as a continu- 
 ous support. The addition of the stiff- 
 ener resulted in the formation of three 
 
 FIGURE 15. • Standard auger-miner cutting head. 
 
 FIGURE 16. - Reduced-noise auger-miner cutting 
 head. (Top: view from front; bottom: view from rear.) 
 
28 
 
 cavities with triangular cross-sections 
 (figs. 16-17). The largest cavity was on 
 the nonconveying (inby) side of the he- 
 lix, but because of space constraints, 
 the two smaller cavities were located 
 on the conveying (outby) side. In-mine 
 tests showed, however, that the addition 
 of the stiffener to the conveying side 
 did not reduce the auger's coal-carrying 
 capability. 
 
 As shown in figure 17, the triangular- 
 shaped cavities were filled with sand and 
 sealed. The sand added mass, which fur- 
 ther reduced the vibration of the helix 
 and helped dissipate its bell-like ring- 
 ing sound at its resonant frequencies. 
 The added mass did not hamper the auger's 
 performance because it merely replaced 
 the mass lost when the conveying helix 
 was removed. 
 
 A set of reduced-noise augers was 
 tested for approximately 6 months in an 
 
 underground coal mine. Although the 
 miners were initially very skeptical 
 about the new auger design, they were 
 completely satisfied with its performance 
 by the end of the test period. More im- 
 portantly, the new augers resulted in 
 substantial noise reductions — 10 dBA at 
 the jacksetter position (98 dBA overall 
 noise) and 6 dBA at the operator position 
 (96 dBA overall noise). This reduction 
 was sufficient to eliminate the cutting 
 head as a contributor to operator noise; 
 no further reduction could be achieved 
 without treating the chain conveyor 
 or motors (17). The 10 dBA reduction at 
 the jacksetter position resulted in a 
 fourfold increase in allowable operating 
 time — from 45 min to almost 3 h. 
 
 The conical helix stiffener and sand 
 are relatively simple to install; they 
 could be installed by most mine shops. 
 However, the larger core is rather dif- 
 ficult to fabricate and does not produce 
 
 Adapter 
 (6'/2-ln-lD by 8-in-OD 
 
 by 5-in-long tube) 
 
 lO-ln-OD by 8-in-ID 
 mechanical tubing 
 
 LEFT END VIEW 
 
 Note: 
 
 All dimensions are in inches. 
 
 L5-| 
 
 -5-1 
 
 3/3- in radius continues. for 180°. 
 Fill with dry sand before closing. 
 
 20 lead - 
 
 53 
 
 65 
 
 PLAN VIEW 
 
 k5- 
 
 1.5 
 
 0.75 
 
 RIGHT END VIEW 
 
 (Right hand auger shown. 
 Left hand opposite.) 
 
 typical 
 
 .^ Fill with dry sand. 
 
 1 
 
 SECTION B-B' 
 
 ~2 typico/ 
 
 SECTION /l/l' 
 
 FIGURE 17. - Fabrication drawing of reduced-noise auger. 
 
29 
 
 as much noise reduction as the stiffen- 
 ing and sand-filling treatments. There- 
 fore, it is recommended that mine shops 
 do not attempt to enlarge the auger core. 
 Detailed instructions for converting 
 a standard two-helix auger into a sand- 
 filled single-helix reduced-noise auger 
 are now available upon request from the 
 Bureau, The reduced-noise augers are al- 
 so available from the auger manufacturer. 
 
 Chain Conveyors 
 
 Part 2 briefly described the purpose 
 of a chain conveyor, its major compo- 
 nents, how it generates noise, and the 
 Bureau's approach to conveyor noise con- 
 trol. Here, the results of Bureau re- 
 search on conveyor noise (16) are dis- 
 cussed more thoroughly. Figure 4 (in 
 part 2) shows the location of the con- 
 veyor on a continuous miner; figure 18 
 shows only the conveyor and points out 
 the components that produce the most 
 noise. The basic noise-generating mecha- 
 nisms on the conveyor are (1) scraping of 
 the chain and flights against the upper 
 and lower decks and (2) impacts at dis- 
 continuities (gaps, misalignments, etc.), 
 such as those at the idler roller, takeup 
 plate, guide plate, and flex plate edge, 
 
 Aboveground Tests 
 
 Most of the Bureau research on conveyor 
 noise control was done in an aboveground 
 
 test facility. The key element of this 
 facility was the conveyor portion of a 
 continuous miner, including all the com- 
 ponents pictured in figure 18, Prior to 
 the installation of any noise-control 
 treatments, the overall noise level at 
 the operator's position (next to the bend 
 of the conveyor) was 101,5 dBA. The fol- 
 lowing paragraphs describe the noise- 
 control treatments developed for each 
 noise source in figure 18 and discuss how 
 each treatment contributed to the reduc- 
 tion of conveyor noise. 
 
 Figure 19 shows the basic components 
 of the noise-control treatments applied 
 to the conveyor decks and sidewalls. The 
 top surface of the upper deck, the bot- 
 tom surface of the lower deck, and the 
 outer faces of the sidewalls received a 
 constrained-layer damping treatment. In 
 addition, the damped deck plates were 
 isolated from the conveyor chain and 
 flights by resilient wear strips. Both 
 the damping treatment and the we'ar strips 
 contained a layer of wear-resistant 
 energy-absorbing polymer material sand- 
 wiched between layers of steel. The wear 
 strips were also backed with rubber to 
 further isolate the decks from the moving 
 conveyor. The upper deck wear strips 
 were originally welded to the deck in 
 one piece, however, the relative motion 
 between adjacent conveyor sections would 
 have damaged these one-piece strips 
 during normal underground operation. 
 
 Flex plate edge 
 
 Idler roller 
 
 Takeup plate 
 
 Guide plate 
 
 Lower deck 
 
 FIGURE 18. - Noise-producing components of o continuous miner chain conveyor. 
 
30 
 
 Damping material 
 
 Damped flex plate 
 
 Upper deck 
 resilient strip 
 
 Damped 
 upper deck 
 
 Lower deck 
 resilient strip 
 
 Damped lower deck 
 
 FIGURE 19. - Noise-control treatments on conveyor decks and sidewalls. 
 
 Therefore, the upper deck wear strips 
 were cut, and their leading edges were 
 tapered to reduce flight-strip Impacts, 
 Since this created dlscoritlnultles be- 
 tween adjacent conveyor sections, it re- 
 duced the quieting effect of the strips; 
 however, cutting the strips in this man- 
 ner Increased their durability. 
 
 Figure 19 also shows several noise- 
 control treatments designed to reduce 
 noise due to Impacts at the conveyor bend 
 point. First, tapered metal strips were 
 welded upstream of the flex plates to 
 minimize impacts between the flight tips 
 and the flex plate edges. Second, the 
 original round-headed carriage bolts 
 holding the flex plates to the sidewalls 
 were replaced by countersunk flathead 
 bolts, thereby reducing Impacts between 
 the flights and bolt heads. Third, the 
 
 flex plates were damped in much the same 
 way as the conveyor decks and sidewalls 
 were damped. Fourth, a piece of leaded 
 vinyl tape was placed over a hole in the 
 sidewall near the operator's ear. Howev- 
 er, the tape was only a temporary treat- 
 ment; the conveyor manufacturer stated 
 that the entire bend point area would 
 have to be redesigned to eliminate the 
 hole permanently. 
 
 Figure 20 shows the noise-control 
 treatments applied to the tail section of 
 the conveyor. The vertical mismatch be- 
 tween the idler roller and the rear edge 
 of the upper deck was improved signifi- 
 cantly by installing a larger diameter 
 roller. This roller was mounted on re- 
 silient support slides to isolate the 
 tail section from the remaining chain- 
 roller impacts. Figure 20 also shows 
 
31 
 
 Modified roller 
 
 Upper tokeup 
 plate resilient pad 
 
 Lower takeup 
 plate resilient pad 
 
 bracket support 
 
 Resilient idler 
 roller support slides 
 
 FIGURE 20. - Noise-control treatments on conveyor idler (toil) roKer and takeup pTate. 
 
 that the takeup plate between the tail 
 roller and the rear portion of the upper 
 deck was mounted on resilient support 
 pads. 
 
 Table 6 summarizes the aboveground 
 noise reductions resulting from the con- 
 veyor noise-control treatments. (Howev- 
 er, as shown, a noise increase rather 
 than a reduction resulted when the upper 
 deck wear strips were cut to make them 
 more durable.) Not including the block- 
 ing of the lower deck hole (a temporary 
 treatment), the noise-control treatments 
 produced an overall noise reduction of 
 10.5 dBA (from 101.5 to 91.0 dBA) at the 
 operator's position. The most effective 
 
 treatments were the constrained-layer 
 damping of the decks and sidewalls, the 
 resilient wear strips, and the larger 
 diameter idler roller; they reduced noise 
 by an estimated 5, 3, and 2 dBA, respec- 
 tively. These results indicate that a 
 fully treated conveyor without a lower 
 deck hole would have a noise level 
 12.0 dBA lower than that of the untreated 
 conveyor with the lower deck hole. 
 
 The installation of the wear strips on 
 the lower deck was one of the last modi- 
 fications made to the conveyor because it 
 was a relatively difficult procedure. 
 The entire lower deck had to be removed 
 from the conveyor assembly, fitted with 
 
32 
 
 TABLE 6. - Summary of aboveground chain conveyor 
 noise-control tests 
 
 (Noise level at operator's position, dBA) 
 
 Treatment Result' 
 
 None (untreated conveyor) 
 
 Lower deck damped 
 
 Upper deck damped 
 
 Installation of upper deck wear strips.,,, 
 
 Sldewalls damped 
 
 Large-diameter idler roller (substitution) 
 
 Isolation of takeup plate 
 
 Upper deck wear strips cut 
 
 Installation of lower deck wear strips,,.. 
 
 Lower deck hole blocked ,,. 
 
 'Cumulative. 
 
 101. 
 
 5 
 
 101, 
 
 
 
 97, 
 
 
 
 96, 
 
 5 
 
 95. 
 
 5 
 
 93, 
 
 
 
 91. 
 
 ,5 
 
 93. 
 
 
 
 91. 
 
 .0 
 
 89. 
 
 .5 
 
 the strips, and rewelded to the conveyor 
 slightly below its original position 
 to compensate for the thickness of the 
 strips. The reason for the comparatively 
 large noise reduction (2.0 dBA) after 
 this modification was that all other ma- 
 jor noise sources on the conveyor had al- 
 ready been treated. With the addition of 
 the lower deck wear strips, the largest 
 single remaining noise source (i.e. , the 
 lower deck) was quieted. 
 
 The aboveground tests showed that noise 
 generated by a chain conveyor can be 
 significantly reduced without drasti- 
 cally changing its original design; how- 
 ever, the noise-control treatments would 
 be even easier to install on newly de- 
 signed conveyors. Allowable operator ex- 
 posure time would increase from 1.6 h 
 (untreated conveyor, 101.5 dBA) to almost 
 7 h (treated conveyor, 91.0 dBA) if the 
 conveyor were the only noise source. The 
 potential effect of these treatments on 
 operating continuous miners would be to 
 make conveyor noise insignificant com- 
 pared to coal-cutting noise. 
 
 Underground Tests 
 
 The original underground test plan 
 called for noise measurements on the same 
 continuous miner before and after con- 
 veyor noise-control treatments were in- 
 stalled. However, this plan proved to be 
 
 unfeasible because too much equipment 
 downtime would have been required to in- 
 stall the noise-control treatments and 
 transport the miner to and from an above- 
 ground shop. (The treatments were too 
 complex to install underground.) There- 
 fore, noise measurements were made on 
 several different machines — both treated 
 and untreated, but of the same model 
 whenever possible. The main disadvantage 
 of this approach was that a more exten- 
 sive series of measurements was required 
 for a statistically proper evaluation of 
 the noise-control treatments. 
 
 The constrained-layer damping of the 
 conveyor decks and sidewalls was the only 
 noise-control treatment evaluated exten- 
 sively in the underground tests. It was 
 the first, simplest, and most effective 
 treatment installed aboveground, so it 
 was readily accepted for underground use 
 by both the equipment manufacturer and 
 the mine operators. The other noise- 
 control treatments described in the 
 "Aboveground Tests" section involved more 
 substantial design changes, so a fully- 
 treated miner conveyor was never manufac- 
 tured for underground use. However, some 
 useful information was obtained on the 
 resilient wear strips and a previously 
 neglected noise source, the guide plates 
 near the bend point of the lower deck 
 (fig. 18). 
 
33 
 
 Table 7 shows the average underground 
 noise levels produced by two groups of 
 continuous miners. Six miners were in 
 the treated group, and eleven were in 
 the "untreated" group; all were in the 
 Jeffrey^ 120 series of models (120 L, 120 
 M, 120 H, 120 H2, or 122). All six of 
 the treated miners had damped decks and 
 sidewalls, and one of these also had wear 
 strips on the upper deck (fig. 19). As 
 shown in table 7, noise measurements were 
 taken with the miners in five different 
 operating modes and with their tail booms 
 in three different positions. 
 
 Since the data in table 7 are average 
 noise measurements rather than "before 
 and after" measurements of the same ma- 
 chine, any conclusions from them about 
 the effectiveness of the noise-control 
 treatments must be made cautiously. Con- 
 siderable variations in noise levels were 
 found within the treated and untreated 
 groups, some as large as the differ- 
 ences between the two groups (average 
 noise reduction) . The variations within 
 groups were largely attributable to dif- 
 ferences in mine conditions, equipment 
 models, equipment age, and quality of 
 maintenance. Nevertheless, the data col- 
 lected during the underground tests re- 
 vealed some general trends that led to 
 these three conclusions: (1) The treated 
 
 "Reference to specific products does 
 not imply endorsement by the Bureau of 
 Mines . 
 
 machines were usually quieter than the 
 untreated machines; (2) Noise-level re- 
 ductions were greater for operations that 
 utilize the conveyor; and (3) Miners 
 whose noise-control treatments were 
 installed by the manufacturer were quiet- 
 er than miners whose treatments were 
 installed in mining company shops. In 
 addition, table 7 shows that the average 
 noise reduction in the "Conveyor only (no 
 coal)" operating mode was much larger 
 than those in the "load only" and "cut 
 and load" modes. This is because the 
 presence of coal on the conveyor had a 
 greater muffling effect on the untreated 
 machines than on the treated machines. 
 In fact, the coal on the conveyor reduced 
 the average noise level more than the 
 damping treatment; that is, the average 
 noise level of the untreated miners in 
 the "load only" mode was 1.2 dBA lower 
 than that of the treated miners in the 
 "conveyor only" mode (97.7 versus 98.9 
 dBA). 
 
 The durability of the constrained-layer 
 damping treatment was satisfactory on all 
 six treated miners. No signs of failure 
 were found in either the viscoelastic 
 damping material or the bond between this 
 material and the steel. In one case, 
 this bond proved to be stronger than the 
 original bond between the upper deck and 
 sidewall; the conveyor continued to oper- 
 ate for 2 weeks after the deck-sidewall 
 bond had failed. 
 
 TABLE 7. - Summary of underground chain conveyor noise-control tests 
 (Noise level at operator's position, dBA) 
 
 
 Operating mode 
 
 
 Conveyor only 
 (no coal) ' 
 
 Load only2 
 
 Cut only 
 
 Cut and 
 load2 
 
 Idle 
 
 Untreated miners^ ................ 
 
 103.8 
 98.9 
 
 97.7 
 95.6 
 
 97.2 
 96.3 
 
 100.6 
 97.8 
 
 91.0 
 
 Treated miners^ 
 
 91.3 
 
 Average noise reduction 
 
 4.9 
 
 2.1 
 
 .9 
 
 2.8 
 
 -.3 
 
 'Average from tests using 3 conveyor tail 
 from operator, angled toward operator. 
 ^Tail boom position: straight. 
 ^Average; 11 miners tested. 
 ^Average; 6 miners tested. 
 
 boom positions: straight, angled away 
 
34 
 
 Although the resilient wear strips 
 were sufficiently durable during several 
 months of aboveground tests, more prob- 
 lems were expected underground because 
 coal could abrade the exposed rubber 
 bases of the strips. However, after a 
 3-month underground trial period, damage 
 had occurred only where the flights 
 abruptly struck the upper deck after 
 rounding the drive sprocket at the front 
 of the conveyor. Upstream and downstream 
 of this point, the strips showed negligi- 
 ble wear. 
 
 The underground noise levels were gen- 
 erally higher when the conveyor was an- 
 gled than when it was straight. This was 
 caused primarily by impacts between the 
 conveyor flights and the guide plates 
 (fig. 18) at the conveyor bend point. 
 The guide plates used in the aboveground 
 tests were welded to the sidewalls and 
 were relatively quiet. However, some of 
 the conveyors observed underground had 
 pinned-on guide plates that rattled nois- 
 ily due to flight-plate impacts. In 
 fact, guide plate rattle was sometimes 
 the dominant noise source at the opera- 
 tor's position. The obvious solution to 
 this problem would be to use welded-on 
 rather than pinned-on guideplates. 
 
 In summary, the underground tests 
 showed that the constrained-layer damping 
 treatments were both durable and effec- 
 tive in reducing noise. Continuous miner 
 manufacturers could easily incorporate 
 these treatments into the design of new 
 conveyors. Although noise reductions 
 were not as dramatic underground as they 
 were in the aboveground test facility, 
 this was partially due to (1) other noise 
 sources (coal cutting, gathering arms, 
 motors, drive train, etc.), (2) the re- 
 verberant nature of the underground 
 environment, and (3) the muffling ef- 
 fect of the coal on the conveyor. One 
 significant finding was the superi- 
 ority of welded-on versus pinned-on guide 
 plates as a means of noise control. 
 Additional underground tests of chain 
 conveyors containing all the Bureau- 
 developed noise-control treatments (table 
 
 6) are needed to further verify their 
 durability and acoustical effectiveness. 
 
 Mantrip Vehicles 
 
 Although the mantrip vehicle (a large 
 rail-mounted personnel carrier) is not 
 one of the loudest noise sources in un- 
 derground coal mines (figure 3, part 2) , 
 almost all face workers are exposed to 
 mantrip noise. Since face workers' regu- 
 lar jobs often expose them to high noise 
 levels , the additional contribution of 
 mantrip noise could easily result in 
 noncompliance with Federal noise regu- 
 lations. The Bureau investigated two 
 slightly different approaches to mantrip 
 noise control — retrofit treatments and 
 the integration of quieting techniques 
 into new mantrip vehicles. The goal of 
 both approaches was to reduce noise to a 
 maximum of 85 dBA for all passengers. 
 
 Retrofit Treatments ( 17 ) 
 
 The mantrip chosen for retrofit treat- 
 ments (fig. 21) had four passenger com- 
 partments and a central battery and motor 
 compartment. Operator controls were lo- 
 cated in both of the end compartments 
 to allow the driver to face the direction 
 of travel. The four major elements of 
 the retrofit noise-control program were 
 (1) aboveground diagnostic tests, (2) in- 
 stallation of noise-control treatments, 
 (3) aboveground testing of the retro- 
 fitted vehicle, and (4) underground 
 tests. 
 
 The three major noise sources on the 
 unmodified mantrip were wheel-rail inter- 
 action, motor noise, and drive train 
 noise. As shown in figure 22, all three 
 sources generated both airborne noise 
 (direct path to passengers' ears) and 
 structureborne noise (vibration of the 
 frame, panels, etc.). Structureborne 
 noise produced by wheel-rail interaction 
 (fig. 22i4) was the largest single noise 
 source, followed by airborne motor noise 
 (fig. 225), structureborne motor noise, 
 and the sum of airborne and structure- 
 borne drive train noise (fig. 22C) . 
 
35 
 
 FIGURE 21. - Mantrip vehicle used in retrofit noise-control program. 
 
 KEY 
 
 Airborne path 
 Structureborne path 
 
 Frame 
 
 Bushing 
 
 Observer 
 
 r 
 
 zi 
 
 Floor 
 
 HD) {O} 
 
 ^^ 
 
 FIGURE 22. - Mantrip noise sources and 
 transmission paths. A, Wheel-rail noise; B, 
 motor noise; C, drive-train noise. 
 
 The overall noise 
 unmodified mantrip 
 93.5 dBA, depending 
 track curvature, and 
 tion. More noise was 
 travel speeds, most 
 creased wheel-rail 
 Noise on curves was 
 
 levels within the 
 ranged from 83.5 to 
 on travel speed, 
 compartment loca- 
 produced at higher 
 ly because of in- 
 interactive forces, 
 higher than noise 
 
 on straight track because of the nature 
 of wheel-rail interaction during curv- 
 ing. That is, a "chattering" noise was 
 produced when the wheels repeatedly 
 climbed the rail and fell back downward 
 while negotiating a turn. Finally, noise 
 in the "passenger compartments" was 
 greater than in the (end) "driver com- 
 partments" because the latter were far- 
 ther away from both the wheels and the 
 central battery-motor compartment. (See 
 figure 21.) 
 
 Three retrofit noise-control treatments 
 were installed: resilient wheels, a 
 sound- absorbing motor enclosure, and 
 damping of the mantrip roof and sidewall 
 panels. The resilient wheels (fig. 23) 
 contained rubber inserts that isolated 
 the wheel-rail interface from the rest 
 of the vehicle, thus decreasing struc- 
 tureborne wheel-rail noise. A layer of 
 sound-absorbing fiberglass was attached 
 to the inner surfaces of the motor enclo- 
 sure to reduce airborne motor noise, and 
 a thin sheet of perforated metal covered 
 the fiberglass to protect it from damage. 
 The roof and sidewall damping treatment 
 consisted of a layer of polymer material 
 constrained by a steel sheet. 
 
 Extensive tests of the modified mantrip 
 were conducted on an aboveground track. 
 Noise levels in the passenger and driver 
 compartments ranged from 80 to 92 dBA, 
 again depending on travel speed, track 
 curvature, and compartment location. 
 
36 
 
 Current 
 conductor 
 
 Wheel 
 center 
 
 Rubber 
 blocks 
 
 FIGURE 23. - Resilient wheels to isolate man- 
 trip from wheel-rail noise. 
 
 Although these noise levels were general- 
 ly lower than in the unmodified mantrip, 
 the noise reductions were not as great as 
 expected. The major reason was that the 
 new, resilient wheels had not yet been 
 "broken in" to conform to the track. 
 That is , the sharp edges of the new re- 
 silient wheels caused them to climb far- 
 ther up the rail during curving than the 
 worn-down, all-steel wheels of the unmod- 
 ified mantrip. 
 
 the materials could be prefabricated 
 and their dimensions could be designed 
 to be compatible with the mantrip. The 
 factory-designed "quiet" mantrip was sim- 
 ilar to the retrofitted vehicle but dif- 
 fered in four basic ways: (1) It was a 
 trolley-powered model (same manufactur- 
 er); (2) the roof and sidewall panels 
 were prefabricated damped plates; (3) a 
 redesigned suspension system (fig. 24) 
 replaced the resilient wheels as the 
 means for isolating the mantrip structure 
 from wheel-rail interactive forces; and 
 (4) the electric motor was mounted on 
 vibration-isolation pads and enclosed in 
 a sound-absorbing structure (fig. 25). 
 
 These treatments were evaluated by com- 
 paring the noise levels of the factory- 
 quieted mantrip with those of an un- 
 treated vehicle of the same model, in the 
 same underground mine , and under the same 
 operating conditions. Noise levels with- 
 in the quieted mantrip were about 84.5 
 dBA, compared to 90 to 92 dBA in the un- 
 treated vehicle, recorded at an average 
 speed of 10 mi/h. The same trends as 
 were previously mentioned with regard to 
 travel speed, track curvature, and com- 
 partment location were observed. 
 
 The retrofitted mantrip was then tested 
 underground. At the end of 3 months of 
 testing in two different coal mines, all 
 three treatments remained basically 
 intact and undamaged. Evidence of wear 
 was beginning to show on the resilient 
 wheels. Noise levels within the mantrip 
 ranged from 83 to 86.5 dBA, measured over 
 a wide variety of tram speeds and track 
 curvatures. These noise levels corre- 
 sponded to an average speed of 10 mi/h, 
 as recorded by a machine-mounted radar 
 unit. 
 
 Factory Integration of Noise 
 Controls ( 15 ) 
 
 The major problems of the retrofit 
 noise-control approach were the high 
 costs of labor and materials associated 
 with one-of-a-kind installation. How- 
 ever, factory-integration of the treat- 
 ments was considered feasible because 
 
 The noise reduction met the goal of the 
 mantrip program — a vehicle with an 85-dBA 
 maximum interior noise level. Important- 
 ly, the factory-quieted mantrip cost less 
 than 5 pet more to build than an unqui- 
 eted machine of the same model. Equip- 
 ment manufacturers could easily incor- 
 porate the same basic noise-control 
 treatments on almost any similar mantrip 
 vehicle. 
 
 Stoper Drills 
 
 As shown in part 2 (fig. 3), stoper op- 
 erators consistently experience noise 
 levels greater than 115 dBA, which is far 
 out of compliance with Federal noise reg- 
 ulations. Control of stoper noise was 
 therefore a high-priority area of Bureau 
 research. Both retrofit and redesign 
 techniques were used, and stoper drills 
 for both coal and hardrock use were 
 addressed. 
 
37 
 
 Rubber seat 
 
 Rubber bushing 
 
 Suspension arm 
 
 Guide plate 
 
 iner 
 
 Rubber 
 sleeve 
 
 Linerite 
 SECTION A-4' SECTION B-B' 
 
 Metal 
 sleeve 
 
 Metal washer 
 
 Rubber washer 
 
 SECTION C-C 
 
 Rubber sheet 
 
 /l'< 
 
 FIGURE 24. - Redesigned suspension system of factory-quieted mantrip. 
 
 I- in-thick fiberglass insulation on top 
 and sides of motor compartment 
 
 Rubber mount 
 FIGURE 25. - Motor enclosure of factory=quieted mantrip. 
 
38 
 
 Retrofit Treatments ( 38 ) 
 
 The most effective retrofit noise- 
 control treatment for a standard stoper 
 (figure 5, part 2) included a jacket-type 
 muffler surrounding the drill body and 
 the air-exhaust port (fig. 26). Urethane 
 end caps held the jacket material (a pol- 
 ymer sheet) in place. Drill-steel noise 
 was partially abated by placing a 6-in- 
 long collar near its bottom end (fig. 
 27). The collar was made of a poured-in 
 
 urethane 
 sheath. 
 
 layer constrained by a steel 
 
 FIGURE 26. - Wraparound jacket-type muffler 
 for stoper drill. 
 
 In tests conducted in the Bureau's 
 (Pittsburgh, PA) experimental coal mine 
 (fig. 28), the jacket muffler caused 
 about a 13-dBA noise reduction, and the 
 drill-steel collar resulted in an addi- 
 tional reduction of about 2 dBA. The 
 quieted noise level, 100 dBA, would per- 
 mit about 2 h of operating time per 
 shift, versus no permissible time for an 
 untreated stoper. The entire retrofit 
 package increased the total drill weight 
 (including feedleg) by about 10 pet. The 
 materials used were fairly inexpensive 
 (about $150 in 1980). Retrofit kits sim- 
 ilar to those tested are now commercially 
 available for several models of stopers. 
 
 Retrofitted stopers (jacket muffler 
 only) were tested in 15 operating under- 
 ground coal mines, and muf f led-versus- 
 unmuffled noise reductions of 7 to 8 dBA 
 were consistently obtained. The 13-dBA 
 experimental noise reduction was not 
 achieved because the underground mines 
 did not control the drill feed rate or 
 maintain the noise-control treatments as 
 diligently as the Bureau did in its ex- 
 perimental mine. Nevertheless, the typi- 
 cal muffled noise levels (105 to 106 dBA) 
 were low enough to permit a doubling of 
 the allowable operating time per shift. 
 Unfortunately, drilling rates with the 
 modified stoper were about 15 to 50 pet 
 slower than those of unmuffled stopers, 
 and freezing was a common problem with 
 the wraparound mufflers. 
 
 Redesign for Noise Control (8_, 13) 
 
 Although the stoper retrofit noise- 
 control treatments were fairly success- 
 ful. Bureau studies showed that greater 
 noise reductions and improved drilling 
 performance could be obtained by rede- 
 signing the drill to incorporate noise- 
 reducing features. The redesign effort 
 included three major steps: (1) redesign 
 of the drill-steel rotation mechanism and 
 other drill parts, (2) development of a 
 more effective muffler-enclosure device, 
 and (3) development of a "shroud tube" to 
 attenuate drill-steel noise. 
 
39 
 
 FIGURE 27. - Damping collar for stoper drill steel. 
 
 Chuck - 
 Shank- 
 
 Piston 
 
 Air port 
 
 Downstroke chamber 
 Valve 
 
 Rock 
 Bit 
 
 Drill rod 
 
 Collar 
 
 Chuck driver nut 
 Drill body 
 
 Return-stroke chamber 
 
 Exhaust port 
 Rifle bar 
 Compressed air 
 Pawl, ratchet 
 
 Compressed air 
 
 Airleg 
 
 FIGURE 28. - Stoper with retrofit noise-control 
 treatments in operation. 
 
 FIGURE 29. - Internal components of standard 
 stoper drill with rifle-bar rotation. 
 
 A standard stoper drill (fig. 29) pro- 
 duces drill-steel rotation through a 
 "rifle-bar" arrangement. The oscillat- 
 ing piston supplies percussive energy to 
 the steel and imparts rotation on its 
 backstroke through a pawl-and-ratchet 
 mechanism. The steel rotates through a 
 15° to 20° arc with each stroke, result- 
 ing in a rotation speed of about 150 to 
 200 rev/min. 
 
40 
 
 Aluminum 
 outer 
 cover 
 Front I 
 
 exhaust ""^ 
 
 Isolation 
 mounts 
 
 Replaceable . , 
 chuck Independent 
 
 rotation 
 
 Valveless 
 hammer 
 
 Flexible liner 
 to prevent 
 
 ice buildup 
 
 FIGURE 30. - Internal components of redesigned "quiet'* stoper drill. 
 
 The rifle-bar rotation mechanism gener- 
 ated a great deal of high-frequency "rat- 
 tling" noise that was beyond the ef- 
 fective noise-attenuating range of the 
 retrofit jacket-type muffler. Therefore, 
 muffler effectiveness was increased by 
 using a quieter motor-and-gear arrange- 
 ment to achieve the same drill-steel ro- 
 tation speed. The piston no longer ini- 
 tiated rotation, so it was redesigned to 
 serve as the valve controlling the flow 
 of compressed air within the drill. The 
 annular clearance between t"he chuck and 
 shank was reduced, and the collar on the 
 drill steel was eliminated to reduce mis- 
 alignment and rattling impacts at the top 
 of the drill body. These design changes 
 improved the efficiency of the drill and 
 also reduced its high-frequency noise. 
 Figure 30 shows the major internal com- 
 ponents of the redesigned stoper drill. 
 
 The new drill— body design necessitated 
 the development of a special muffler- 
 enclosure device consisting of an alumi- 
 num outer cover and rubber isolation 
 mounts at each end of the drill that pre- 
 vented the transfer of vibration from the 
 drill cylinder to the outer aluminum 
 
 shell (fig. 30). During drilling, the 
 exhaust air from the piston chamber and 
 rotation motor left the acoustical enclo- 
 sure through an exhaust hole near the top 
 of the drill. The muffler enclosure at- 
 tenuated both drill-body and air-exhaust 
 noise, and a flexible deflector plate 
 near the exhaust port of the piston cham- 
 ber helped reduce icing. 
 
 Another important component of the re- 
 designed stoper drill was the steel 
 "shroud tube" that surrounded the drill 
 steel like a sheath and acted as a bar- 
 rier against the noise produced by drill- 
 steel vibration. The outer diameter of 
 the shroud tube was small enough to allow 
 it to follow the drill bit into the hole, 
 and its inner diameter was large enough 
 to keep it from touching the drill steel. 
 The tube was connected to the top of the 
 drill body through a "shroud tube damper" 
 to provide isolation from drill-body 
 vibration. 
 
 Figure 31 shows the "feedleg version" 
 of the redesigned handheld drill. Six of 
 these "quiet" drills were manufactured 
 and tested in operating mines. All six 
 
41 
 
 FIGURE 31. - Redesigned "quiet" stoper on feedleg: 
 
 produced noise levels of 98 to 105 dBA at 
 the operator's position, reflecting a 10- 
 to 17-dBA reduction versus the noise 
 of standard stopers. Importantly, the 
 weight and penetration rates of the rede- 
 signed drills were approximately equal to 
 those of standard stopers. In addition, 
 the three machine control levers — thrust, 
 hammer, and rotation — were located to- 
 gether on the drill backhead (fig. 32) , 
 allowing simultaneous operator control of 
 all three functions. These design and 
 performance features greatly aided opera- 
 tor acceptance of the redesigned drill 
 and illustrated the advantages of the re- 
 design approach versus the retrofit ap- 
 proach to stoper noise control. 
 
 The only disadvantage of the redesigned 
 stoper (other than its initial cost) was 
 
 that the shroud tube required removal 
 and replacement during the drill-steel 
 changing process. Since this proved 
 to be time-consuming, operators often 
 drilled without the shroud tube, partial- 
 ly negating the effectiveness of the re- 
 designed drill. However, noise levels 
 without the shroud tube were still about 
 108 dBA, which was substantially lower 
 than those of standard stopers or ret- 
 rofitted stopers with untreated drill 
 steels. 
 
 UNDERGROUND HARDROCK MINING 
 
 Part 2 briefly reviewed the noise prob- 
 lems associated with underground hardrock 
 mines. The two major underground hard- 
 rock mining noise sources the Bureau has 
 investigated (other than handheld drills) 
 
42 
 
 Light hammer 
 only (collar) 
 
 1/2 hammer 
 1/2 rotation 
 
 Full hammer 
 full rotation 
 
 Air to leg 
 and water on 
 
 Rotation only 
 and air to leg 
 
 Rotation rate 
 control 
 
 Leg air control 
 
 Hammer, rotation, 
 water control 
 
 FIGURE 32. - Drilling controls of redesigned 
 "quiet" stoper drill. 
 
 are jumbo-mounted pneumatic percussion 
 drills and diesel-powered LHD machines. 
 These machines have been identified by 
 the Bureau as the two most serious noise 
 offenders in terms of both the noise lev- 
 els produced and the number of workers 
 overexposed. 
 
 Jumbo-Mounted Percussion Drills 
 
 As with mantrip vehicles and stoper 
 drills, the Bureau has investigated both 
 retrofit and redesign measures for reduc- 
 ing jumbo drill noise. A potentially 
 workable retrofit package was developed 
 under Bureau contract and is now being 
 tested in-house to determine its long- 
 term durability. The redesigned jumbo 
 drill is now being field-tested by anoth- 
 er contractor. As with stoper drills, 
 the two major components of the noise- 
 controlled jumbo drills were (1) a muf- 
 fler enclosure to attenuate air-exhaust 
 and drill-body noise and (2) a shroud 
 tube to attenuate drill-steel noise. 
 
 Retrofit Treatments (11) 
 
 Figures 33 and 34 show the retrofit 
 muffler enclosure designed by the Bureau 
 contractor for a drifter with rifle-bar 
 
 rotation. This muffler enclosure had to 
 surround the drifter completely because 
 there were three air-exhaust ports — a 
 main port on the top and two auxiliary 
 ports on the underside of the drill body. 
 The halves of this two-piece, boxlike en- 
 closure fit together snugly around a hor- 
 izontal centerline. Its octagon-shaped 
 profile was a compromise reached after 
 considering the requirements of exterior 
 slimness and light weight and require- 
 ments regarding interior volume and 
 noise-attenuating properties. The top 
 portion of the enclosure was hinged to 
 the bottom portion to allow easy access 
 to the drill (fig. 33). Figure 34 shows 
 the retrofitted drill in its normal 
 drilling position (cover closed). 
 
 Figure' 35 shows the exit path of the 
 main exhaust airflow (arrows). The ex- 
 haust air exited the drill radially, 
 struck a silicone rubber deflector (not 
 shown) at the top of the enclosure, and 
 moved forward to escape through an open- 
 ing at the front of the enclosure. Be- 
 cause the deflector was very flexible, it 
 shook off any ice that began to form on 
 it. The fiberglass in the muffler sec- 
 tion at the front of the enclosure was 
 held in place by a perforated metal 
 plate, and a thin layer of Mylar polyes- 
 ter film prevented it from absorbing oil 
 and water. This muffler section can also 
 be seen in figure 33. The three major 
 advantages of this muffler-enclosure de- 
 sign were that (1) exhaust noise was 
 directed away from the operator and ab- 
 sorbed; (2) the cold exhaust air cooled 
 the coupling and shank at the front of 
 the drifter; and (3) the warm drill com- 
 ponents heated the exhaust air, thus in- 
 hibiting ice formation. 
 
 Figure 36 shows the components of the 
 shroud tube surrounding the drill steel. 
 The outer diameter of the shroud tube was 
 slightly smaller than the bit diameter, 
 allowing the tube to enter the hole be- 
 hind the bit. The inner poljoner layer 
 rode loosely on the drill steel, causing 
 the tube to rotate slightly during opera- 
 tion. The foam interlayer absorbed some 
 of the vibration imparted to the polymer. 
 
43 
 
 FIGURE 33. - Jumbo drill within retrofit muffler enclosure (cover open). 
 
 FIGURE 34. - Jumbo drill within retrofit muffler enclosure (cover closed). 
 
44 
 
 Perforated plate 
 muffler section 
 
 Enclosure 
 
 Muffler section 
 
 (fiberglass retained by 
 
 perforated plate) 
 
 Tapered exhaust exit 
 
 transition 
 
 Z-bar 
 clamp 
 
 "^Muffler section 
 
 Enclosure 
 
 'Feed channel 
 FIGURE 35. - Schematic views of retrofit muffler enclosure for jumbo drill. 
 
 Drifter 
 enclosure 
 
 Drill steel 
 Plastic liner- 
 Foam interlayer7 
 Steel tube. 
 
 Coupling cover 
 'Coupling 
 
 FIGURE 36. - Retrofit shroud tube for control- 
 ling jumbo drill-steel noise. 
 
 and the steel outer layer protected the 
 two inner layers from dacaage. Exhaust 
 air from the muffler enclosure traveled 
 forward through the annulus between the 
 steel and the shroud tube, escaping just 
 behind the bit. 
 
 Performance of the jumbo drill with and 
 without the retrofit noise-control treat- 
 ments was evaluated first in the labora- 
 tory, then at an aboveground test site. 
 Laboratory tests in a reverberation room 
 (figures 33 and 34) showed that the sound 
 
 power level of the treated drill was 19.3 
 dBA lower than that of the untreated 
 drill. At the aboveground test site, 
 noise reductions of 16.5 to 18.5 dBA were 
 recorded at the operator's position (ta- 
 ble 8). Diagnostic tests showed that the 
 muffler enclosure accounted for about 11 
 dBA of this reduction; the shroud tube 
 and/or the rock mass surrounding the 
 drill hole accounted for the remainder. 
 Ice formation and damage to the noise- 
 control treatments were almost negligible 
 during these tests. 
 
 The retrofitted drill was then tested 
 in an operating underground zinc mine 
 (fig. 37). As shown in table 8, noise 
 levels at the operator's position were 
 12.5 to 15 dBA lower than those of the 
 untreated drill. One of the reasons for 
 the more modest noise reductions in 
 the underground tests was that the con- 
 fined, reverberant underground environ- 
 ment resulted in reflections that par- 
 tially negated the advantage of directing 
 the exhaust air away from the operator. 
 Also, the overall noise levels were much 
 higher underground than aboveground. The 
 noise levels of the treated drill indi- 
 cate that it could have been operated for 
 about 8 h per shift aboveground (88 to 92 
 dBA) or about 1-1/2 h underground (101 to 
 105 dBA). 
 
45 
 
 FIGURE 37. - Jumbo drill with retrofit noise-control treatments in underground zinc mine. 
 
 TABLE 8. - Results of retrofit jumbo drill noise-control tests 
 (Noise level at operator's position, dBA) 
 
 
 Position of drill' 
 
 
 Collaring hole 
 (10 ft steel) 
 
 Middle of hole 
 (5-6 ft steel) 
 
 End of hole 
 (1-2 ft steel) 
 
 ABOVEGROUND TESTS 
 
 Untreated drill 
 
 108.5 
 92 
 
 107 
 88.5 
 
 105.5 
 
 Treated drill 
 
 88 
 
 Noise reduction, . . . 
 
 16.5 
 
 18.5 
 
 17.5 
 
 UNDERGROUND TESTS 
 
 Untreated drill 
 
 117.5 
 105 
 
 116 
 101 
 
 115.5 
 
 Treated drill 
 
 101.5 
 
 Noise reduction. , , , 
 
 12.5 
 
 15 
 
 14 
 
 'Holes were drilled to 10 ft. 
 
 The durability of the noise-control 
 treatments was evaluated by drilling ap- 
 proximately 10,000 ft of hole in the un- 
 derground zinc mine. Although only about 
 500 ft was drilled with the shroud tube, 9 
 the muffler enclosure was used during 
 the entire test period. Overall, the 
 
 ^The mine did not have the 10-ft-long 
 drill steels for which the shroud tube 
 was designed; by the time they were ob- 
 tained, the test was almost completed. 
 
 components of the muffler enclosure were 
 quite durable; the outside was not dam- 
 aged, the fiberglass baffles were in good 
 condition, the protective film had only 
 two small holes, and the rubber exhaust 
 deflector showed no signs of wear. The 
 only damaged acoustical component was a 
 rubber seal at the drilling-air inlet 
 (fig. 35) that came off when the bolts 
 supporting the drill mounting bracket 
 failed. This failure, however, was not 
 the fault of the acoustical treatments. 
 
46 
 
 Mine personnel reported very good oper- 
 ator acceptance of the partially quieted 
 drill (muffler enclosure only) during un- 
 derground tests, despite the need to fix 
 damaged drill parts and support brackets 
 on several occasions. The presence of 
 the muffler enclosure did not interfere 
 significantly with either the replacement 
 of broken parts or routine drill mainte- 
 nance. Operators generally agreed that 
 the treated machine drilled just as fast 
 or faster than the unmodified drills used 
 at the mine. 
 
 The Bureau is presently testing the 
 fully quieted drill to evaluate the dura- 
 bility of the shroud tube depicted in 
 figure 36 and other similar shroud-tube 
 designs. It is also evaluating the ef- 
 fect of the shroud tube on operator ac- 
 ceptance (e.g. , with respect to abil- 
 ity to observe a stoppage of drill-steel 
 rotation) . 
 
 Redesign for Noise Control (14) 
 
 Although the retrofit muffler-enclosure 
 described above would be quite effective 
 for most jumbo drills with rifle-bar ro- 
 tation, it would not be appropriate for 
 drifters containing independent drill- 
 steel rotation motors. This is because 
 independent rotation drills are usually 
 somewhat larger than rifle-bar drills and 
 would require larger, heavier muffler en- 
 closures. The problem of air exhaust 
 from the rotation motor would also have 
 to be addressed. Therefore, the Bureau 
 sponsored a program to redesign an inde- 
 pendent-rotation drill for the purpose of 
 reducing noise. 
 
 Figure 38 is a photograph of the proto- 
 type redesigned jumbo drill immediately 
 prior to drilling. In order to make a 
 simple, compact muffler enclosure for the 
 drifter, the rotation motor was removed 
 from the drifter body and relocated at 
 the front end of the feed channel. This 
 design change required the use of a very 
 long drill shank called a kelly bar. The 
 drifter percussed the rear end of the 
 kelly bar while the new rotation mecha- 
 nism (an air motor, belt drive, and 
 gears) imparted rotation to its front 
 
 end. The kelly-bar drive mechanism was 
 then fitted with a muffler enclosure (not 
 shown in figure 38) to attenuate its 
 noise. 
 
 The muffler enclosure for the drifter 
 was a two-piece boxlike structure made 
 entirely of molded pol5nner material. The 
 top half fit snugly atop the bottom half 
 and could be removed for easy access 
 to the drill. The drifter was mounted 
 within the bottom half of the enclosure 
 through rubber bushings that isolated the 
 feed channel from drifter vibration. 
 
 The shroud tube, which is also shown in 
 figure 38, was a collapsible steel coil 
 approximately 8 in in diameter. Unlike 
 the shroud tube on the retrofitted jumbo 
 drill, it did not touch the drill steel 
 or enter the hole during drilling. In- 
 stead, it was suspended firmly between 
 the front portion of the drifter enclo- 
 sure and the rear face of the kelly- 
 bar rotation mechanism. The springlike 
 shroud tube was completely extended at 
 the start of the drilling (fig. 38) and 
 collapsed as the drifter moved toward the 
 face (fig. 39). Exhaust air from the 
 drifter moved forward through the shroud 
 tube and a plunger-shaped rubber "sting- 
 er" that was pressed against the rock 
 face to attenuate noise produced by 
 bit-rock interaction. Both the drifter 
 exhaust air and the hole-flushing air 
 exited through the small gap between the 
 "stinger" and the rock face (fig. 39). 
 
 Initial testing of the redesigned drill 
 was conducted in a surface rock quarry 
 (figures 38 and 39) and in a nonproduc- 
 tion setting at the Colorado School of 
 Mines' underground experimental mine to 
 determine the acoustical performance and 
 durability of the redesigned drill compo- 
 nents. Noise levels at the operator's 
 position were about 96.5 dBA on the sur- 
 face and 100 dBA underground, reflecting 
 a substantial improvement over those of 
 standard jumbo drills. The redesigned 
 drill has not yet been tested in an un- 
 derground producing mine. At present, it 
 is being modified to accommodate a semi- 
 automatic drill-steel changing device 
 that should facilitate long-hole drilling 
 
47 
 
 FIGURE 38. - Redesigned, noise-controlled jumbo drill at start of hole. 
 
 FIGURE 39. - Redesigned, noise-controlled jumbo drill at completion of hole. 
 
48 
 
 and increase 
 acceptance. 
 
 the likelihood of operator 
 
 Load-Haul-Dump Machines 
 
 As stated in part 2, the Bureau desig- 
 nated the diesel-powered LHD vehicle as a 
 high-priority machine in terms of noise 
 control because (1) almost all LHD op- 
 erators are overexposed to noise; (2) the 
 LHD is one of the most common machines in 
 underground hardrock mines; and (3) the 
 LHD is one of the most difficult diesel- 
 powered machines to noise-control. As 
 with other equipment types, the Bureau 
 investigated both retrofit and redesign 
 approaches to the LHD noise problem. 
 
 Retrofit Treatments (22) 
 
 Figure 40 shows the location of the 
 major noise sources of a typical LHD 
 vehicle with respect to its operator. 
 Diagnostic tests conducted at the LHD 
 manufacturer's shop produced operator 
 noise levels of 99 to 101 dBA, depending 
 on the vehicle operating mode. Table 9 
 shows that the three major noise sources 
 on the LHD, in decreasing order of impor- 
 tance, were the transmission, the diesel 
 engine, and the engine cooling fan. The 
 engine intake and exhaust were also sig- 
 nificant noise sources; however, they 
 were not treated in the retrofit program 
 so that efforts could be concentrated on 
 
 the three main sources. In table 9, the 
 estimated underground noise levels of the 
 untreated LHD are 1 dBA tower than the 
 noise levels recorded during diagnostic 
 tests in the aboveground shop because the 
 underground environmrnt was expected to 
 be slightly less reverberant than the 
 shop. The "Required reduction" column in 
 table 9 shows the noise reductions needed 
 to result in a combined noise level of 
 90 dBA at the operator's position during 
 underground operation (85 dBA from each 
 source) . 
 
 Transmission noise was dominant be- 
 cause the transmission compartment was 
 immediately adjacent to the operator's 
 compartment (fig. 40). Airborne trans- 
 mission noise was treated by sealing sev- 
 eral holes between the two compartments 
 and installing comp6site foam-and-plate 
 linings on the top and forward sides of 
 the transmission compartment (fig. 41). 
 Structureborne transmission noise was 
 treated by mounting the transmission on 
 specially designed rubber vibration-iso- 
 lation pads and installing similar mate- 
 rials around the underside of the top 
 cover. 
 
 Engine airborne noise, the second 
 largest source on the LHD, was treated 
 primarily by building an acoustical 
 enclosure around the engine (fig. 42). 
 The top, left, and right sides of the 
 
 TABLE 9. - Breakdown of noise sources on unmodified 
 LHD vehicle 
 
 (Noise level at operator's position, dBA) 
 
 Noise source 
 
 Recorded 
 aboveground 
 
 Underground 
 (estimated) 
 
 Required 
 reduction ' 
 
 Transmission: 
 
 Airborne 
 
 99 
 96 
 
 101 
 
 96 
 88 
 97 
 94 
 
 98 
 
 95 
 
 100 
 
 95 
 87 
 96 
 93 
 
 ND 
 
 Structureborne 
 
 Combined. 
 
 ND 
 15 
 
 Engine: 
 
 Airborne. 
 
 ND 
 
 Structureborne 
 
 Combined 
 
 ND 
 11 
 
 Cooling fan 
 
 8 
 
 ND Not determined. 
 
 'Reduction needed to achieve 90-dBA overall level under- 
 ground (85 dBA from each source) . 
 
49 
 
 Water combustion 
 and air intake 
 
 tanks - 
 
 Engine exhaust 
 
 Cooling fan 
 
 Engine 
 Torque 
 converter Hydraulic pumps 
 
 PLAN 
 
 Note: 
 
 All dimensions in inches. 
 
 ELEVATION 
 
 FIGURE 40. - Noise sources on typical 
 diesel-powered LHD vehicle. 
 
 enclosure were lined with noise-absorbing 
 polyurethane foam. Two foam-lined cut- 
 outs for air exhaust were added to the 
 "belly pan" beneath the engine. Exten- 
 sive analysis of this enclosure configur- 
 ation showed that it would not result in 
 engine overheating. Structureborne en- 
 gine noise was treated by welding trian- 
 gular stiffening gussets to the frame 
 rails on which the engine was mounted. 
 
 The cooling fan was located at the rear 
 of the engine compartment, A fold-down 
 baffle that covered the cooling fan 
 outlet received the same f oam-and-plate 
 treatment as the transmission cover, and 
 the walls surrounding the fan outlet were 
 treated with polyurethane foam (fig. 43). 
 
 Similar acoustical foam treatments were 
 installed on the interior walls of the 
 torque converter compartment and the com- 
 partment containing the water and fuel 
 tanks. (See figure 40 for the locations 
 of these components.) In addition, nu- 
 merous existing LHD components had to 
 be relocated or modified slightly to fa- 
 cilitate the installation of the noise- 
 control treatments. The total cost of 
 
 FIGURE 41o - Sound-absorbing foam lining 
 within LHD transmission compartmento 
 
 the acoustical materials was approximate- 
 ly 5 to 7 pet of the selling price of a 
 new LHD vehicle. 
 
 The noise-controlled LHD was then 
 tested in an underground gypsum mine; un- 
 fortunately, noise levels within the 
 operator's compartment could not always 
 be measured directly. ^^ However, opera- 
 tor noise levels for the two loudest sta- 
 tionary LHD modes (high idle and torque 
 converter stall) ranged from 92.5 to 93 
 dBA, which was approximately 7 to 9 dBA 
 quieter than in the untreated vehicle. 
 In addition, the operator NEI was approx- 
 imately 103 pet when measured shortly 
 after the retrofitted LHD was placed into 
 
 "■OAt the time of this study (1977), 
 sound level recording equipment was too 
 bulky to place in the operator's compart- 
 ment during tramming without interfering 
 with operator movement. 
 
50 
 
 FIGURE 42. - Retrofit acoustical enclosure for LHD engine. A, Top cover; B, right side cover; C, 
 left side cover; D, belly pan treatment. 
 
51 
 
 FIGURE 43. 
 ment lining). 
 
 Retrofit noise-control treatments on LHD engine cooling fan (cover and fan compart- 
 
 underground service; 1 yr later, the NEI 
 was only 89 pet. Although reduced oper- 
 ating time per shift may have been par- 
 tially responsible for this decrease, it 
 showed that the noise-control treatments 
 had not deteriorated over time. Mine 
 officials reported that the noise-control 
 treatments did not pose serious mainte- 
 nance problems or hinder machine 
 performance. 
 
 The LHD retrofit noise-control treat- 
 ments were successful only because the 
 individual noise sources were proper- 
 ly diagnosed and the treatments were 
 carefully designed, installed, and 
 
 maintained. Each LHD vehicle is slightly 
 different, so preliminary diagnostic work 
 would have to be performed before similar 
 noise-control treatments could be in- 
 stalled on a different LHD model. Engine 
 enclosures must be designed with special 
 care to prevent overheating. 
 
 The Bureau is presently sponsoring a 
 program to equip four different LHD vehi- 
 cles with retrofit noise controls. Pre- 
 liminary results have been only moderate- 
 ly successful (noise reductions of 4 to 5 
 dBA) , but the program has been signifi- 
 cantly affected by problems unrelated to 
 the noise-control treatments (machine 
 
52 
 
 breakdowns, mine closings, poor mainte- 
 nance of noise-control treatments, diffi- 
 culty in locating mines that will cooper- 
 ate, etc.). The Bureau, in cooperation 
 with MSHA, has also initiated an in-house 
 program dealing with LHD retrofit noise 
 controls. 
 
 Redesign for Noise Control (40) 
 
 Although retrofit LHD noise-control 
 treatments can be successful, incorpo- 
 rating these treatments in a newly de- 
 signed vehicle would proably be a more 
 cost-effective approach. Under Bureau 
 contract, a major LHD manufacturer incor- 
 porated noise-control treatments into two 
 of its new machines — one with an 8-yd^ 
 bucket capacity and one with a 2-yd-^ 
 bucket capacity. The redesign approach 
 was somewhat similar to the retrofit 
 approach; preliminary diagnostic tests 
 were performed on the manufacturer's 
 standard-design LHD vehicles before 
 treatments were designed. Since the man- 
 ufacturer was not constrained by existing 
 machine dimensions, better fitting acous- 
 tical treatments and mechanical compo- 
 nents were designed and Installed. 
 
 Diagnostic tests revealed that the 
 three major noise sources on the stan- 
 dard-design LHD's were again the trans- 
 mission, engine (including intake and 
 exhaust) and engine cooling fan. There- 
 fore, the manufacturer installed noise- 
 control treatments similar to the retro- 
 fit treatments described earlier. In 
 addition, the manufacturer placed exhaust 
 mufflers on both sides of the engine 
 (fig. 44), lined the interior surfaces of 
 the operator's compartment with acousti- 
 cal foam (fig. 45) , and mounted the 
 transmission on rubber vibration-isola- 
 tion mounts (fig. 46) . These treatments 
 were much easier to incorporate in the 
 redesigned machines than in the retro- 
 fitted machine. 
 
 Aboveground tests of the redesigned 
 LHD's revealed a 9.3-dBA noise reduction 
 for the 8-yd^ LHD (from 99.0 to 89.7 dBA) 
 and a 10.3-dBA reduction for the 2-yd-^ 
 LHD (from 101.1 to 90.8 dBA). The S-yd^ 
 
 FIGURE 44„ = Exhaust muffler on redesigned, 
 noise-controlled LHD vehicle, 
 
 LHD was then sent to an underground mine 
 for extended testing. The results are 
 shown in table 10. Although the Initial 
 underground tests were encouraging (94- 
 dBA operator noise level) , the noise- 
 control treatments were not maintained 
 properly and were gradually removed dur- 
 ing the 8-month evaluation period. The 
 final noise level of 102 dBA corresponded 
 to that of an untreated machine, imply- 
 ing that an 8-dBA noise reduction was 
 achieved by the manufacturer during the 
 redesign process. The 8-dBA noise in- 
 crease over the evaluation period re- 
 sulted in more than a threefold decrease 
 in allowable operating time (1.5 to 4.7 h 
 per shift); for this reason, the lack of 
 proper maintenance of the noise-control 
 treatments was particularly harmful. 
 
53 
 
 FIGURE 45o => LHD operator-compartment noise=control treotmentSo A, Canopy; B, foot-pedal area. 
 
 Transmission 
 mount 
 
 Fibromount 
 
 Cinch washer 
 
 Bolt assembly 
 
 Main frame 
 
 Spacers 
 
 FIGURE 46. - Vibration-isolation mount for LHD 
 transmission. 
 
 TABLE 10. - Results of underground tests 
 of redesigned, noise-controlled LHD 
 
 
 
 Operator 
 
 Status of treatments 
 
 Months 
 
 noise 
 
 
 in use 
 
 level, 
 dBA 
 
 All treatments in place 
 
 1 
 
 94 
 
 Some treatments removed 
 
 4 
 
 97 
 
 All treatments removed. 
 
 8 
 
 102 
 
 yr. In general, mine operators were sat- 
 isfied with the durability and effective- 
 ness of the noise-control treatments. 
 
 SURFACE MINING 
 
 As explained in part 2, the Bureau's 
 surface mining noise-control research 
 program has concentrated on retrofit 
 acoustical cab treatments for bulldozers 
 and front-end loaders. Two models of 
 both machine types were selected for 
 these treatments, based on their overall 
 popularity in the surface mining indus- 
 try. The retrofitted dozers and front- 
 end loaders were field tested in surface 
 coal mines for a period of about 1-1/2 
 
 Detailed fabrication manuals, contain- 
 ing photographs and illustrations that 
 show how the noise-control treatments 
 were installed, have been prepared for 
 both bulldozers and front-end loaders. 
 Numerous Bureau-sponsored workshops were 
 held throughout the country to provide 
 equipment users with a closer look at the 
 retrofit process. Most of the workshop 
 attendees found them beneficial, and many 
 equipment users have since applied the 
 noise-control treatments to their own 
 bulldozers and front-end loaders. 
 
54 
 
 FIGURE 47. - Caterpillar D-9G bulldozer (ROPS-FOPS only) with retrofit noise-control treatments. 
 
 ROPS-FOPS canopy absorption .. ,,, 
 
 " Muffler 
 
 Seat seals 
 
 Hydraulic valve 
 cover and tank 
 seal 
 
 Dashboard seals 
 and vibration 
 isolation 
 
 Floormat and seals 
 
 FIGURE 48. - Noise-control treatments installed on Caterpillar D-9G bulldozer (ROPS-FOPS only). 
 
55 
 
 , Bulldozers (6^) 
 
 A breakdown of the 1977 bulldozer popu- 
 lation in U.S. surface mines (table 11) 
 shows that the Caterpillar model D-9 was 
 by far the most popular model, compris- 
 ing 47 pet of the population (40) . For 
 this reason, the Bureau treated two dif- 
 ferent varieties of D-9 dozers, one with 
 only a roll-over-protective and falling- 
 object-protective structure (ROPS-FOPS) 
 and one with a complete (but not acousti- 
 cal) cab. To show that retrofit noise- 
 control treatments could also be applied 
 successfully to another manufacturer's 
 bulldozer, an International Harvester 
 model TD-25C (ROPS-FOPS only) was also 
 treated. 
 
 TABLE 11. - Breakdown of bulldozers 
 used in U.S. surface mines, 1977, 
 by model 
 
 Pet of 
 Model t otal 
 
 47 
 17 
 11 
 25 
 
 Caterpillar D-9 
 
 Caterpillar D-8 
 
 International Harvester TD-25. . . . 
 All others 
 
 Although the design details of the 
 three machines were somewhat different, 
 the same four basic treatments were used: 
 (1) installing a muffler on the diesel 
 engine exhaust, (2) sealing numerous 
 holes in the floor and dashboard of the 
 operator's station, (3) adding sound- 
 absorbing materials under the ROPS-FOPS 
 and under the cover of the hydraulic 
 tank, and (4) installing vibration-isola- 
 tion materials between the engine and 
 dashboard. In addition, windshields were 
 installed on the two dozers that origin- 
 ally contained only ROPS-FOPS. The wind- 
 shields were extremely important because 
 they blocked the direct path between the 
 diesel engine (the largest single noise 
 source on the miachines) and the dozer 
 operators. Seals were also installed 
 around the doors of the cab-equipped D-9 
 dozer. 
 
 Tables 12 and 13 summarize the noise 
 reductions achieved through the bulldozer 
 retrofit treatments, the cost of the 
 
 acoustical materials and hardware, and 
 the labor time it took to install them. 
 Table 12 shows that the operator noise 
 levels after treatment were 89 to 94 dBA, 
 low enough to permit 6 to 8 h of daily 
 operating time without violating Federal 
 noise regulations; before treatment, only 
 1 to 2 h of operating time was allowed. 
 The effects of the individual treat- 
 ments on the three machines are described 
 below. 
 
 Caterpillar D-9G With ROPS-FOPS Only 
 
 Figure 47 is a photograph of the 
 treated dozer, and figure 48 shows the 
 seven major components of the retrofit 
 noise-control package. Diagnostic tests 
 of the untreated dozer indicated that the 
 windshield would be the single most ef- 
 fective noise-control treatment, followed 
 by the ROPS-FOPS canopy absorption and 
 the engine exhaust muffler; therefore, 
 these three treatments were installed 
 first. 
 
 Figure 49 shows how the operator noise 
 level decreased as each of the seven 
 treatments was added. The overall noise 
 reduction was 11 dBA, but the reduction 
 obtained through one treatment depended 
 on the presence of the previous treat- 
 ments. It can be seen from figure 49 
 that the windshield alone would have 
 
 X 
 
 T" 
 
 Basellne (no treatment) 
 
 Windshield 
 
 Windshield and absorption 
 
 Windshield, absorption, muHler 
 
 Windshield, absorption, 
 muffler, dash treatment 
 
 Windshield, absorption, 
 muffler, dash, floor seals 
 
 Full treatment 
 
 90 92 94 96 98 100 102 
 
 SOUND LEVEL AT OPERATOR'S RIGHT EAR, 
 AT HIGH IDLE, dBA 
 
 104 
 
 106 
 
 FIGURE 49. - Step-by-step noise reduction of 
 Caterpillar D-9G bulldozer (ROPS-FOPS only). 
 
56 
 
 reduced the noise by about 4 dBA, 
 canopy absorption alone would 
 reduced the noise by about 3 dBA, 
 the exhaust muffler alone would 
 reduced the noise by about 1.5 dBA. 
 
 the negligible effect on operator noise if 
 
 have the windshield, canopy absorption, and 
 
 and muffler had not been installed; there- 
 have fore, these three treatments were by far 
 
 The the most important components of the ret- 
 
 a rofit package. 
 
 remaining treatments would have had 
 
 TABLE 12. - Summary of bulldozer retrofit noise-control treatment results 
 
 
 Operator noise level. 
 
 Noise 
 
 reduction, 
 
 dBA 
 
 Cost 
 
 Model 
 
 dBA 
 
 Materials, 
 1978 dollars 
 
 Labor, 
 
 
 Before 
 
 After 
 
 h 
 
 
 treatment 
 
 treatment 
 
 
 
 
 Caterpillar D-9G: 
 
 
 
 
 
 
 ROPS-FOPS only 
 
 105 
 
 94 
 
 11 
 
 825 
 
 106 
 
 Cab (nonacoustical) 
 
 99I-IOO2 
 
 892-911 
 
 9-11 
 
 725 
 
 88 
 
 International Harvester TD- 
 
 
 
 
 
 
 25C; ROPS-FOPS only 
 
 102 
 
 91 
 
 11 
 
 912 
 
 80 
 
 'Cab doors open. ^c^^ doors closed. 
 
 TABLE 13. - Summary of treatments and costs for bulldozer noise-control treatments 
 
 Treatment component 
 
 Materials (approx) , 
 1978 dollars 
 
 Labor (estimated) , h 
 
 Welder Mechanic 
 
 CATERPILLAR D-9G WITH ROPS-FOPS ONLY 
 
 
 
 Windshield 
 
 275 
 
 115 
 190 
 25 
 80 
 55 
 80 
 5 
 
 29 
 
 4 
 
 1 
 
 
 4 
 2 
 
 16 
 
 Sound absorption materials for FOPS 
 canopy 
 
 10 
 
 Exhaust muffler. .......................... 
 
 2 
 
 Dashboard seals (and vibration isolation) . 
 Floormat and seals 
 
 4 
 8 
 
 Seat seals 
 
 8 
 
 Hydraulic valve cover and tank seal 
 
 Miscellaneous 
 
 16 
 2 
 
 Total 
 
 825 
 
 40 
 
 66 
 
 CATERPILLAR D 
 
 -9G WITH CAB 
 
 
 
 FOPS canopy absorption. 
 
 150 
 75 
 80 
 
 105 
 
 95 
 
 25 
 
 5 
 
 4 
 2 
 2 
 4 
 
 
 2 
 2 
 
 8 
 
 Cab wall seals 
 
 22 
 
 Floormat and seals 
 
 8 
 
 Seat and hydraulic valve seals 
 
 12 
 
 Sound absorption materials for cab 
 surfaces 
 
 16 
 
 Dashboard seals (and vibration isolation) . 
 Miscellaneous 
 
 4 
 2 
 
 Subtotal 
 
 535 
 190 
 
 16 
 
 
 
 72 
 
 Muffler ' 
 
 2 
 
 Total 
 
 725 
 
 16 
 
 74 
 
 INTEEINATIONAL HARVESTER TD 
 
 -25C 
 
 WITH ROPS-FOPS ONLY 
 
 
 Windshield 
 
 537 
 
 182 
 
 43 
 
 150 
 
 8 
 1 
 
 .5 
 
 53 
 
 FOPS canopy— absorption materials. ......... 
 
 8 
 
 Floormat 
 
 2 
 
 Seat seals and hydraulic box seal 
 
 7.5 
 
 Total. 
 
 912 
 400 
 
 9.5 
 
 
 
 70.5 
 
 Optional: Dashboard barrier seal 
 
 7 
 
 'Needed only if existing muffler is ineffective. 
 
57 
 
 Caterpillar D-9G With Cab 
 
 Figure 50 shows the six major compo- 
 nents of the retrofit noise-control pack- 
 age applied to the cab-equipped D-9G 
 dozer. This machine already had a 
 relatively new muffler, so none was 
 installed; however, this would ordinarily 
 have been included in the package. Since 
 a windshield was already a part of the 
 cab, the simpler "cab wall seal" treat- 
 ment replaced the windshield treatment 
 required for the D-9G dozer with ROPS- 
 FOPS only. The interior walls of the 
 cab were treated with the same sound- 
 absorbing materials as the underside of 
 the canopy. As shown in table 12, the 
 operator noise level in the untreated 
 cab was higher when the doors were 
 closed than when they were open; this was 
 because the untreated doors tended to 
 
 rattle in their sockets when they were 
 closed. After door seals were added, 
 however, the operator noise level was 
 lower when the doors were closed. 
 
 Figure 51 shows how the operator noise 
 level decreased as the treatments were 
 added (11-dBA total reduction, cab doors 
 closed). As with the D-9G dozer with 
 ROPS-FOPS only, the noise reduction at 
 each stage of treatment depended on the 
 presence of the previously installed 
 treatments. 
 
 International Harvester TD-25C 
 With ROPS-FOPS Only 
 
 Figure 52 shows the major components of 
 the retrofit noise-control package for 
 the TD-25C dozer. Since the manufacturer 
 had already installed an exhaust muffler 
 
 ROPS-FOPS canopy absorption 
 
 Seat and hydraulic 
 valve seals 
 
 Dashboard seals and 
 vibration isolation 
 
 Sound absorption on 
 cab interior 
 
 Floormat and seals 
 
 Cab wall seals 
 
 FIGURE 50. - Noise-control treatments on cab-equipped Caterpillar D-9G bulldozer. 
 
58 
 
 1 — ' — \ — ' — r 
 
 ^ r 
 
 -I 1 1 1 r- 
 
 Basellne (muffler only) 
 
 Muffler and ROPS absorption 
 
 Muffler, absorption, cab seals 
 
 Muffler, absorption, cab seals, floor treatment 
 
 Muffler, absorption, cab seals, floor, 
 seat treatment 
 
 Muffler, absorption, cab seals, floor, 
 seat treatment, additional absorption 
 
 Muffler, absorption, cab seals, floor, 
 seat traatmsnt, additional absorption, 
 dash traatmant 
 
 L 
 
 J. 
 
 _L 
 
 80 82 
 
 84 
 
 86 
 
 88 
 
 90 
 
 92 
 
 94 
 
 96 
 
 98 100 
 
 SOUND LEVEL AT OPERATOR'S RIGHT EAR, 
 AT HIGH IDLE, dBA 
 
 FIGURE 51. - Step-by-step noise reduction of 
 cab-equipped Caterpillar D-9G bulldozer. 
 
 on this machine, none was needed; howev- 
 er, a muffler would have to be installed 
 if none were present. Figure 53 shows 
 how the operator noise level decreased as 
 the treatments were added (11-dBA total 
 reduction) , and table 13 summarizes the 
 material costs and labor hours associated 
 with each component of the package. 
 
 Table 13 shows that the cost of the 
 windshield was the biggest difference be- 
 tween the retrofit packages for the In- 
 ternational TD-25C and the Caterpillar 
 D-9G with ROPS-FOPS. More materials and 
 labor were needed for the TD-25C wind- 
 shield because it was larger and more 
 difficult to fabricate, but it provided 
 more noise reduction (5 dBA) than the 
 D-9G windshield (4 dBA). As with the 
 
 other two bulldozers , the noise reduc- 
 tions achieved with individual treatments 
 on the TD-25C depended on the presence of 
 the other treatments. The windshield and 
 the ROPS-FOPS canopy absorption were the 
 two most important treatments; the others 
 would have been ineffective without them. 
 The dashboard barrier was considered to 
 be an optional treatment because its cost 
 was high compared to the noise reduction 
 resulting from its installation. 
 
 Front-End Loaders (7^) 
 
 The front-end loader ranked second af- 
 ter the bulldozer, as a noise offender in 
 the surface raining industry (40) (table 
 4) . Although about 40 pet of the loaders 
 identified during a 1977 census ( 40 ) were 
 equipped with factory-designed acoustical 
 cabs, the remaining 60 pet required some 
 type of retrofit noise-control treatment. 
 The Bureau chose two of the most popular 
 loader models for the retrofit program — a 
 Caterpillar 988 and an International Har- 
 vester H-400 B. Both machines had non- 
 acoustical operator cabs, and this made 
 the retrofit treatments easier to install 
 than if they had been equipped only with 
 ROPS-FOPS. 
 
 The treatments themselves were simi- 
 lar to those installed on the cab- 
 equipped bulldozer: (1) exhaust muf- 
 flers; (2) seals around openings in the 
 cab walls, doors, seats, and floors; and 
 (3) sound-absorbing materials on all in- 
 terior cab surfaces, including the can- 
 opies. Tables 14 and 15 summarize the 
 noise reductions, material costs, and la- 
 bor hours associated with the retrofit 
 treatments. As with the cab-equipped 
 bulldozer, the noise levels in the 
 
 TABLE 14. - Summary of front-end loader retrofit noise-control 
 treatment results 
 
 
 Operator noise level, dBA 
 
 Noise 
 
 reduction, 
 
 dBAl 
 
 Cost 
 
 Model 
 
 Before 
 treatment 
 
 After 
 treatment 
 
 Materials , 
 1978 dollars2 
 
 Labor, 
 h 
 
 Caterpillar 988 
 
 International Harvester 
 H-400 B 
 
 993-1011 
 
 95I.3 
 
 901-9P 
 83'-873 
 
 11 
 12 
 
 410 
 580 
 
 29 
 19 
 
 'Cab doors closed. ^Not including exhaust muffler. ^Cab doors open. 
 
59 
 
 ROPS-FOPS canopy absorption 
 
 Windshield 
 
 Dashboard barrier 
 
 Floormat 
 
 Hydraulic box seals 
 
 FIGURE 52. - Noisercontrol treatments installed on International Harvester TD-25C bulldozer. 
 
 
 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 
 
 1 1 
 
 
 Baseline (no treatment) 
 
 
 
 
 
 
 
 Windshield 
 
 
 
 
 
 
 1- 
 z 
 
 lU 
 
 Windshield and absorption 
 
 
 t- 
 
 
 
 
 < 
 111 
 
 ec 
 
 Windshield, absorption, 
 floormat 
 
 
 
 
 
 
 
 WIndthIdd, abMrption, 
 lloormat, (ttt and hydraulic 
 
 M«lt 
 
 
 
 
 
 
 
 Full treatment 
 
 
 
 1 1 1 1 1 
 
 1 
 
 1 L. 
 
 _l 
 
 1 1 
 
 1 ■ . 1 . 
 
 I.I 
 
 85 
 
 90 
 
 95 
 
 100 
 
 105 
 
 SOUND LEVEL AT OPERATOR'S RIGHT EAR, 
 AT HIGH IDLE, dBA 
 
 FIGURE 53. - Step-by-step noise reduction of Inter- 
 national Harvester TD-25C bulldozer. 
 
 untreated loader cabs were the same or 
 greater when the doors were closed than 
 when they were open, due to the doors 
 rattling in their sockets. After treat- 
 ment, however, the loader cabs were 
 quieter with the doors closed. The costs 
 of installing exhaust mufflers are not 
 included in tables 14 and 15 because the 
 loaders already had mufflers. If muf- 
 flers had not been present, they would 
 have been essential components of the 
 noise-control packages. 
 
 Figures 54 and 55 show the retrofit 
 package on the Caterpillar 988 loader and 
 its results, and figures 56 and 57 do the 
 same for the International H-400 B. As 
 with the bulldozer retrofit packages, the 
 effectiveness of each successive noise- 
 control treatment depended on the pres- 
 ence of previous treatments. 
 
60 
 
 TABLE 15. - Summary of treatments and costs for front-end loader 
 noise-control packages 
 
 Treatment component 
 
 Materials (approx) 
 
 Labor (mechanic, 
 estimated) , h 
 
 CATERPILLAR 
 
 988 
 
 
 
 Cab wall seals .................•.••••••».••... 
 
 $58 
 
 200 
 39 
 50 
 
 63 
 
 7 
 
 Sound absorption materials for canopy and 
 rear cab wall. ............................... 
 
 8 
 
 Floormat *.••.•••••••••••••••••••••••••••••••.. 
 
 3 
 
 Pedestal seals. ............................... 
 
 6 
 
 Additional sound absorption materials for cab 
 interior 
 
 5 
 
 Total i 
 
 410 
 
 29 
 
 INTERNATIONAL HARVESTER H-400 B 
 
 Cab wall seals 
 
 $138 
 
 393 
 
 49 
 
 8 
 
 Sound absorption materials for cab. 
 
 8 
 
 Floormat 
 
 3 
 
 Total 
 
 580 
 
 19 
 
 ^In 1978 dollars for Caterpillar 988; in 1979 dollars for International Harvester 
 H-400 B. 
 
 Canopy and rear cab wa! 
 sound absorption 
 
 FloornnatXc'^ 
 
 Additional sound 
 absorption on cab 
 interior 
 
 Pedestal seals 
 
 Cab wall seals 
 
 FIGURE 54o - Noise^control treatments installed on Caterpillar 988 front-^end loader. 
 
61 
 
 
 1 1 1 1 1 1 > 1 1 1 1 1 1 1 i 1 ' 1 > 1 • 
 
 
 Baseline (no treatment) 
 
 
 
 
 
 Cab seals 
 
 
 
 
 
 
 1- 
 z 
 
 Ul 
 
 Seals and absorption 
 
 
 K 
 
 
 
 
 < 
 
 Seals, absorption, floormat 
 
 
 
 
 
 
 
 Seals, absorption, floormat, 
 pedestal treatment 
 
 
 
 
 
 
 
 Full treatment 
 
 
 
 II 
 
 L_l 
 
 „.I..,J.. 
 
 J—i 
 
 1 1 1 
 
 1 1 . 1 
 
 80 82 84 86 88 90 92 94 96 98 100 102 
 
 SOUND LEVEL AT OPERATOR'S RIGHT EAR, 
 AT HIGH IDLE, dBA 
 
 FIGURE 55. - Step-by-step noise reduction 
 of Caterpillar 988 front-end loader. 
 
 PREPARATION AND PROCESSING PLANTS 
 
 Bureau studies in the 1970' s identified 
 the most serious noise problems in coal 
 and mineral processing plants. Noise in 
 the coal industry has been regulated 
 since 1969, so most of the early Bureau 
 work dealt with coal preparation plants. 
 Federal noise regulations were extended 
 to the metallic and nonmetallic mining 
 industries in 1977, so several recent Bu- 
 reau programs have addressed noise prob- 
 lems in these types of processing plants. 
 Since the use of nonmetallic screen decks 
 is one way to reduce noise in all types 
 of processing plants, the Bureau spon- 
 sored an in-depth study to determine the 
 screening efficiencies attainable with 
 nonmetallic screen decks. The following 
 sections summarize the results of Bureau 
 research efforts in these areas. 
 
 Canopy absorption 
 
 Cab wall seal 
 
 Cab door seals 
 Floormat 
 
 Cab wall absorption 
 
 FIGURE 56. - Noise-control treatments installed on International Harvester H-400 B front-end loader. 
 
62 
 
 
 1 1 1 I 1 1 1 1 1 1 1 1 1 1 1 I 1 1 1 1 
 
 
 t- 
 
 Baseline (no treatment) 
 
 
 z 
 
 
 
 
 s 
 
 1- 
 < 
 
 Seals and absorption 
 
 
 lU 
 
 rr 
 
 
 
 
 i- 
 
 Full treatment 
 
 
 
 .1 
 
 1 
 
 1 1 1 1 1 1 1 1 1 
 
 u 
 
 74 
 
 76 78 80 82 
 
 84 
 
 86 88 90 92 94 96 
 
 SOUND LEVEL AT OPERATOR'S RIGHT EAR, 
 AT HIGH IDLE, dBA 
 
 FIGURE 57. - Step-by-step noise reduction of 
 International Harvester H-400 B front-end loader. 
 
 Coal Preparation Plants 
 
 Retrofit Treatments (32-33) 
 
 The Bureau's most successful program 
 in coal preparation plant noise control 
 was a retrofit project conducted at 
 
 the Georgetown Preparation Plant, Con- 
 solidation Coal Co., Cadiz, OH. The 
 work consisted of three major elements: 
 (1) identification of the most serious 
 noise sources, (2) selection and instal- 
 lation of retrofit noise-control treat- 
 ments, and (3) both short- and long-term 
 evaluations of the durability and acous- 
 tical effectiveness of the retrofit 
 treatments. 
 
 Figure 58 shows the flow chart of the 
 Georgetown preparation plant and depicts 
 most of its noise-generating equipment. 
 Since workers moved throughout the plant 
 during the normal working shift, the 
 noise sources could not be ranked in 
 terms of noise level alone; the amount of 
 time spent in the noisiest areas also 
 had to be considered. The plant was 
 
 -From 1.500-ton raw coal bin 
 
 Plus 7 in 
 
 McNally 
 
 Crusher 
 
 _ i^^rusiier 
 nq screens ▼ ^ 
 
 •u^ .1^ t:^ n:^^' u^ 
 
 1 \ 1 I 1 
 
 1 11 1 1 
 
 Flash 
 
 dryers ^^^ 
 
 Loading tracks 
 and loading booms 
 
 FIGURE 58. - Flow chart of Georgetown coal preparation plant. 
 
63 
 
 therefore divided into 25 different 
 "noise areas," of which 13 were iden- 
 tified as either "continuous-exposure 
 areas" (i.e., at least one worker was 
 present during the entire shift) or "fre- 
 quent-exposure areas" (workers were pres- 
 ent for about 50 pet of the shift) . Ta- 
 ble 16 shows the noise levels in these 13 
 areas and the equipment primarily respon- 
 sible for the noise in each area. Mate- 
 rial falling on the chutes and bins in 
 these areas also contributed heavily to 
 the recorded noise levels. 
 
 Most of the noise sources in table 16 
 received one or more of the following 
 noise-control treatments: (1) resilient 
 screen decks, (2) resilient impact pads, 
 (3) chute liners, and (4) loaded-vinyl 
 curtains. The resilient screen decks 
 
 consisted of elastomeric top surfaces 
 (rubber or urethane) bonded to metal bot- 
 tom layers, and the impact pads and chute 
 liners were also made of elastomeric ma- 
 terials. These three treatments reduced 
 the noise generated by material (coal or 
 rock) falling or sliding on bare steel. 
 The loaded-vinyl curtains were used to 
 enclose several particularly noisy pieces 
 of equipment. In most cases, these cur- 
 tains consisted of a series of vertical 
 strips of the loaded-vinyl material, held 
 together along the edges with Velcro 
 fasteners (fig. 59). This construction 
 allowed workers to inspect the enclosed 
 equipment without removing the entire 
 curtain. Table 17 describes the treat- 
 ments applied to each piece of equipment 
 listed in table 16. 
 
 FIGURE 59. - Curtain around screening area in Georgetown coal preparation plant. 
 
64 
 
 TABLE 16. - Short term and long term effectiveness of noise-control treatments 
 at Georgetown coal preparation plant, A-weighted decibels 
 
 Equipment 
 
 Before 
 
 Shortly after 
 
 3-4 yr after 
 
 treatment 
 
 treatment 
 
 treatment 
 
 111-113 
 
 (') 
 
 C) 
 
 102-104 
 
 96-97 
 
 (2) 
 
 95-103 
 
 89-92 
 
 91-92 
 
 101 
 
 96 
 
 (2) 
 
 93-100 
 
 91-92 
 
 93-95 
 
 96- 97 
 
 93 
 
 94 
 
 95- 97 
 
 93 
 
 96 
 
 94- 97 
 
 91-93 
 
 93-94 
 
 94- 97 
 
 92 
 
 93-94 
 
 96 
 
 90 
 
 95-99 
 
 94- 95 
 
 92-93 
 
 93 
 
 94 
 
 90-92 
 
 93-94 
 
 94 
 
 91 
 
 92-94 
 
 Railcar shaker , 
 
 Classifying screens , 
 
 Centrifugal dryers , 
 
 Vibrating screens , 
 
 Rock crusher , 
 
 Clean coal desanding screens, 
 Secondary sizing screens...., 
 
 Baum j igs 
 
 Middling shaker screens , 
 
 Coal crushers 
 
 Flight conveyor , 
 
 Picking table 
 
 Primary screens 
 
 ^Not treated; shaker operation discontinued. 
 Not measured; equipment not operating at full capacity, 
 
 The noise-control treatments listed in 
 table 17 had to be evaluated in terms of 
 both short-term noise reduction and long- 
 term durability. Short-term noise re- 
 ductions were relatively easy to assess 
 through bef ore-and-af ter noise measure- 
 ments in the same areas. However, the 
 long-texrm acoustical performance of the 
 treatments was difficult to measure quan- 
 titatively because of (1) normal wear of 
 the acoustical materials and (2) changes 
 in the plant's operating procedures dur- 
 ing the course of the project. Table 17 
 describes the relative durability of many 
 of the treatments; but of course, the 
 listed equipment "lifetimes" would have 
 to be compared to those of standard (usu- 
 ally steel) components to assess their 
 true value. In addition, the size compo- 
 sition of the material handled by the 
 plant became much coarser as surface- 
 mined coal replaced underground coal^^ as 
 the primary plant feed. This increased 
 the wear rates of the acoustical materi- 
 als and increased the "inherent noisi- 
 ness" of the plant (because large parti- 
 cles produce larger impacts than small 
 particles). Finally, the addition of a 
 
 ^ ^ Coal produced by continuous miners is 
 more finely sized than that produced by 
 surface coal mining equipment. 
 
 coal froth-flotation circuit and thermal 
 dryers decreased the amount of mate- 
 rial handled by some of the original 
 equipment . 
 
 Table 16 shows the noise levels mea- 
 sured in the critical areas of the plant 
 before treatment, shortly after treat- 
 ment (when all treatments were relatively 
 undamaged) , and about 3 to 4 yr after 
 treatment. The noise increases after 3 
 to 4 yr were due to treatment wear, in- 
 creased throughput size, and/or other 
 factors (including material failures) de- 
 scribed in table 17. Changes in plant 
 operating procedures accounted for the 
 lack of long-term noise reduction data 
 (table 16) for some equipment types. 
 
 The long-term physical performance of 
 the noise-control treatments varied 
 greatly. The loaded-vinyl curtains gen- 
 erally lasted longer than the other 
 treatments simply because they were not 
 exposed to physical impacts; however, 
 some of the elastomeric impact pads and 
 chute liners were both quieter and longer 
 lasting than their steel counterparts. 
 The following sections summarize the 
 physical performance of the four major 
 types of treatments used at the George- 
 town plant: 
 
65 
 
 TABLE 17. - Noise-control treatments used at Georgetown coal preparation plant 
 
 Equipment and treatment 
 
 Comments 
 
 Rail car shaker: No treatment, 
 
 Classifying screens: Resilient impact pad 
 beneath infeed chute; rubber-clad steel 
 decks. 
 
 Centrifugal dryers: Loaded vinyl curtain en- 
 closures; Velcro fastener strips sewn on 
 curtain material. 
 
 Vibrating screens: Rubber liners in dis- 
 charge chutes; ure thane-clad steel decks. 
 
 Rock crusher: 2 rubber impact pads in infeed 
 chute (2-3/16 in thick); pads had ribbed 
 profile to achieve 90° impact angle. 
 
 Clean coal desanding screens: Plastic liners 
 in receiving hoppers and discharge chutes; 
 loaded-vinyl curtains separated screens from 
 main aisle. 
 
 Secondary sizing screens: All-elastomer and 
 rubber-clad decks; partial covers over open 
 discharge chutes; loaded-vinyl curtain 
 enclosures. 
 
 Baum jigs: Ribbed rubber impact pads at in- 
 feed chutes; flat urethane impact pads in 
 refuse chutes; plastic and ceramic liners in 
 refuse chutes; partial covers on open chute 
 tops; mufflers for air blowers. 
 
 Middling shaker screens: Loaded-vinyl cur- 
 tains around screening area (both clear and 
 opaque) . 
 
 Coal crushers : Loaded-vinyl curtains between 
 crushers and main aisle; Velcro fastener 
 strips glued to curtain material. 
 
 Flight conveyor: Loaded-vinyl curtains sep- 
 arated conveyor from main aisle; sewn-on 
 Velcro fasteners. 
 
 Picking table: Urethane impact pad placed 
 where material dropped from upper to lower 
 portion of table; plastic layer downstream 
 of impact pad. 
 
 Primary screens: Elastomer-clad steel decks; 
 all resilient decks (polymer topped by rub- 
 ber) ; impact pads between screen sections. 
 
 Shaker no longer in use; plant 
 switched from rail-carried to 
 trucked-in feed coal. Acoustic booth 
 for operator may be best treatment. 
 
 Impact pad successful for 2 yr. 
 Rubber-clad screens caused blinding. 
 
 Enclosures still in place 4-5 yr 
 after initial installation. 
 
 Rubber liners successful. Urethane- 
 steel bond separated after 7 months. 
 
 Pads completely successful , still in- 
 tact 3 yr after initial installation. 
 
 Hopper liner wore out due to impacts, 
 but discharge chute liner remained 
 intact. Curtains still intact 4 yr 
 after installation. 
 
 All-elastomer decks caused blinding. 
 Rubber-clad decks marginally success- 
 ful. Curtains very successful. 
 
 Both types of impact pads lasted 
 longer than steel. Plastic chute 
 liners wore out after 2-1/2 months, 
 but ceramic liners lasted 4 yr. No 
 problems with mufflers. 
 
 Clear curtains became obscured with 
 dirt, but reduced noise successfully. 
 
 Moderately successful; glued-on fast- 
 eners peeled off after 6 months, 
 Sewn-on fasteners recommended. 
 
 Curtains still intact 4 yr after ini- 
 tial installation. 
 
 Impact pad lasted 2 yr before replace- 
 ment. Plastic layer wore out after 
 6 months. 
 
 Elastomer-steel bond separated easily. 
 All-resilient decks remained intact 
 but caused blinding. Impact pads 
 lasted over 3 yr before wearing out. 
 
66 
 
 Resilient Screen Decks 
 
 The tests demonstrated that elastomer- 
 clad and all-elastomer screen decks could 
 reduce the noise produced by impacts of 
 material on the decks. However, their 
 acoustical advantage over all-steel decks 
 could not be measured directly because of 
 the noise generated by impacts at the 
 screen feed and discharge chutes, scrap- 
 ing of the screen bottom decks, and vi- 
 bration of the screen drive mechanisms. 
 
 Two operational problems also occurred 
 with the resilient screen decks — "blind- 
 ing" and delamination. Although "blind- 
 ing" (plugging of screen holes by near- 
 sized pieces of material) can occur on 
 any screen, depending on screen loading 
 and the amount of open area initially 
 present, the resilient decks seemed to be 
 more susceptible to this problem than 
 all-steel decks. Older, crank-arm-type 
 screens (e.g., primary screens) were par- 
 ticularly vulnerable. Delamination (sep- 
 aration of the elastomer and steel layers 
 of the elastomer-clad screens) sometimes 
 occurred before the elastomer top sur- 
 faces began to show significant wear due 
 to material impacts. However, several 
 types of elastomer-clad screens tested at 
 other preparation plants (under the same 
 overall program as the Georgetown tests) 
 did not cause blinding, showed no signs 
 of delamination, and lasted longer than 
 all-steel decks. Reasons for these con- 
 flicting results could not be identified 
 conclusively, 
 
 Resilent Impact Pads 
 
 In many cases, an elastomeric impact 
 pad at the inlet or discharge point of a 
 belt, screen, or chute was a very cost- 
 effective means of noise control. When 
 designed and installed properly, the 
 long service life of an elastomeric pad 
 more than compensated for its high ini- 
 tial cost. However, the material impact 
 angle and the elastomer thickness had to 
 be chosen carefully to achieve maximum 
 performance. Since a 90° impact angle 
 causes the minimum elastomer wear, impact 
 pads placed on inclined surfaces (not 
 perpendicular to material flow) were 
 
 ribbed to create a 90° impact angle. A 
 smooth impact pad was used where the ma- 
 terial flow was perpendicular or nearly 
 perpendicular to the pad surface. Thick- 
 er impact pads were successful in some 
 areas where thinner pads had failed. 
 
 Chute Liners 
 
 Resilient chute liners (rubber and ure- 
 thane) were more durable and more ef- 
 fective in reducing noise than rigid 
 (plastic and ceramic) liners due to the 
 resilient liners' ability to absorb mate- 
 rial impact forces. Furthermore, plastic 
 liners wore out quickly when exposed to 
 tumbling or impacting material flows; al- 
 though ceramic tiles were more durable, 
 they showed evidence of cracking over 
 time. Both the resilient and plastic 
 chute liners lasted longer when exposed 
 to smooth, sliding material flows. 
 
 In general, the chute liners were only 
 marginally effective in open chutes due 
 to the noise inherent in the material 
 flow. In these cases, partial chute cov- 
 ers were just as effective and some- 
 what cheaper than chute liners. However, 
 chute covers could only be installed 
 where frequent visual monitoring was not 
 essential and where surges of material 
 flow did not occur. 
 
 Loaded-Vinyl Curtains 
 
 The flexible curtains proved to be the 
 longest lasting and most effective noise- 
 control treatments in the plant. They 
 were effective in reducing noise produced 
 by large, complicated machines and could 
 be easily opened or removed when equip- 
 ment inspection and maintenance were 
 needed. The only conditions that would 
 preclude the use of these curtains in 
 other plants would be (1) extremely lim- 
 ited clearance around the noisy equipment 
 and/or (2) the need for constant, unob- 
 structed visual monitoring of the equip- 
 ment from a distance. The clear loaded- 
 vinyl curtains used at the Georgetown 
 plant were just as effective as opaque 
 curtains in reducing noise, but they soon 
 became obscured with dirt. 
 
67 
 
 Designing New Plants for 
 Noise Control (34) 
 
 The Bureau's Georgetown study showed 
 that noise levels in coal preparation 
 plants can be reduced through retrofit 
 treatments; however, it was obvious that 
 the noise exposures of many workers in 
 these types of plants could be lowered if 
 new plants were designed in accordance 
 with standard engineering noise-control 
 principles. For this reason, the Bureau 
 prepared a manual containing noise- 
 control guidelines for designers of new 
 coal preparation plants. The two major 
 subjects addressed in this manual were 
 
 (1) new plant layout and design and 
 
 (2) noise control of new plant equipment. 
 
 In the area of new plant layout and 
 design, three basic approaches were 
 suggested: (1) isolation of high-noise 
 areas, (2) isolation of operator loca- 
 tions and walkways, and (3) choosing in- 
 herently quieter preparation processes. 
 The most obvious example of the first ap- 
 proach is to design the plant such that 
 the noisiest equipment is located far 
 away from the highest concentration of 
 plant workers. Limiting the number of 
 openings (walkways, chutes, drains, etc.) 
 between relatively noisy floors and quiet 
 floors of the plant and sealing all non- 
 essential openings between these floors 
 are two other techniques that can be used 
 to isolate high-noise areas. The second 
 approach, isolation of operator loca- 
 tions and walkways, is similar to the 
 first except that the worker rather than 
 the equipment is the primary focus. A 
 good example of worker isolation would 
 be a completely enclosed gallery-type 
 walkway to allow visual observation of 
 process equipment (fig. 60). Remote con- 
 trol and computer monitoring of all plant 
 processes is probably the most logical 
 combined application of the first two ap- 
 proaches. The third approach, choosing 
 inherently quieter processes, is limited 
 to situations where two types of process 
 machinery perform the same basic function 
 (e.g. , "louder" vacuum disc filters ver- 
 sus "quieter" bowl-type centrifuges for 
 
 fines dewatering). Choosing one large, 
 easy-to-isolate process machine over sev- 
 eral smaller machines of the same type 
 is a combination of the first and third 
 approaches. 
 
 Noise control of new preparation plant 
 equipment can often involve the use of 
 the retrofit treatments installed in the 
 Georgetown plant — resilient screen decks, 
 impact pads , chute liners , and loaded- 
 vinyl curtains. In many cases, however, 
 these and other treatments are more cost- 
 effective when incorporated into the 
 equipment before it is installed in the 
 plant. The manual described above con- 
 tains detailed descriptions of 13 differ- 
 ent types of coal preparation equipment, 
 discusses the noise sources on each type, 
 and suggests potential noise-control al- 
 ternatives. Table 18 summarizes this 
 information. 
 
 Taconite Processing Plants 
 
 The Bureau's noise-control efforts in 
 metal processing plants were directed at 
 taconite (iron ore) operations. Since 
 taconite is harder than most other metal- 
 lic ores, it generates more noise when 
 crushed and requires more powerful and 
 more durable processing and handling 
 equipment. Noise-control treatments for 
 taconite processing plants could be ap- 
 plied to other processing plants with 
 similar equipment. 
 
 The approach for controlling noise in 
 taconite processing plants was about the 
 same as for coal processing plants — 
 the most serious noise sources within a 
 typical plant were identified, the noise- 
 generating mechanisms of each source 
 were analyzed, and potential noise-con- 
 trol treatments for each source were sug- 
 gested. The most important noise sources 
 identified by the Bureau were (1) second- 
 ary crushers, (2) grinding mills, and 
 (3) f ines-dewatering screens. Several 
 treatments were subsequently installed 
 and evaluated, and other practical noise- 
 control treatments were identified. 
 
68 
 
 TABLE 18. - Noise-control alternatives for new coal preparation equipment 
 
 Equipment type and 
 typical noise sources 
 
 Electric motors: Cooling fan, elec- 
 tromagnetic forces, mechanical noise 
 (bearing, bushings). 
 
 Air compressors: Compressed air 
 exhaust, internal piston or screw 
 impacts. 
 
 Enclosed gear drives: Gear meshing, 
 cooling fan. 
 
 Vacuum pumps: Pulsating air expan- 
 sion, external gear reducers. 
 
 Fans and blowers; 
 drive motors. 
 
 Air pulsations, 
 
 Centrifugal pumps: Water pulsations, 
 
 particle impacts within pump , drive 
 
 motors. 
 Vibrating screens: Product-to-screen 
 
 impacts, material flow above screen, 
 
 drive motors. 
 Chutes: Product-to-chute impacts, 
 
 material flow within chute. 
 
 Centrifugal dryers: Material flow 
 within centrifuge, motor and gear 
 noise. 
 
 Crushers: Product-to-crusher im- 
 pacts, material flow within crusher, 
 motor and gear noise. 
 
 Jigs and heavy-media vessels: Jig 
 blowers and exhaust, jig elevator 
 discharge, material and water flow 
 within vessel. 
 
 Cyclones: Material and water flow 
 within cyclone. 
 
 Feeders and conveyors: Material 
 flow and discharge, motor and gear 
 noise. 
 
 Alternatives 
 
 Use low-horsepower, low-speed motors whenever 
 possible. "Motor mute" fan mufflers, uni- 
 directional cooling fans. Sound-absorbing 
 motor enclosures. 
 
 Sound absorbing enclosures (standard on newer 
 models) . 
 
 "Quieter" gear designs (helical type, tighter 
 tolerances, low ratios). Cooling-fan noise 
 controls; see "electric motors." Complete 
 gearbox acoustical enclosures. 
 
 Use liquid-ring pumps (instead of lobe-type 
 displacement pumps). Wrap mufflers and 
 silencers with pipe lagging. Low-speed 
 impellers and gearbox noise controls. 
 
 Use centrifugal (rather than positive- 
 displacement) blowers. Intake and exhaust 
 mufflers. Motor noise controls. 
 
 Use V-belt (rather than gear-drive) motors. 
 Avoid concentrating pumps in one location. 
 Motor and gear noise controls. 
 
 Resilient screen decks. Barrier-type enclo- 
 sures around screen area (loaded-vinyl cur- 
 tains). Motor and gear noise controls. 
 
 Reduce product impact velocity (by reducing 
 drop height). Resilient chute liners (in- 
 ternal. External chute-damping treatments 
 or barrier-type enclosures. 
 
 Acoustic seals for centrifuge shell. Exte- 
 rior lagging or damping treatments. Motor 
 and gear noise controls. 
 
 Locate crushers away from personnel. Barrier- 
 type enclosures (loaded-vinyl curtains). 
 Vibration-isolated crusher mounts. 
 
 Mufflers for jig blowers and exhaust. Chute 
 noise control (e,g. , impact pads). 
 
 Cover (with inspection doors) over cyclone 
 
 sump. 
 Cover open-topped feeders and conveyors. 
 
 Chute noise controls (impact pads). Motor 
 
 and gear noise controls. 
 
69 
 
 
 FIGURE 60. - Schematic of enclosed gallery-type walkway in preparation plant. 
 
 ^ 
 
 Secondary Crusher Enclosure (23) 
 
 Figure 61 shows that the secondary 
 crusher occupied several vertical levels 
 in the processing plant. For this rea- 
 son, two separate enclosures were con- 
 structed — one for the lower (drive) lev- 
 el, and one for the upper (adjustment) 
 level. The walls of both enclosures were 
 made of sheet-metal panels lined with 
 sound-absorbing material; the metal kept 
 noise from radiating outward, and the 
 sound-absorbing material prevented re- 
 verberant sound buildup within the enclo- 
 sure. The drive-level enclosure had 
 
 several doors and access panels to facil- 
 itate crusher maintenance. The panels 
 of the adjustment-level enclosure were 
 mounted on an overhead sliding track sys- 
 tem because workers frequently needed ac- 
 cess to the crusher at the adjustment 
 level. Ceiling panels were also used on 
 the adjustment-level enclosure. 
 
 Because of the close proximity of other 
 noise sources in the crusher area (terti- 
 ary crushers , screens , crusher feed sys- 
 tem, etc.), the total noise reduction 
 provided by the secondary crusher enclo- 
 sures was difficult to assess. However, 
 
 \ 
 s 
 
 'i 
 
70 
 
 FIGURE 61. ' Secondary crusher urea in taconite processing plant. 
 
 noise levels measured at 1 ft outside the 
 enclosures during normal operation were 
 as much as 6 to 8.5 dBA lower than when 
 the enclosures were absent. These reduc- 
 tions were recorded on the side of the 
 enclosure opposite the side facing the 
 "open," noisy area of the plant (i.e., 
 the enclosures also served as a barrier 
 against other noise sources). Noise re- 
 ductions on the "open" sides of the en- 
 closures were 1 to 3 dBA, despite the re- 
 flection of tertiary crusher and screen 
 noise from the exterior enclosure walls. 
 Even though the overall noise level at 1 
 ft outside the enclosure was still 97 to 
 101,5 dBA, the enclosures were very ef- 
 fective in reducing the noise contribu- 
 tion of the secondary crusher. Other 
 
 noise sources would have to be treated to 
 achieve further noise reduction in the 
 secondary crusher area. 
 
 Noise levels within the adjustment- 
 level enclosure were about the same as 
 those measured before it was installed 
 (85 dBA while the crusher was idling, 100 
 to 101 dBA while it was crushing ore). 
 However, these levels would have been 
 much higher if the interior enclosure 
 walls had not contained sound-absorbing 
 materials. The noise level within the 
 enclosure would have been about 95 dBA if 
 the secondary crusher were shut off com- 
 pletely (with other noise sources still 
 operating) , or about 5 dBA lower than the 
 noise level with the enclosure absent. 
 
71 
 
 The adjustment-level enclosure could thus 
 protect workers from external noise dur- 
 ing crusher maintenance operations. 
 
 Grinding Mill Treatments (J_0, 29 ) 
 
 Rod mills, ball mills, and autoge- 
 nous mills are large rotating cylindrical 
 structures (figure 9, part 2) used to 
 grind taconite ore. As the mill rotates 
 (fig. 62) , the ore particles and/or the 
 metal grinding media (balls or rods) rise 
 along one side of the cylinder and fall 
 back to the bottom of the other side. 
 Impacts between the ore, grinding media, 
 and cylinder linings cause the lower por- 
 tion of the falling side of the mill to 
 radiate about three times as much noise 
 as the top and rising sides. For this 
 reason, an 8-ft-high acoustical barrier 
 (fig, 62) covering only the lower, fall- 
 ing side of a semiautogenous mill was de- 
 signed, installed, and evaluated. 
 
 Falling side 
 
 Ore particles 
 
 and 
 
 grinding media 
 
 Rising side 
 
 Mill shell 
 
 4-in-thicl< fiberglass 
 supported by studs 
 
 /4-in plywood with 
 %-lb/ft^ loaded vinyl 
 
 Outline of 
 mill shell 
 
 Existing steel grating 
 
 FIGURE 62. - Taconite grinding mill (top) and 
 noise barrier (bottom) on falling side of mill. 
 
 At 15 ft away from the falling side of 
 the mill, noise levels were about 6 dBA 
 lower than before the barrier was in- 
 stalled; in the "shadow zone" close to 
 the barrier, noise was reduced by about 
 10 dBA, Noise reductions behind the bar- 
 rier at the feed end of the mill were 
 only 2 to 6 dBA because of noise produced 
 by the feed chute. After installation of 
 the barrier, overall noise levels at most 
 locations on the falling side of the 
 mill were less than 95 dBA and were lower 
 than noise levels on the rising side. 
 However, since most of the noise on the 
 rising side was actually "falling-side 
 noise" reflected upward from the mill 
 floor through the grating shown in fig- 
 ure 62, a loaded-vinyl mat covering the 
 rising-side grating would have reduced 
 this noise significantly. 
 
 Another practical grinding mill noise- 
 control treatment, rubber shell liners, 
 has been used successfully by several 
 European and North American companies. 
 Although the main reason for their use 
 is for longer wear, reduced noise is an 
 added benefit. Scandanavian processing 
 plants have reported noise reductions of 
 about 6 dBA due to rubber mill liners, 
 and indirect measurements in North Ameri- 
 can plants indicated a 3- to 4-dBA reduc- 
 tion. Direct noise-reduction measure- 
 ments could not be obtained in North 
 American plants because (1) noise levels 
 of standard metal-lined mills were not 
 available for comparison and (2) other 
 noise sources in the vicinity of rubber- 
 lined mills were often dominant. 
 
 Rubber shell liners have been used only 
 in autogenous and semiautogenous mills 
 because metal grinding media would abrade 
 the rubber too quickly. In addition, 
 they have been effective only in "wet" 
 mills, where harmful temperature in- 
 creases (due to warming of the liners) 
 can be avoided. However, a rubber layer 
 constrained between the standard metal 
 liner plates and the outer shell could 
 significantly reduce the noise produced 
 by ball and rod mills without the adverse 
 effects of abrasion and temperature in- 
 creases. Rubber washers for the liner 
 
72 
 
 mounting bolts on the exterior shell sur- 
 face would provide additional vibration 
 isolation. Although these two noise- 
 control treatments have not yet been 
 tested, they would be relatively easy to 
 design and install and would not inter- 
 fere greatly with mill maintenance. 
 
 Fines-Dewatering Screens 
 
 A typical f ines-dewatering screen cir- 
 cuit consists of a parallel arrangement 
 of boxes in which the concave screen sur- 
 faces are contained. To prevent blind- 
 ing of the narrow (0.004-in) screen open- 
 ings, a pneumatic rapping device strikes 
 the screenbox at 2- to 10-s intervals. 
 Most of the screening noise is generated 
 by the rapping action and the flow of 
 material across the screen. Substantial 
 noise reductions of 11 to 13.5 dBA were 
 achieved simply by installing sliding 
 covers over the screen box, shrouding the 
 rapper arm, and isolating the rapper head 
 from its mounting bar at the back of the 
 screenbox (fig. 63). Resilient screen 
 decks such as those used in coal prepara- 
 tion plants would also help reduce noise 
 produced by f ines-dewatering screens, 
 
 Nonmetallic Processing Plants (30) 
 
 The nonmetallic mining industry is 
 unique in that both stationary and port- 
 able processing plants are used. The 
 
 stationary plants contain the same types 
 of noise-producing equipment as coal 
 and metal processing plants — crushers, 
 screens, chutes, etc. Small portable 
 processing plants are common in the sand, 
 gravel, and crushed stone industries. 
 These plants consist of a single compo- 
 nent or unit operation, such as a crusher 
 and screen, mounted on a chassis that can 
 be moved around the mining site or towed 
 on public highways. Typical operator 
 noise levels in nonmetallic processing 
 plants are 95 to 110 dBA, with exposure 
 times as high as 6 to 7 h. 
 
 As with the other types of processing 
 plants, a three-step approach to noise 
 control was taken; it included (1) noise 
 source survey and characterization, 
 
 (2) design of retrofit treatments, and 
 
 (3) field installation and evaluation of 
 treatments. After a survey of eight typ- 
 ical plants showed that the crushing and 
 screening process was the primary cause 
 of overexposure, retrofit treatments were 
 installed in three different plants. Ta- 
 ble 19 describes these treatments and 
 summarizes their effectiveness. 
 
 As shown in table 19, the treatments 
 were very effective in two of the three 
 processing plants. In the first plant 
 (which utilized a primary jaw crusher) , 
 an independently mounted air-conditioned 
 operator's booth replaced an ineffective 
 
 Bolt 
 Modified grommet 
 
 Counterweight 
 arm 
 
 Modified grommet 
 
 Washer 
 Strikeplate cover 
 
 Rubber isolator 
 Washer 
 
 FIGURE 63. - Noise-control treatments for rapper on taconite fines screen. 
 
73 
 
 TABLE 19. - Noise-control treatments installed in three nonmetallic 
 processing plants 
 
 Treatment 
 
 Result 
 
 Cost 
 
 Materials , 
 1981 dollars 
 
 Labor, 
 h 
 
 PRIMARY JAW CRUSHER 
 
 Air-conditioned control booth on 
 steel structure next to crusher, 
 with door to catwalk to permit 
 operator inspection of crusher. 
 
 80-dBA noise level in booth 
 (compared to 97 dBA on 
 catwalk) . 
 
 5,000 
 
 40 
 
 INCLINED SCREEN AND SECONDARY CONE CRUSHER 
 
 Resilient screen components (feed- 
 box liner, top deck, wing liners, 
 discharge lip), resilient crusher 
 components (feed hopper liner, 
 feed plate, feed cone liners), 
 and flexible curtain around 
 crusher. 
 
 3- and 7-dBA overall noise 
 reduction. Treatments 
 showed only minor wear 
 problems over 7 months 
 (198,000 tons of crushed 
 stone, sand, and gravel). 
 
 66 
 
 HORIZONTAL SCREEN AND SECONDARY CONE CRUSHER 
 
 Resilient screen (impact pads on 
 feed chute, feedbox liner, top 
 and bottom decks, wing liners, 
 discharge lip) and resilient 
 crusher feed hopper liner. 
 
 No significant noise 
 reduction. ^ 
 
 17,000 
 
 69 
 
 'Treatment ineffectiveness was directly traceable to operator's change of equipment 
 models after completion of original treatment designs. 
 
 crusher-mounted booth that was not air- 
 conditioned. The only treatments showing 
 significant wear in the second plant (in- 
 clined screen and secondary cone crusher) 
 were the resilient crusher feed cone 
 liners and feed plate; however, the lat- 
 ter failure was corrected when the origi- 
 nal, undersized feed plate was replaced 
 with a properly sized plate. Improperly 
 sized components were also responsible 
 for the ineffectiveness of the noise- 
 control treatments at the third plant 
 (horizontal screen and secondary cone 
 crusher). Since the plant operator had 
 changed both the models of the equipment 
 used and the nature of the crusher feed 
 after the original retrofit treatments 
 were designed, extensive field modifica- 
 tions and remanuf acturing were necessary. 
 For example, the resilient impact pad on 
 the screen feedbox failed before the dis- 
 charge lip could be installed. 
 
 Despite the failure of the retrofit 
 treatments at the third plant, the Bu- 
 reau's program showed that retrofit 
 
 chase price of a 
 pacity of 200 to 
 
 noise-control treatments could be applied 
 successfully to portable nonmetallic 
 crushing and screening plants. Treatment 
 costs were only 5 to 7 pet of the pur- 
 new plant (with a ca- 
 300 tons/h) ; this is 
 roughly equal to the annual repair and 
 replacement costs of a reasonably well- 
 maintained plant. Since crusher and 
 screen performance was not adversely af- 
 fected by the retrofit treatments, they 
 appeared to be a cost-effective means of 
 noise control. 
 
 Additional Research on 
 Screen-Noise Abatement 
 
 Resilient, nonmetallic screen decks 
 have been utilized extensively as a means 
 to reduce noise generated by impacts of 
 material (coal, rock, etc.) on the screen 
 surface. Two other promising screen 
 noise-abatement techniques are isolation 
 of the screen drive mechanism and damping 
 of the screen sidewalls. The Bureau and 
 a major screen manufacturer conducted a 
 
74 
 
 joint research effort to detennine the 
 effects of these three treatments on 
 screen performance, durability, and noise 
 reduction (20) . This effort involved the 
 testing of six different commercially 
 available nonmetallic screen decks using 
 three different products — coal, granite 
 and dolomite. The results of this re- 
 search corroborated the results of previ- 
 ous studies. It was found that (1) coal 
 screening is a quieter process than gran- 
 ite or dolomite screening because coal is 
 a softer material; (2) vibration isola- 
 tion and sidewall damping are durable, 
 cost-effective methods of controlling the 
 portion of screening noise that is unre- 
 lated to material impacts; and (3) resil- 
 ient screen decks can be about 2 to 7 dBA 
 quieter (and more durable) than all-steel 
 decks, but can cause up to a 10-pct re- 
 duction in screening efficiency (i.e. , 10 
 pet more blinding) . 
 
 In order to investigate the blinding 
 problem more closely, a digital computer 
 model was developed to evaluate screening 
 efficiency under a wide variety of oper- 
 ating conditions (21). A unique feature 
 of this model was its use of explicit 
 fundamental relationships rather than 
 empirical quantities (percent oversize 
 and undersize) to predict screening effi- 
 ciency. The phenomena that affect the 
 approach of a single particle toward the 
 screen deck (particle size, hole size, 
 bed depth, etc.) were investigated in or- 
 der to derive a mathematical expression 
 for the probability that it would pass 
 through. "Probability-of -passage" equa- 
 tions simulated the percentage of mate- 
 rial passing through the screen on a 
 given approach, and the niomber of ap- 
 proaches on a given screen was used to 
 determine screening efficiency. Figure 
 64 is the generalized flow chart of the 
 screening-simulation algorithm. 
 
 Read in physical 
 operating data 
 
 Calculate: 
 Probabilities of passage (P) 
 Number of opportunities (N) 
 
 T 
 
 Start simulation 
 (1= I) 
 
 -l 
 
 Calculate bed depth(BD) 
 and active depth (AD) 
 
 Feed X If X P 
 
 Materiel removed 
 
 Remaining material 
 for next opportunity 
 
 1=1+1 
 
 I < N 
 
 Overall performance 
 (output) 
 
 FIGURE 64. 
 algorithm. 
 
 Flow chart of screening simulation 
 
 the computer-predicted values were accu- 
 rate to within ±3 pet, although discrep- 
 ancies were greater for screens with non- 
 standard shaking strokes and frequencies. 
 To allow interested parties to make quick 
 estimates of screening efficiency without 
 running the entire program, a handbook 
 consisting of the basic program elements 
 was developed. Users of this handbook 
 can make quick comparisons between the 
 efficiencies attainable with standard 
 versus nonmetallic screen decks for the 
 same type of material. 
 
 Input to the computer program consisted 
 of detailed information on the screen 
 and the material being processed. Output 
 consisted of the predicted screening ef- 
 ficiency and the size distribution of the 
 overproduct and underproduct. Laboratory 
 tests with actual screens verified that 
 
 USE OF HEARING PROTECTORS 
 IN THE MINING ENVIRONMENT 
 
 As explained in part 1 , the Bureau 
 views hearing protectors (earplugs, ear- 
 muffs, etc.) as only a partial, tempo- 
 rary solution to mining noise problems. 
 
75 
 
 Historically, it has been more effective 
 to control industrial health problems at 
 their sources than to require workers to 
 use personal protective devices. For ex- 
 ample, early attempts to reduce coal 
 miners' exposure to dust through the use 
 of respirators did not prevent coal work- 
 ers' pneumoconiosis. Although large dust 
 particles were trapped by the respirator, 
 respirable dust particles (those smaller 
 than 5 ym) passed through and around the 
 respirator and were inhaled by workers. 
 The same is true of hearing protective 
 devices; unless they achieve a perfect 
 seal around the ear (earmuffs) or ear 
 canal (earplugs), noise will pass through 
 the openings and render them ineffective. 
 
 However, Federal regulations do provide 
 for the use of hearing protectors by 
 requiring mine operators to make them 
 available to miners in situations where 
 engineering or administrative noise con- 
 trols are not available, have not yet 
 been developed, or fail to reduce noise 
 to within levels of compliance. There- 
 fore, the Bureau has conducted research 
 to investigate the noise-reducing charac- 
 teristics of personal hearing protectors 
 and assess their potential usefulness in 
 the mining environment. 
 
 Limitations of Hearing 
 Protector Effectiveness 
 
 No hearing protective device can elimi- 
 nate all sound transmission to the ear. 
 Figure 65 shows the four most common 
 noise pathways: (1) air leaks around the 
 protector, (2) transmission of external 
 sound through the protector material, 
 (3) vibration of the protector in re- 
 sponse to external noise, and (4) bone 
 and tissue conduction — sound transmitted 
 directly through the skull. Even if a 
 "perfect" hearing protector could be de- 
 veloped to eliminate the first three 
 pathways, bone and tissue conduction 
 would still be present. Although the 
 amount of bone and tissue conduction 
 varies among individuals and different 
 sound frequencies, the maximum noise re- 
 duction attainable with a "perfect" hear- 
 ing protector would be about 50 dB (18) . 
 Even under well-controlled laboratory 
 conditions, the maximum noise reduc- 
 tion achieved through the use of present- 
 ly available hearing protectors has 
 been about 35 dB. Furthermore, these 
 maximum reductions would occur only in 
 the frequency range of 2,000 to 4,000 
 Hz, In real-world situations (with non- 
 perfect protector fit, etc.), actual 
 
 
 Bone 
 and tissue 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Protector 
 vibration 
 
 
 ' 
 
 r 
 
 
 ' 
 
 ' 
 
 
 ' 
 
 ' 
 
 Noise 
 
 
 — ^ 
 
 Outer 
 ear 
 
 
 Middle 
 ear 
 
 
 Inner 
 ear 
 
 
 
 Material 
 leaks 
 
 — »- 
 
 
 
 
 1 
 
 1 
 
 
 
 
 
 
 
 
 
 
 Air leaks 
 
 
 
 
 
 
 
 FIGURE 65. - Noise pathways to ear protected by hearing protective device. 
 
76 
 
 noise reductions were found to be 20 dB 
 or less ( 4_) , again depending on the fre- 
 quency examined. 
 
 Assessing Earmuff Attenuation 
 
 Two basic techniques have been wide- 
 ly accepted for measuring the noise- 
 reducing properties of hearing protective 
 devices. The "real-ear" method (American 
 National Standards Institute (ANSI) stan- 
 dard S3. 19-1974) measures the hearing 
 threshold of a human subject both with 
 and without hearing protection. The dif- 
 ference in the hearing threshold of the 
 protected and unprotected subject indi- 
 cates the effectiveness of the hearing 
 protector. Although the real-ear and 
 other "psychophysical" methods of this 
 type are used most often, several nonsub- 
 j active, "physical" methods are also 
 used. Physical methods often involve two 
 microphones, one outside the hearing pro- 
 tector and one between the protector and 
 the subject's eardrum. ^2 The difference 
 in sound level at the two microphone lo- 
 cations is then measured. In both the 
 psychophysical and physical approaches, 
 the noise-reducing properties of the 
 hearing protectors are measured over the 
 entire audible frequency range. Af- 
 ter applying corrections for A-weighting 
 and the frequency spectrum of the inci- 
 dent noise, a noise-reduction rating 
 (NRR) is assigned to the hearing protec- 
 tive device. 
 
 The psychophysical (real-ear) method 
 is undoubtedly more accurate than the 
 physical (microphone) method of measuring 
 hearing protector effectiveness. Howev- 
 er, the psychophysical method requires 
 the use of a laboratory and is very time 
 consuming, so the physical method is pre- 
 ferable for field evaluation of hearing 
 protectors. In order to determine the 
 exact differences between the two meth- 
 ods, the Bureau conducted a series of 
 controlled laboratory experiments on ear- 
 muffs commonly worn by miners (36). 
 
 "■^ANSI standard S3. 19-1 974 specifies a 
 test procedure in which a dummy head with 
 a simulated human skin is the "subject." 
 
 First, real-ear attenuations were mea- 
 sured for 5 different types of miners' 
 earmuff s, worn by 12 different subjects 
 with normal hearing "Unprotected" mea- 
 surements (without muffs) were also re- 
 corded. Microphones were then placed in- 
 side and outside the muffs, and the same 
 12 subjects were tested. 
 
 At sound frequencies below 2,000 Hz, 
 the measured earmuff attenuations were 
 approximately the same in both the physi- 
 cal (microphones) and psychophysical 
 (real-ear) tests. Above 2,000 Hz, how- 
 ever, greater attenuations were measured 
 in the psychophysical tests than in the 
 physical tests (3- to 7-dB difference). 
 These results were consistent among all 
 12 subjects and for all 5 types of ear- 
 muffs. The differences probably resulted 
 from the inherent differences in the two 
 test procedures. In the "unprotected" 
 portion of the psychophysical tests, the 
 high-frequency external sounds caused a 
 standing-wave resonance phenomenon to oc- 
 cur within the subjects' ear canals, thus 
 increasing the per-ceived sound levels. ^^ 
 In the "protected" portion of the psycho- 
 physical tests, both the exterior noise 
 and the standing-wave phenomenon in the 
 ear canal were drastically reduced. In 
 contrast, the standing-wave phenomenon 
 did not occur in the "unprotected" por- 
 tion of the physical test because the un- 
 protected microphone was completely out- 
 side the muff. The attenuation measured 
 at the protected microphone, therefore, 
 included only the reduction of exterior 
 noise. 
 
 The existence of the standing-wave res- 
 onance phenomenon in the ear canal points 
 out an important limitation of the physi- 
 cal method of measuring earmuff effec- 
 tiveness; in fact, this limitation is 
 common to almost all conventional methods 
 of sound measurement: Since the pro- 
 tected microphones could not be placed 
 
 ^ -^The standing-wave resonance phenome- 
 non was prominent only at high frequen- 
 cies because these sounds had wavelengths 
 that were similar to the lengths of the 
 subjects' ear canals. 
 
77 
 
 Inside the subjects' ear canals, they 
 did not record the sound produced by 
 the standing waves. Therefore, the two- 
 microphone earmuff system actually under- 
 estimated the earrauffs' ability to atten- 
 uate high-frequency noise. 
 
 In order to compensate for the differ- 
 ences between the psychophysical and 
 physical methods of measuring earmuff 
 attenuation, the Bureau designed a cor- 
 rection filter for the protected micro- 
 phone and used a microphone with linear 
 response outside the muff. When tested 
 in the laboratory, this system produced 
 results that were similar to the results 
 of the psychophysical tests (37). How- 
 ever, the system would not reliably mea- 
 sure actual earmuff attenuation in the 
 field because the protected microphone 
 and its cable would undoubtedly move 
 around and contact the subject's body, 
 thereby producing a misleadingly high 
 sound level under the muff. Exterior 
 sound levels would have to be at least 
 100 dB at all audible frequencies to 
 override this under-the-muf f noise. In 
 most field situations, exterior sound 
 levels would not exceed 100 dB at higher 
 frequencies; therefore, applications of 
 the "corrected" physical technique would 
 be extremely limited. 
 
 The Bureau is still engaged in research 
 to devise a reliable physical method for 
 measuring the effectiveness of miners' 
 hearing protectors as worn in the field. 
 Even if the two-microphone muff system 
 described above could be perfected, the 
 same technique would be extremely diffi- 
 cult to apply to earplugs. The Bureau 
 plans to attempt to improve the earmuff 
 system and devise a physical measuring 
 system applicable to earplugs and other 
 types of hearing protectors. Until such 
 systems are developed, the true benefits 
 of hearing protectors in the mining envi- 
 ronment cannot be determined. 
 
 Hearing Protector Interference 
 With Required Acoustical Cues 
 
 One of the miners' greatest concerns 
 about hearing protectors is the potential 
 elimination of sounds they need to hear, 
 such as verbal communications and "roof 
 talk" — the noise preceding an impending 
 roof fall. For this reason, the Bureau 
 conducted research to determine the 
 amount of interference caused by hearing 
 protectors. One study (35) showed that 
 when the ambient noise level in the mine 
 was less than 90 dBA, hearing protectors 
 did indeed inhibit the miners' ability to 
 discriminate speech and roof talk from 
 other noises. However, when the ambient 
 noise level in the mine was greater than 
 90 dBA, discrimination of speech and roof 
 talk was the same or slightly better when 
 hearing protectors were used. The basic 
 conclusion of this study was that hearing 
 protectors would benefit miners only when 
 overall noise levels exceed 90 dBA. 
 
 In an effort to provide a hearing pro- 
 tector that would work only when noise 
 exceeded 90 dBA, the Bureau developed a 
 "discriminating earmuff" (12). An elec- 
 trical system incorporated into the muff 
 allowed all sounds lower than 83 dBA to 
 pass unattenuated to the ear. Sounds 
 louder than 83 dBA were progressively at- 
 tenuated up to a maximum attenuation of 
 30 dBA (90 dBA under the muff at 120 dBA 
 exterior noise). In underground tests, 
 subjects wearing the discriminating ear- 
 muff were able to hear low-level sounds 
 such as speech and roof talk more easily 
 than subjects wearing standard muffs. 
 The discriminating muffs and standard 
 muffs provided equal attenuation in high- 
 noise environments. The only disadvan- 
 tage of the discriminating muffs was that 
 their electronics made them rather expen- 
 sive and too fragile for in-mine use. 
 
 PART 4. —FUTURE BUREAU NOISE-CONTROL EFFORTS 
 
 Parts 2 and 3 documented the most sig- 
 nificant and beneficial results of past 
 Bureau noise-control research programs 
 and illustrated their broad scope. Most 
 
 of these programs were directed toward 
 solving the immediate noise problems of 
 the mining industry and involved field 
 demonstrations of retrofit techniques 
 
78 
 
 whenever possible. When redesign of 
 equipment appeared to be a more viable 
 long-term solution, the end product 
 sought by the Bureau was usually a field 
 demonstration of a prototype "quiet" ma- 
 chine or process. 
 
 Recently, however, the scope and nature 
 of Bureau research in all areas, includ- 
 ing noise control, has changed dramati- 
 cally. Demonstration projects such as 
 those described in part 3 have been de- 
 emphasized in favor of long-term in-house 
 programs in basic noise studies and ap- 
 plied noise-control research. The rea- 
 sons for this change in emphasis were 
 both philosophical and monetary. Limited 
 resources have made it necessary for the 
 Bureau to adopt the point of view that 
 since a successful field demonstration of 
 an inherently quieter piece of equipment 
 requires substantial assistance from its 
 manufacturer and user, these parties, 
 rather than the Government , should take 
 the leading role. Furthermore, inherent 
 design differences between different 
 models of the same type of mining equip- 
 ment make it nearly impossible for the 
 Bureau to design a "generic" noise-con- 
 trol treatment suitable for any single 
 equipment type. Although past Bureau 
 noise-control projects have successfully 
 addressed specific models, the cost of 
 developing numerous model-specific treat- 
 ments is prohibitive. Unfortunately, 
 this is exactly the type of effort needed 
 to achieve widespread, immediate solu- 
 tions to the many noise problems in min- 
 ing, but neither the Bureau nor private 
 industry can afford this effort at the 
 present time. Therefore, the Bureau is 
 now pursuing long-term strategies that 
 will use its limited resources more 
 effectively. 
 
 Until recently, the desire to achieve 
 immediate noise reductions in many mining 
 areas forced the Bureau to rely heavily 
 on contracting firms, mostly noise spe- 
 cialists and equipment manufacturers. 
 This approach helped introduce noise- 
 control technology to the mining industry 
 but did not allow Bureau personnel to 
 pursue basic, long-term goals in noise 
 
 control. However, the Bureau has now 
 begun to concentrate on acquiring the 
 capability to perform most of its noise- 
 control research in-house. Research ef- 
 forts will consist of long-term studies 
 of mining noise problems that cannot be 
 solved by the industry itself and will 
 focus only on those problems that require 
 in-depth scientific study. 
 
 FACILITIES AND EQUIPMENT 
 
 High-quality facilities and equipment 
 are essential in any noise-research pro- 
 gram, and the Bureau is now establishing 
 these at its Pittsburgh Research Center. 
 The centerpiece of the in-house facility 
 is a large reverberation building^ ^ capa- 
 ble of housing a piece of mining equip- 
 ment as large as a continuous miner, 
 jumbo drill rig, or bulldozer. The phys- 
 ical dimensions and interior surfaces of 
 this building were designed specifical- 
 ly to allow precise measurements of the 
 sound power levels of noise sources lo- 
 cated within. A small anechoic chamber, 
 a room whose walls, floors, and ceiling 
 reflect almost no noise, has been built 
 outside the larger reverberation room 
 for experiments with personal hearing 
 protectors. All laboratory instrumenta- 
 tion needed to conduct detailed acousti- 
 cal experiments in these rooms (sound 
 level meters, tape recorders, micro- 
 phones , accelerometers , frequency ana- 
 lyzers, etc.) have been obtained. 
 
 In order to allow mining machines to 
 operate "normally" (i.e., with all noise- 
 generating mechanisms present) within the 
 reverberation building, it will contain 
 several different cutting media. Noise 
 produced by coal-cutting machines will 
 be investigated by cutting into a synthe- 
 tic coal seam similar to the one shown 
 in figure 13 (part 3). Noise produced 
 by rock drills will be investigated 
 by drilling into replaceable blocks of 
 extra-hard concrete. Specific tests to 
 be conducted in the reverberation build- 
 ing are summarized below. 
 
 ^ '^Scheduled for completion by the end 
 of 1984. 
 
79 
 
 RESEARCH PROGRAMS 
 
 Because of the relatively recent change 
 in emphasis from contract to in-house re- 
 search, the programs described below are 
 still in their early stages. The four 
 major areas of emphasis — coal cutting, 
 conveying, percussion drilling, and hear- 
 ing protectors — were chosen because con- 
 tract research efforts have shown that 
 long-term studies are still needed in 
 these areas. 
 
 Coal Cutting 
 
 Although machine components for quieter 
 coal cutting and conveying were designed 
 and developed under Bureau contracts 
 (part 3) , only the sand-filled auger- 
 miner cutting head received extensive 
 testing under operating conditions in 
 underground coal mines. Quieter cutting 
 heads for drum-type continuous miners and 
 longwall shearers are now being tested 
 underground under two remaining Bureau 
 contracts. Future work in this area will 
 include basic studies into the effects 
 of bit lacing and geometry on noise 
 and the investigation of alternative, 
 quieter cutting technologies (e.g. , water 
 jetting). 
 
 Conveying 
 
 The "quiet" chain conveyor components 
 that performed well in aboveground tests 
 could not be tested extensively under- 
 ground due to problems in locating coop- 
 erating mines, manufacturers' reluctance 
 to incorporate the unproven components in 
 their product lines, and the short-term 
 nature of the contract under which the 
 components were developed. The long-term 
 durability and acoustical performance of 
 these "quiet" components are now being 
 tested in a closed-loop conveying setup. 
 In the future, the Bureau plans to con- 
 duct detailed, quantitative studies of 
 the impacts that generate conveyor noise. 
 It is believed that a better understand- 
 ing of these phenomena is needed to 
 achieve further conveyor noise reduction. 
 
 Percussion Drilling 
 
 Future Bureau work on percussion drill 
 noise will consist of basic studies to 
 gain a better understanding of the per- 
 cussion drilling process and its rela- 
 tionship to noise. Work is planned in 
 the following areas: 
 
 Drill Sound-Power Studies 
 
 Sound-power measurements of drills and 
 their noise-producing components should 
 provide a simple, accurate indication of 
 how drill operating parameters (hammer 
 pressure, feed thrust, etc.) influence 
 the overall noise level. Relationships 
 between sound power and drill size, power 
 source (pneumatic versus hydraulic) , and 
 internal design (valveless versus valved, 
 etc.) will be explored. Noise diagnostic 
 work is expected to be made much easier 
 through these studies. 
 
 Drill Energetics 
 
 Laboratory experiments will be con- 
 ducted to determine the relationships 
 between noise and blow energy, blow 
 frequency, off-centered impacts, bit 
 sharpness, etc. 
 
 Drill-Body Noise 
 
 Preliminary studies of hydraulic drills 
 indicate that drill-body noise is equal 
 to or greater than drill-steel noise. 
 The effects of drill-body design and ma- 
 terials on drill-body noise levels will 
 be investigated. 
 
 Small-Diameter In-the-Hole Drill 
 
 Placing the percussive tool inside the 
 drill hose will obviously reduce the 
 amount of noise reaching the operator. 
 Work is now underway to explore the 
 feasibility of extending down-the-hole 
 drilling technology to smaller size tools 
 (2 in diam or less). 
 
80 
 
 Alternative Drilling Technologies 
 
 TECHNOLOGY TRANSFER 
 
 At some point in the future, the Bureau 
 plans to investigate new drilling tech- 
 nologies that would perform the same 
 function as percussion drills (i.e., 
 drilling small holes in hard rock) in an 
 inherently quieter manner. Candidate 
 technologies include rotary drilling and 
 water jets. 
 
 Perhaps the most promising drill-steel 
 noise-control concept now being investi- 
 gated by the Bureau is the "concentric 
 drill steel." The two-piece concentric 
 steel consists of (1) an inner pulse- 
 transmission rod to deliver the blow to 
 the bit and (2) an outer torque tube to 
 rotate the bit and attenuate the noise 
 produced by the inner pulse-transmission 
 rod. The outer tube is acoustically iso- 
 lated from the inner rod by elastomer in- 
 serts, and hole-flushing water passes 
 through an annulus between the two sec- 
 tions. Although the prototype concentric 
 steel is being developed under contract 
 (39) , much of the testing and evaluation 
 will be done in-house, using the pneumat- 
 ic and hydraulic jumbo drills. 
 
 Hearing Protectors 
 
 The goal of the Bureau's hearing pro- 
 tector research program is to develop a 
 reliable, reproducible method of evaluat- 
 ing the effectiveness of hearing protec- 
 tors worn by miners in the field. The 
 previous Bureau efforts discussed in part 
 3 showed that much work remains to be 
 done in this area. Future Bureau re- 
 search will attempt to define (1) the 
 theoretical minimum noise level attain- 
 able through the use of hearing protec- 
 tors, (2) the ability of hearing protec- 
 tors to attenuate impact noise, and 
 (3) the exact effects of "noise leaks" 
 (air gaps, etc.) on hearing protector ef- 
 fectiveness. Plans are for MSHA to coop- 
 erate closely with the Bureau, and the 
 in-house anechoic chamber will be a valu- 
 able tool in these investigations. 
 
 Technology transfer is one Bureau 
 activity that will continue during the 
 shift from short-term contracts to long- 
 term in-house research. The only antic- 
 ipated difference between past and fu- 
 ture practices is that contract final 
 reports will be replaced by in-house Re- 
 ports of Investigations (RI's) and Infor- 
 mation Circulars (IC's) as the primary 
 technology-transfer vehicles. As before, 
 Bureau personnel will continually review 
 new noise-control developments in mining 
 and other industries and apply them to 
 in-house programs. 
 
 In addition, information concerning the 
 mining equipment noise-control technol- 
 ogy described in part 3 will be supplied 
 to the industry in a more usable form. 
 Although the contract final reports cov- 
 ering the projects described in part 3 
 contain substantial amounts of informa- 
 tion, most are much too detailed for 
 potential users of the technology. 
 Therefore, the Bureau has published a 
 handbook (2^) covering all its mining 
 machinery noise-control programs in a 
 clear and concise format. Figures 66 and 
 67 have been taken from this handbook; 
 they summarize the current noise-control 
 technology for underground mining equip- 
 ment (fig. 66) and preparation and 
 processing plants (fig. 67). Some of the 
 "quieted" noise levels of the machines in 
 these figures resulted from Bureau ef- 
 forts, some resulted from manufacturers' 
 efforts, and others represent the noise 
 levels that could be achieved if current- 
 ly available noise-control techniques 
 were applied to previously untreated 
 machines. The Bureau's handbook also 
 contains extensive, easy-to-use listings 
 that tell how the noise-control treat- 
 ments are applied, how noise-control ma- 
 terials can be obtained, and where to go 
 for further information. 
 
 SUMMARY 
 
 Hearing loss due to noise in the min- 
 ing environment is a very serious 
 
 occupational health hazard. The Bureau 
 of Mines has addressed this problem by 
 
81 
 
 KEY 
 
 Unquiet ed 
 Quieted 
 
 TYPICAL WORKER EXPOSURE, dBA 
 80 90 100 110 120 
 
 Loaders 
 
 ^ 
 
 Hand -held pneumatic 
 percussion drills 
 
 Jumbo- mounted percussion 
 drills 
 
 Rotary face drills 
 
 Cutting machines 
 
 Continuous miners 
 (drum type) 
 
 Continuous miners 
 (auger type) 
 
 Shuttle cars 
 
 Longwall shearers and plows 
 
 Continuous haulage chain 
 conveyors 
 
 Roof bolters 
 
 Diesel-powered load-haul-dump 
 
 ^" 
 
 Diesel -powered haulage 
 trucks 
 
 D'lesel -powered personnel 
 carriers and aux. equipment 
 
 ^^^"^^ 
 
 Rail-mounted mantrips and 
 locomotives 
 
 '^^B^ 
 
 Face ventilation systems 
 (fans and blowers) 
 
 Pneumatic slushers and 
 tuggers 
 
 ^^^^ 
 
 ^^" 
 
 iWp5^-^'N 
 
 ^mA. 
 
 80 90 100 110 120 
 
 FIGURE 66. - Quieted versus unquieted noise levels of underground mining equipment. 
 
82 
 
 KEY 
 
 Unquieted 
 Quieted 
 
 TYPICAL WORKER EXPOSURE, dBA 
 70 80 90 100 110 
 
 120 
 
 Crushing and breaking equipment 
 (jaw and cone crushers) 
 
 Crushing and breaking equipment 
 (autogenous grinders and mills) 
 
 FIGURE 67. - Quieted versus unquieted noise fevels of preparation and processing plant 
 equipment. 
 
 conducting a wide variety of noise- 
 control research programs. During the 
 past 10 yr, these programs have involved 
 all four major segments of the mining 
 industry — underground coal mining, hard- 
 rock mining, surface mining, and coal and 
 mineral processing. 
 
 Mining machinery is primarily respon- 
 sible for the high noise levels in the 
 mining workplace. For this reason, and 
 beause the workplace cannot be easily 
 
 treated to reduce noise levels, Bureau 
 research has focused on noise-control 
 treatments for the machines used in 
 mines. Retrofit techniques for existing 
 equipment have been pursued whenever pos- 
 sible; however, in many cases equipment 
 redesign has been found to be a more ef- 
 fective approach. 
 
 Retrofit treatments have been developed 
 for (1) the cutting heads of auger-type 
 continuous miners, (2) chain conveyors. 
 
83 
 
 (3) handheld and jumbo -mounted pneumatic 
 percussion drills, (4) dlesel-powered LHD 
 vehicles, (5) bulldozers and front-end 
 loaders, and (6) coal and taconite pro- 
 cessing plants. Redesign efforts have 
 been directed toward (1) pneumatic per- 
 cussion drills, (2) LHD vehicles, and 
 (3) preparation plants. Basic studies 
 of coal-cutting dynamics have provided 
 valuable information that is now being 
 used to design reduced-noise cutting 
 heads for drum-type continuous miners and 
 longwall shearers. Studies of the effec- 
 tiveness of hearing protectors in the 
 mining environment are underway. 
 
 The Bureau's emphasis in the past 2 yr 
 has shifted from field demonstrations and 
 contract research to long-term basic in- 
 house research. A reverberation building 
 has been constructed to provide Bureau 
 personnel with a well-controlled acousti- 
 cal environment in which detailed studies 
 of the noise-generating mechanisms of 
 mining equipment can be conducted. This 
 approach is expected to be more cost- 
 effective than field demonstrations and 
 should yield results that can be imple- 
 mented in the future by equipment manu- 
 facturers and users to create a quieter 
 mining environment. 
 
 REFERENCES 
 
 1. Bartholomae, R. C, J. G. Kovac, 
 and J. Robertson. Measuring Noise from 
 a Continuous Mining Machine. BuMines IC 
 8922, 1983, 17 pp. 
 
 2. Bartholomae, R. C. , and R. P. 
 Parker. Mining Machinery Noise Control 
 Guidelines, 1983. BuMines Handbook, 
 1983, 87 pp. 
 
 3. Becker, R. S. , G. R. Anderson, and 
 J. G. Kovac. An Investigation of the 
 Mechanics and Noise Associated With Coal 
 Cutting. Pres. at ASME Winter Meeting, 
 Chicago, IL, Nov. 16-21, 1980. ASME pre- 
 print 80-WA/NC-l, 15 pp. 
 
 4. Berger, E. H. Using the NRR to 
 Estimate the Real-World Performance of 
 Hearing Protectors. Sound and Vib. , Jan. 
 1983, pp. 12-18. 
 
 5. Bobick, T. G. , and D. A. Giardino. 
 The Noise Environment of the Underground 
 Coal Mine. MESA (Dep. Interior) Inf. 
 Rep. 1034, 1976, 26 pp. 
 
 6. Bolt, Beranek, and Newman, Inc. 
 Bulldozer Noise-Control Manual. Ongoing 
 BuMines contract J0177049; for inf., con- 
 tact R. C. Bartholomae, TPO, BuMines, 
 Pittsburgh, PA. 
 
 7. . Loader Noise-Control Man- 
 ual. Ongoing BuMines contract J0395028; 
 
 for inf., contact R. C. Bartholomae, TPO, 
 BuMines, Pittsburgh, PA. 
 
 8. Creare Products, Inc. Develop- 
 ment of a Prototype Quiet Hard Rock 
 Stoper Drill. Ongoing BuMines contract 
 H0113034; for inf., contact W. W. Aljoe, 
 TPO, BuMines, Pittsburgh, PA. 
 
 9. Daniel, J. H. , J. A. Burks, R. C. 
 Bartholomae, R. Madden, and E, E, Ungar. 
 The Noise Exposure of Operators of Mobile 
 Machines in U.S. Surface Coal Mines. 
 BuMines IC 8841, 1981, 24 pp. 
 
 10. Dixon, N. R. Development and 
 Evaluation of Noise-Control Techniques 
 for Taconite Processing Equipment. On- 
 going BuMines contract J0377014 (Bolt, 
 Beranek, and Newman, Inc.); for inf., 
 contact T. G. Bobick, TPO, BuMines, 
 Pittsburgh, PA. 
 
 11. Dixon, N. R. , and M. N. Rubin. 
 Development of a Prototype Retrofit 
 Noise-Control Treatment for Jumbo Drills 
 (contract H0387006, Bolt, Beranek, and 
 Newman, Inc.). BuMines OFR 111-83, 1983, 
 95 pp.; NTIS PB 83-218800. 
 
 12. Durkin, J. Discriminating Ear- 
 muff. Paper in Noise Control. Proceed- 
 ings of Bureau of Mines Technology Trans- 
 fer Seminar, Pittsburgh, PA, January 22, 
 1975. BuMines IC 8686, 1975, pp. 97-108. 
 
84 
 
 13. Dutta, P. K. Development of Com- 
 mercial Quiet Rock Drills. Ongoing Bu- 
 Mines contract J0177125 (Creare Products, 
 Inc.); for inf., contact W. W. Aljoe, 
 TPO, BuMines, Pittsburgh, PA. 
 
 14. Dutta, P. K. , and P. R. Runstad- 
 ler. Development of Prototype Quiet 
 Jumbo Drills. Ongoing BuMines contract 
 H0395025 (Creare Products, Inc.); for 
 inf., contact W. W. Aljoe, TPO, BuMines, 
 Pittsburgh, PA. 
 
 15. Ferrari, V., and A. G. Galaitsis. 
 Integration of Quieting Technology Into 
 New Mantrip Vehicles (contract J0199068, 
 ESD Corp.). BuMines OFR 62-82, 1982, 162 
 pp.; NTIS PB 82-203241. 
 
 22. Huggins , G. G. , R. Madden, and 
 B. S. Murray. Noise Control of an Under- 
 ground Load-Haul-Dump Machine (contract 
 H0262013, Bolt, Beranek, and Newman, 
 Inc.). BuMines OFR 78-125, 1978, 76 pp.; 
 NTIS PB 288-854. 
 
 23. Industrial Acoustics Co., Inc. 
 Taconite Crusher Noise Reduction — Study 
 of Acoustical Enclosure for Symons 
 7-Foot, Standard Head, Extra-Heavy Duty 
 Cone Crusher (contract H0387016) . Bu- 
 Mines OFR 82-064, 1982, 33 pp.; NTIS PB 
 82-202649. 
 
 24. Lord, H. W. , W. S. Gatley, and 
 H, A. Evenson. Noise Control for Engi- 
 neers. McGraw-Hill, 1980, p. 331. 
 
 16. Galaitsis, A, G. Noise Reduction 
 of Chain Conveyors, Volume II (contract 
 H0155113, Bolt, Beranek, and Newman, 
 Inc.). BuMines OFR 171-83, 1983, 62 pp.; 
 NTIS PB 83-262634. 
 
 17. Galaitsis, A. G. , P. J. Remington, 
 and M. M. Myles. Noise Control of a Mine 
 Operated Personnel Carrier — Volume I, De- 
 sign and Performance of Noise-Control 
 Treatments (contract H0166090, Bolt, Ber- 
 anek, and Newman, Inc.). BuMines OFR 
 133-78, 1978, 112 pp.; NTIS PB 289-711. 
 
 18. Giardino, D. A., T. G. Bobick, and 
 L. C. Marraccini. Noise Control of an 
 Underground Continuous Miner, Auger-Type. 
 MESA (Dep. Interior) Inf. Rep. 1056, 
 1977, 57 pp. 
 
 19. Harris, C. M. Handbook of Noise 
 Control. McGraw-Hill, 1979, p. 12-10. 
 
 25. National Institute for Occupation- 
 al Safety and Health. HEW (now HSS) 
 Publ. 76-172, June 1976, 70 pp. 
 
 26. Patterson, W. N. , G. G. Huggins, 
 and A. G. Galaitsis. Noise of Diesel- 
 Powered Underground Mining Equipment — 
 Impact, Prediction, and Control (con- 
 tract H0346046, Bolt, Beranek, and 
 Newman, Inc.). BuMines OFR 75-058, 1975, 
 210 pp.; NTIS PB 243-896. 
 
 27. Pettitt, M. R. Development of a 
 Reduced-Noise Auger Miner Cutting Head. 
 Ongoing BuMines contract H0188065 (Wyle 
 Laboratories); for inf., contact W. W. 
 Aljoe, TPO, BuMines, Pittsburgh, PA. 
 
 28. Pettitt, M. R. , and W. W. Aljoe. 
 Fabrication Manual for a Reduced-Noise 
 Auger Miner Cutting Head. BuMines IC 
 8971, 1984, 9 pp. 
 
 20. Hennings , K. Noise Abatement of 
 Vibrating Screens Using Non-Metallic 
 Decks and Vibration Treatments (contract 
 H0387018, Allis-Chalmers Corp). BuMines 
 OFR 120-82, 1982, 61 pp.; NTIS PB 82- 
 251919. 
 
 21. Hennings, K. , and D. Grant. A 
 Simulation Model for Predicting the Per- 
 formance of Vibrating Screens (contract 
 J0395138, Allis-Chalmers Corp.). BuMines 
 OFR 137-83, 1983, 120 pp.; NTIS PB 83- 
 238386. 
 
 29. Phillips, W. G. Source Diagnosis 
 and Abatement Techniques for Noise 
 Control in Taconite Plants (contract 
 J0377014, Bolt, Beranek, and Newman, 
 Inc.). BuMines OFR 79-079, 1979, 115 pp. 
 
 30. Pokora, R. J., and T. L. Muldoon. 
 Demonstration of Noise-Control Techniques 
 for the Crushing and Screening of Non- 
 Metallic Minerals (contract J0100038, 
 Foster-Miller, Inc.). BuMines OFR 50-83, 
 1983, 185 pp.; NTIS PB 81-237646. 
 
85 
 
 31, Roepke, W, W. , D. P. Lindroth, and 
 T. A. Myren. Reduction of Dust and Ener- 
 gy During Coal Cutting Using Point-Attack 
 Bits. BuMines RI 8185, 1976, 53 pp. 
 
 32. Rubin, M. N. Demonstrating the 
 Noise Control of a Coal Preparation 
 Plant, Volume I: Initial Installation 
 and Treatment Evaluation (contract 
 HO 155 155, Bolt, Beranek, and Newman, 
 Inc.). BuMines OFR 79-104, 1979, 179 
 pp.; NTIS PB 299-963. 
 
 33. 
 
 Demonstrating the Noise 
 
 Control of a Coal Preparation Plant, Vol- 
 ume II: Long-Term Treatment Evaluation 
 (contract HO 155 155, Bolt, Beranek, and 
 Newman, Inc.). BuMines OFR 143-83, 1983, 
 91 pp.; NTIS PB 83-237354. 
 
 34. . Noise-Control Techniques 
 
 for the Design of Coal Preparation 
 Plants (contract J0100018, Roberts and 
 Schaefer Co.). BuMines OFR 42-84, 1984, 
 135 pp.; NTIS PB 84-166172. 
 
 35. Saperstien, L. W. , and W. W. Kauf- 
 man. Audible Warning Signals in Under- 
 ground Coal Mines. Trans. Soc. Min. Eng. 
 AIME, V. 258, No. 1, pp. 1-7. 
 
 36. Stewart, K. C. , and E. J. Burgi. 
 Noise-Attenuating Properties of Earmuffs 
 Worn by Miners. Volvime 1: Comparison of 
 Earmuff Attenuation as Measured by Psy- 
 chophysical and Physical Methods (con- 
 tract J0188018, Univ. Pittsburgh). Bu- 
 Mines OFR 152(l)-83, 1980, 46 pp.; NTIS 
 PB 83-257063. 
 
 37. . Noise-Attenuating Proper- 
 ties of Earmuffs Worn by Miners, Volume 
 
 2: Development of a Laboratory Procedure 
 for the Physical Measurement of Earmuff 
 
 Attenuation (contract J0188018, Univ, 
 Pittsburgh), BuMines OFR 152(2)-83, 
 1980, 37 pp,; NTIS PB 83-257071, 
 
 38. Summers, C, R, , and J, N, Murphy. 
 Noise Abatement of Pneumatic Rock Drill. 
 BuMines RI 7998, 1974, 45 pp. 
 
 39. Technological Enterprises, Inc. 
 Development of Concentric Drill Steels 
 for Noise Control of Percussion Drills. 
 Ongoing BuMines contract J0338022; for 
 inf., contact W. W. Aljoe, TPO, BuMines, 
 Pittsburgh, PA. 
 
 40. Ungar, E. E. A Census of Mobile 
 Machines Used in U.S. Surface Coal Mines 
 (contract J0166057, Bolt, Beranek, and 
 Newman, Inc.). BuMines OFR 77-78, 1977, 
 159 pp.; NTIS PB 284-112. 
 
 41. Walch, R. H. , and G. L. Beech. 
 Noise Control of Underground Load-Haul- 
 Dump (LHD) Machines. Ongoing BuMines 
 contract H0395076 (Eimco Mining and Ma- 
 chinery Corp.); for inf., contact T. G. 
 Bobick, TPO, BuMines, Pittsburgh, PA. 
 
 42. Wyle Laboratories, Investigation 
 and Control of Noise Generated During 
 Coal Cutting, Ongoing BuMines contract 
 J0387229; for inf,, contact W. W, Aljoe, 
 TPO, BuMines, Pittsburgh, PA. 
 
 43. . Noise Control of Longwall 
 
 Mining Systems. Ongoing BuMines contract 
 J0188072; for inf., contact W, W. Aljoe, 
 TPO, BuMines, Pittsburgh, PA. 
 
 Ti-U.S. CPO: 1985-505-019/20,012 
 
 INT.-BU.OF MINES, PGH., PA. 27881 
 
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