Assessment of noise controls commonly 

used on jumbo drills and bolters in 

Western United States underground 

metal mines 

E.R. REEVES 

E.R. Reeves is Senior Service Fellow with the U.S. National 

Institute for Occupational Safety and Health, Pittsburgh, PA. 

Abstract 

The mining industry recognizes the 

importance of engineering controls as 

a primary means of preventing hearing loss.The noise-control treatments 

most commonly observed on drills 

and bolters during this study were 

windshields, sound-absorbing material and hydraulic motor covers. 

These controls were evaluated on 

machines at underground metal 

mines to determine the amount of 

noise reduction achieved by each control.The results indicate that absorbing material has very little effect on 

the noise level. The noise reduction 

attributable to the motor covers was 

dependent on the material used to 

create the cover. Properly installed 

windshields were the most consistent 

control. 

Introduction/background 

Noise-induced hearing loss is 

now the most common occupational 

illness in the United States, with 30 

million workers exposed to excessive 

noise levels (NIOSH, 1996). This is 

of particular concern to the mining 

industry, where many workers are 

exposed to damaging noise levels.As 

a result, approximately 90 percent of 

coal miners and nearly 70 percent of 

metal/nonmetal miners exhibit a hearing disability by age 

50 (Franks 1996, 1997). 

One of the many reasons for the prevalence of hearing loss is the lack of successful engineering noise controls for the equipment used in the underground mining 

industry. The relatively small market for mining equipment, combined with the unique requirements imposed 

by the sometimes-hostile mining environment, has limited manufacturer innovation and the transfer of technology from other industries. 

Fortunately, the mining industry has recognized the 

importance of engineering noise controls as a primary means of reducing 

noise exposure and preventing noiseinduced hearing loss among mine 

workers. Even though a lack of 

readily available proven control 

technology has hindered the implementation of controls, potential 

noise-control solutions are being 

crafted and tried at the mine level by 

mine workers, operators, manufacturers, consultants and government 

personnel. 

A downside to this initiative is 

that in an attempt to reduce worker 

noise exposure, many operators install noise treatments without knowing how much noise reduction to 

expect from the treatment before 

installation or how much noise reduction is actually achieved after installation. In some cases, due to 

improper material selection, placement or installation, little to no

sound reduction occurs after the application of noise treatments. In 

other cases, noise treatments are applied when the source sound level 

does not warrant treatment, thus 

wasting valuable resources. Not only 

do unsuccessful noise controls cost 

the industry time and money, they 

also do nothing to decrease the 

equipment operator’s risk of noiseinduced hearing loss. 

Approach 

A short-term goal of the U.S. National Institute for 

Occupational Safety and Health (NIOSH) is to identify 

possible noise-control solutions that are being applied 

to pieces of machinery and to characterize the noise reductions attributed to those controls. In addition to locating and assessing existing controls, NIOSH is also 

identifying processes or machines in need of noise controls, identifying gaps in technology that impede the use 

of noise controls and identifying barriers to the use of noise controls, including collateral hazards. To 

commence this task, NIOSH surveyed seven underground metal 

mines in the western part of United 

States that had installed noise controls on pieces of mining equipment. 

During the course of the study, 

noise controls installed on locomotives, haul trucks, face drills, roof 

bolters and load-haul-dumps (LHD) 

were evaluated using several acoustical-measurement techniques. The 

results can be used by the industry 

as a guide to focus noise reduction 

and hearing-conservation efforts on 

controls that show the most promise. 

Space does not permit reporting 

all of the findings from all of the 

equipment tested, and space does not 

permit reporting all the results from 
each measurement technique to be presented here. Therefore, only the sound-level results of the machines with

the most installed engineering controls, specifically the

jumbo face drills and roof bolters, are discussed. The remainder of the findings will be published elsewhere, but

a summary of all of the acoustical techniques used is presented below.

FIGURE 1

Two-channel sound level measurement.

FIGURE 2

Sound-intensity measurements.

Methods

To determine the actual amount of noise attenuation achieved by each applied noise control at the machine-operator position, several acoustic measurement

techniques were used take measurements both with and

without the noise-control treatments in place. Sound-level

and sound-intensity measurements were performed using Bruel & Kjaer (B&K) 2260 Investigators Running

Enhanced Sound Analysis and Sound Intensity software,

respectively. The B&K Investigator is a precision Type I

instrument (ANSI S1.4) that has a tolerance of 1 dB between 500 and 4,000 Hz. Noise exposure was measured

using a Quest Q-400 Noise Dosimeter. The Quest Q-400

is a single microphone, dual-channel device that allows

for the simultaneous measurement of noise using two different evaluation criteria.

Table 1

Dosimeter settings.

Dosimeter number Parameter Settings Designation

Dosimeter 1 Weighting A MSHA

Threshold level 90 dB permissible

Exchange rate 5 dB exposure

Criterion level 90 dB limit

Response Slow

Upper limit 140 dB

Dosimeter 2 Weighting A Wide

Threshold level 40 dB range

Exchange rate 3 dB

Criterion level 85 dB

Response Slow

Upper limit 140 dB

Sound-level measurements. Sound-level measurements were conducted on each piece of equipment underground and, when possible, on the surface. On

machines with more than one applied noise treatment,

the testing was done in a manner that allowed for the

determination of the degree of reduction due to each

treatment as well as the overall reduction due to combinations of treatments.

For every machine tested, a one-third-octave linear

equivalent sound pressure level (Leq) spectrum was

measured. The Leq is the average sound level over a

measurement period. For these experiments, the sound

was averaged for at least 15 seconds. Before actual readings were recorded, background noise measurements

were taken to insure that no extraneous noise was present that could

corrupt the data.

Because the noise produced by

the drilling and bolting processes can

be highly variable, care had to be

taken to insure a fair comparison was

being made across conditions. To do

this, a two-channel measurement system was used. One channel, used as

a reference, measured the sound

level outside of the operator area

toward the drill steel; the other channel simultaneously measured the

sound level at the operator’s ear. This

can be seen in Fig. 1. Performing the

measurements in this manner allowed changes in sound level at the

operator to be attributable to the application or absence of the noise control and not a variation in source

 
 

 

level caused by a change in geological conditions while 

drilling. 

Table 2 

Noise reductions due to hydraulic motor noise 

controls. 

Uncontrolled Controlled Reduction 

Motors level dB(A) level dB(A) dB(A) 

Bolter 1 84.9 83.2 1.7 

Bolter 2 77.3 76.9 0.4 

Face drill 1 79.4 77.2 2.2 

Face drill 2 79.9 79.5 0.4 

Face drill 3 84.3 81.9 2.4 

Table 3 

Noise reductions due to absorptive material in 

canopy. 

Canopy 

Bolter 2 

Face drill 1 

Face drill 2 

Face drill 2

 (no windshield) 

Uncontrolled Controlled 

level dB(A) level dB(A) 

97.4 97.3 

99.1 99.3 

99.6 99.6 

100.3 100.1 

Reduction 

dB(A) 

0.1 

-0.2 

0.0 

0.2 

Sound-intensity measurements. To quantify the reduction achieved by some of the noise-control devices, 

sound-intensity measurements were made on the machine surface. Sound intensity is a vector quantity that 

describes the rate of energy flow through a unit area 

(ANSI S12.12). From the intensity data, the localized 

sound power can be calculated. Comparing the sound 

power calculations with and without the noise control in 

place can be used to directly measure the performance 

of the control. 

To measure sound intensity a special probe is used. 

The probe consists of two microphones mounted faceto-face separated by a solid spacer of known length. Based 

on the measured pressure gradient between the microphones and the average pressure, sound intensity is calculated. This two-microphone technique produces a 

vector that has magnitude and direction. 

Before beginning the sound-intensity measurement, 

the desired measurement surface was measured and divided into a grid consisting of rows and columns.The grid 

dimensions varied depending on the size of the measurement surface and the desired degree of frequency and 

spatial resolution. A 15-second measurement was taken 

100-mm (4 in.) from the surface at each grid point using 

the intensity probe with a 12-mm (0.5-in.) spacer. 

Noise dosimetry. To determine the amount of noise 

the machine operators were exposed 

to during the course of their day, 

noise dosimeters (also called noise 

dose meters) were used. The Quest 

Q-400 Noise Dosimeters used for exposure monitoring have two channels, referred to as Dosimeter 1 and 

Dosimeter 2. Therefore, two sets of 

noise exposure parameters can be 

measured simultaneously. Dosimeter 

1 was set to monitor the U.S. Mine 

Safety and Health Administration’s 

(MSHA) Permissible Exposure 

Limit (PEL) of 90 dBA, and Dosimeter 2 was set for wide range data 

collection. The settings for each dosimeter are shown in Table 1. 

FIGURE 3 

Dosimeter mounted for monitoring a face drill. 

The microphone is located just above the 

operator’s ear. 

FIGURE 4 

Dual-boom jumbo face drill. 

For monitoring operator noise 

exposure, the microphone was 

clipped to the shoulder of the equipment operator and worn for a full 

shift. To determine when and where the workers were 

receiving most of their noise dose, time-motion studies 

were performed. 

In addition to monitoring worker exposure, dosimeters were used to monitor equipment sound levels.This 
 

 

method gives an indication of noise levels at a location 

on the machine, independent of operator movement. Figure 3 shows a dosimeter mounted in the cab of a face 

drill. 

FIGURE 5 

Heavy conveyor belt barrier, fiberglass blanket and Plexiglas motor covers. 

Results 

The noise controls implemented on the drills and 

bolters were very consistent across the seven mines visited. All of the mines had implemented some, if not all, 

of the following controls: covers over the hydraulic motors; absorptive material in the canopy; absorptive material around the seat; absorptive material in the lower 

portion of the operator’s area; and a windshield. Most of 

the controls were installed in such a way that they could 

be removed and replaced by a trained mechanic without 

degradation of performance. 

Ten machines were tested: five roof bolters and five 

jumbo face drills. Because the noise reduction achieved 

by the installed controls was being evaluated rather than 

the machine itself, each machine is referenced according 

to a generic identifier, e.g., Bolter 3. Also, it should be 

mentioned that these results are part of a larger study 

that measured and documented noise controls installed 

on most of the machines used in underground metal mining.The documented controls are a representation of how 

seven mine operators attempted to reduce worker noise 

exposure and not an all-inclusive list of available technology. 

Table 4 

Noise reductions due to absorptive material around
 

operator.
 

Seat Uncontrolled Controlled Reduction 

adsorption level dB(A) level dB(A) dB(A) 

Bolter 1

 (drilling) 97.5 97.6 -0.1 

Bolter 1

 (bolting) 98.4 98.7 -0.3 

Face drill 2

 (motor) 78.1 77.2 0.9 

Hydraulic-motor covers. All of the tested drills and 

bolters were equipped with at least one hydraulic motor; 

the dual-boom face drills were equipped with two motors.The motors were located directly behind the operator area as shown in Fig. 4. Five of the tested machines, 

two roof bolters and three jumbo drills, had engineering 

noise controls installed around the hydraulic motors.All 

of the reported measurements were made underground 

at the operator’s ear position with only the hydraulic motors operating. 

Table 2 shows the results from the noise controls used 

on the hydraulic motors. Bolter 1 had a motor cover constructed from 6.35-mm- (0.25-in.-) thick heavy rubber 

conveyor belt, Bolter 2 had a cover constructed from 38­

mm- (1.5-in.-) thick fiberglass blanket, Drill 1 had a motor cover constructed from 12.7-mm- (0.5-in.-) thick heavy 

rubber conveyor belt, Drill 2 had a cover constructed from 

38-mm- (1.5-in.-) thick quilted fiberglass absorptive material and Drill 3 had a motor cover constructed from 

6.35-mm- (0.25-in.-) thick Plexiglas. Several of the tested 

controls are shown in Fig. 5. 

The results shown in Table 2 indicate that the motor 

covers constructed of a heavy barrier material, as opposed 

to an absorptive material, produce the most significant 

level reductions. However, it should be noted that the Aweighted sound levels created by the uncontrolled motors would produce a noise exposure below the Action 

Level of 85 dB(A).Therefore, noise control efforts should 

be directed elsewhere. 

Absorptive material in the canopy. Most of the machines tested had sound-absorbing material under the 

canopy. However, only three of the machines had the 
 

 

material installed in such a way that it could be removed 

and easily replaced to directly measure the noise reduction attributable to it as a control. In all of the reported 

cases, the absorptive material was a 25-mm- (1-in.-) thick 

quilted fiberglass blanket. Figure 6 shows the material 

being removed for testing. 

Table 3 shows the noise reduction achieved due to 

the absorptive material applied to the canopy. The facedrill results were acquired underground during the drilling cycle, while the bolter results were measured above 

ground with the percussive hammer operating. All measurements were performed at the operator’s ear location. 

Face Drill 2 had a movable windshield, so the effect 

of the absorptive material in the canopy was measured 

with and without the windshield present. The other reported results are with the windshield in place. In all cases, 

the results shown in Table 3 indicate that absorption 

placed in the canopy has a minimal to no effect on the 

sound levels that the operator experiences. 

Absorptive material around operator area. One 

tested bolter and one tested face drill had a 25-mm- (1­

in.-) thick quilted fiberglass blanket around the operator’s 

area that could be removed for testing.To determine what 

affect the material had on the sound level reaching the 

operator’s ear, measurements were performed underground with the windshield in place while Bolter 1 was 

drilling and bolting. The operator’s area of Bolter 1 is 

shown in Fig. 7. On Face Drill 2, the sound level reduction attributable to the material was measured while only 

the hydraulic motors were operating. 

Table 4 shows the results from the absorptive material placed around the operator.The results indicate that 

the absorption around the seat has essentially no affect 

on the sound level during the drilling process. During the 

bolting process, the measured sound level at the 

operator’s ear was actually 0.3 dB(A) higher with the 

material in place. However, this difference is so small that 

it would have a negligible affect on the noise exposure of 

the operator. When only the hydraulic motor is in operation, a reduction of nearly 1 dB(A) was achieved.This 

reduction occurred because the material around the operator area on Face Drill 2 was located between the motor, which was the noise source, and the operator. 

Absorptive material in lower front of cab. Figure 8 

shows a 25-mm- (1-in.-) thick quilted fiberglass blanket 

applied to the lower front of the operator area on Bolter 

2. This material is located where the operator’s knees 

would be positioned while operating the drill boom. 

Table 5 shows the effect that the absorptive material 

placed in the lower front of the operator cab had on sound 

levels measured at the operator’s ear during drilling and 

bolting. The results indicate that absorptive material 

placed in the lower portion of the operator cab has essentially no effect on the sound levels that the operator 

experiences. 

Windshields. The most common engineering noise 

control installed on the tested drills and bolters was a 

windshield. The amount of noise attenuation achieved 

varied greatly depending on how the windshield was designed and installed. Several examples of windshields are 

shown in Fig. 9. 

FIGURE 6 

Quilted fiberglass blanket, 25-mm- (1-in.-) 

thick, being removed for testing. 

Most of the windshields are designed to be flipped 

up into the canopy. This feature allows the operator to 

have an unobstructed view while tramming the machine, 

and it also allows for the sound levels to be measured at 

the operator position both with and without the windshield in place. Table 6 shows the effect that the tested 

windshields had on sound levels reaching the operator’s 

ear underground. 

Table 5a 

Noise reduction due to absorption placed in lower 

front of cab with windshield. 

Lower Uncontrolled Controlled Reduction 

cab level dB(A) level dB(A) dB(A) 

absorption 

Bolter 2

 (drilling) 98.1 97.9 0.2 

Bolter 2

 (bolting) 99.9 99.9 0 

Table 5b 

Noise reduction due to absorption placed in lower 

front of cab without windshield. 

Lower Uncontrolled Controlled Reduction 

cab level dB(A) level dB(A) dB(A) 

absorption 

Bolter 2

 (drilling) 98.5 98.5 0 

Bolter 2

 (bolting) 101.5 101.2 0.3 

The results indicate that a windshield designed so 

that gaps, or flanking paths, are minimized will reduce 

sound levels at the operator’s ear more so than a windshield designed to have a gap between panes of glass. 
 

Discussion 

The application of fiberglass absorptive material to 

the canopy, seat area and lower portion of the cab had 

little to no effect on the sound level at the operator’s ear 

during the drilling and bolting process. To reduce the 

sound level reaching the operator, sound-absorbing materials must be placed on surfaces that reflect sound towards the operator’s hearing zone. In an open cab, such 

as those installed on the face drills and roof bolters tested, 

absorptive materials are of limited benefit. 

The well-designed windshields, in general, were the 

most consistent noise control implemented on the drills 

and bolters. This is because the windshields form a barrier between the noise source, which is the drilling and 

bolting process, and the operator. Also, the noise generated by drilling and bolting is predominantly high frequency in content. High frequencies are easier to block 

and absorb due to their shorter wavelength. The windshields that were designed with a space between an upper and a lower pane of glass did not reduce the sound 

levels reaching the operator as much as the solid windshields. This is because the gap allows sound energy to 

“leak” through to the operator. 

On Bolter 5, an attempt was made to supplement the 

windshield with an operator enclosure constructed from 

conveyor belt strips. This can be seen on the far right in 

Fig. 9. Unfortunately, due to gaps between the strips, the 

conveyor belting had no effect on the sound levels reaching the operator’s ear. 

FIGURE 7 

Quilted fiberglass material in the operator’s 

area of Bolter 1. 

FIGURE 8 

Quilted fiberglass material in the lower front of 

the operator’s area of Bolter 2. 

Table 6 

Noise reductions due to windshields. 

Uncontrolled Controlled Reduction 

Windshields level dB(A) level dB(A) dB(A) 

Bolter 2 (drilling) 98.5 97.9
 0.6 

Bolter 2 (bolting) 101.2 99.9
 1.3 

Bolter 5 (drilling) 100.6 99
 1.6 

Bolter 3 (drilling) 99.2 96
 3.2 

Bolter 3 (bolting) 105.7 102.5
 3.2 

Face drill 1 101.7 99.3
 2.4 

Face drill 2 100.3 99.6
 0.7 

Face drill 3 97.1 95.3
 1.8 

Face drill 4 (single boom) 94 91.9
 2.1 

Face drill 4 (dual boom) 98.9 95.6 3.3 

Face drill 5 101.9 100.6 1.3 

 

Conclusions 

In summary, when applying noise-control treatments, 

care should be taken to use the right product for the job. 

The 13-mm- (0.5-in.-) thick rubber conveyor belt mats 

and the 6.35-mm- (0.25-in.-) thick Plexiglas motor covers reduced motor noise because they are barrier materials, which is the correct choice for the application. The 

motor controls used on Bolter 2 were sound absorbers 

that had almost no effect on the noise output from the 

motors. Sound-absorbing material is much more effective when backed by a noise barrier. 

However, hydraulic motor covers on 

the tested face drills and roof bolters are not necessary as the noise levels produced by the uncovered

motors are below 85 dB(A), much

lower than the levels produced by

other noise generating mechanisms

to which the operator will be exposed.

The use of absorptive materials 

in the operator’s area slightly reduces sound levels underground on

the machines tested. Due to the operating environment and the openness of the operator area a large

reduction in noise levels from this 

control is not expected. 
 

 

The windshields reduced the noise reaching the operator during the drilling/bolting cycle. The generated 

noise from the aforementioned processes is relatively 

high frequency in content.Therefore, the windshield provides a protective barrier between the noise source and 

the operator. Care should be taken to seal gaps in the 

windshield and between the windshield and the structure of the machine. 

In the future, this project will evaluate noise controls 

used in other mining sectors, as well as revisit the underground metal sector as new noise controls are implemented. 

FIGURE 9 

The windshields installed on Bolter 2, Bolter 3 and Bolter 5, respectively. 

Acknowledgments 

The author would like to thank the NIOSH field 

crews that participated in the collection of the data for 

this report: Gerald Finfinger, senior scientist; Rudy J. 

Matetic, section chief, HLPS;Alexander E. Prokop, engineering technician;Timothy Matty, electronics technician; 

David Yantek, mechanical engineer; Mike DiMartino, 

electronics technician; Dan Babich, mining engineer; 

Michael Durr, information technology specialist; Peter 

Kovalchik, electronics engineer; J. Shawn Peterson, electrical engineer and Patrick Hintz, industrial hygienist. A 

special acknowledgement is given to Jeffery Kohler, director of the Pittsburgh Research Laboratory, for his vision and insight in developing the hearing loss prevention 

research program and for his technical guidance in conducting this research project. 

The author and NIOSH appreciate the assistance and 

cooperation of the mining companies and would like to 

acknowledge the special role of the mechanics for their 

help in removing and replacing the noise controls, the 

safety supervisors for setting up the trips and taking care 

of the machine logistics and the machine operators for 

their patience and cooperation.This work would not have 

been possible without the full support and cooperation 

of the Nevada Mining Association and its noise task force 

— their commitment to this work and their contributions 

to it are sincerely appreciated. 

References 

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for the Determination of Sound Power Levels of Noise Sources Using 

Sound Intensity,” New York, New York, ANSI S12.12-1992 

Franks, J.R., 1996, “Analysis of audiograms for a large cohort of 

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from Linda Rosenstock, Aug. 6. 

Franks, J.R., 1997, “Prevalence of hearing loss for noise-exposed 

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from Gregory Wagner, October 7. 

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