Determining Underground Roof Bolting Machine Operators 

Noise Exposure Using Laboratory Results 

R. J. Matetic, Ph.D.a 

Peter Kovalchikb 

Gregory Colec 

Jeffrey Petersond 

NIOSH/PRL 

626 Cochrans Mill Road 

Pittsburgh, PA 15236 

Syd Peng, Ph.D.e 

West Virginia University 

PO Box 6070, Mineral Resources Building 

Morgantown, WV 26506 

ABSTRACT 

Mine Safety and Health Administration (MSHA) data indicates that the roof bolting machine is 

third among all equipment and second among equipment in underground coal mining whose 

operators exceed the MSHA-PEL (Mine Safety and Health Administration-Permissible Exposure 

Limit). In response to this, the National Institute for Occupational Safety and Health (NIOSH) 

has conducted a study to reduce overexposures of noise to operators of roof bolting machines. 

An important segment of the research is to determine, characterize and measure sound power 

levels radiated by a roof bolting machine during different drilling configurations. The 

determined sound power levels generated during the drilling cycle are of major interest because 

the levels represent the overall sound power generated by the machine. These sound power 

levels, determined from laboratory tests, can be used to accurately assess the effectiveness of 

proposed controls for reducing noise exposure to roof bolting machine operators. By using the 

sound power level results obtained from the laboratory and a commercially available acoustical 

modeling package, a method for predicting sound pressure levels to roof bolting machine 

operators for differing drilling types and parameters could be determined. This paper provides a 

method for predicting sound pressure levels at the operator’s position of a roof bolting machine 

in an underground coal mine using sound power levels determined in the laboratory. To 

determine validity, underground coal mine data was also collected to compare predicted and 

measured sound pressure levels. The results of the predicted sound pressure level data were the 

same as the underground measured results. This research will provide the mining industry with a 

valid method for predicting a roof bolting machine operator’s noise dosage underground given 

any type of drilling configuration or drilling method utilized from laboratory testing. 

a

 Email address: RMatetic@cdc.gov

b

 Email address: PKovalchik@cdc.gov 

c

 Email address: GPCole@cdc.gov 

d

 Email address: JPeterson@cdc.gov 

e

 Email address: sspeng@mail.wvu.edu 
1. INTRODUCTION 

Noise is one of the most pervasive health hazards in mining. NIOSH identified occupational 

noise-induced hearing loss (NIHL) as one of the ten leading work-related diseases and injuries. 

Mine Safety and Health Administration coal noise sample data (Title 30 CFR, Part 62) collected 

from 2000 to 2002 show that 65% of the equipment whose operators exceeded 100% dosage 

comprise only seven different types of machines: auger miners, bulldozers, continuous miners, 

front end loaders, roof bolters, shuttle cars (electric) and trucks (MSHA, 2000-2002). In addition, 

the MSHA data indicate that the roof bolter is third among all the equipment and second among 

equipment in underground coal mines whose operators exceed 100% dosage1,2. 

NIOSH is addressing this at the Pittsburgh Research Laboratory (PRL) to reduce noise exposure 

to roof bolter operators in coal mining. This paper concentrates on determining the sound power 

noise emissions during the drilling cycle of a roof-bolting machine. Laboratory sound power 

measurements were collected and through the utilization of an acoustic model, an operator’s 

noise exposure or dosage could then be determined. This research effort will provide the mining 

industry with the opportunity to predict noise dosage to roof bolting machine operators without 

the laborious effort of conducting numerous underground measurements. 

2. BACKGROUND 

Underground mining machines are subject to many variables that can affect the noise levels. 

Some of these variables cannot be controlled while others can be influenced or even controlled 

by the machine operator. The acoustic environment in which the mining machines operate is a 

critical factor affecting the sound pressure levels. Underground mines are enclosed areas, which 

usually represent diffused sound fields. An ideal diffuse sound field is a sound field in which the 

time average of the mean-square sound pressure is everywhere the same and the flow of acoustic 

energy in all directions is equally probable. The geometry and the composition of the surfaces 

influence the overall sound level by the way sound waves (treated as rays in the acoustic model) 

are reflected or absorbed. 

Mine entries also have various shapes, rectangular, square, or arched, and various dimensions as 

well. These variations in shape and size affect the overall sound energy that is reflected or 

absorbed. These are the variables that cannot be controlled in the acoustic environment, and 

include geometry and composition of the surfaces, shape of the mine opening, and the 

compressive strength of the affected medium. 

A methodology and model was developed to predict sound pressure levels at the operator 

position of a roof bolting machine using full-octave band frequency measurements obtained from 

laboratory and in-mine testing. The objective was to determine a miners’ effective noise dosage 

related to measured and determined engineering noise control tests determined from laboratory 

trials dependent upon full-octave band sound power levels. Upon completion of the model, the 

measured sound power determined in the laboratory, with a calculated or predicted sound 

absorption coefficient, was then entered into the model to determine an operator’s noise dosage 

relative to the drilling cycle of the roof bolting machine. To achieve this, an acoustic modeling 

program based on ray-tracing techniques was utilized to predict the steady-state sound pressure 

level and the associated sound decay in a mine environment. The information then provided a 

snapshot of the environment and calculated the noise levels throughout the environment. 
 

 

Ultimately, this effort is providing the mining industry with a method to model a simple event 

within a mine section and determine the noise dosage of the roof bolting machine operator. 

3. APPROACH 

Equation 1 can be used to relate the sound power of the noise source to the sound pressure level. 

This equation is a function of the directivity of the source, the distance from the source, the 

geometry and absorption of the environment. Listed below is the equation: 

Lp = Lw + 10 Log10 [(Q / 4� r2

) + (4(1-�) / S�)] + 10.3, (1) 

where 

Lp = sound pressure at a distance r from the source 

Lw = sound power level of source 

Q = directivity of the source 

r = distance from the source 

S = surface area 

� = average absorption coefficient 

If the directivity, the distance, and the geometry and absorption of the environment are kept 

constant the equation shows that the sound pressure level is directly proportional to the sound 

power level. By knowing this, a commercially available software package, Raynoise3

, can be 

used to show how the sound power relates to sound pressure level at a given point with a known 

geometry and absorption coefficient. The only requirements for Raynoise are octave sound 

power levels, octave absorption coefficients, room or entry dimensions, and measurement 

locations in x, y, and z coordinates. 

Because of all the variations, both geometrically and acoustically in underground mines it would 

be difficult to measure sound power in these conditions and achieve constant test results. It 

would be impossible to control the acoustic environment, which would make it difficult to 

evaluate the roof bolting machine and its components for noise levels. Therefore, sound power 

testing was conducted in the Pittsburgh Research Lab (PRL) NVLAP accredited reverberation 

chamber. This allowed sound power measurements to be determined in a controlled acoustic 

environment and independent of variables of underground mines. The PRL reverberation 

chamber (Illustration 1) was designed for sound power testing of large equipment in 

conformance with ISO 374.4

 The chamber meets ISO 374 in the frequency range of 100Hz to 

10,000 Hz. 

Illustration 1: PRL reverberation chamber 
Once the sound power level of the roof bolting machine is determined, the other requirements 

necessary for sound level prediction are the absorption coefficients, the characteristics of the 

specific roof bolting machine, room or entry dimensions, and measurement locations in x, y, and 

z coordinates. The sound absorption coefficients used for this research were determined by past 

NIOSH research5

. The height measurement of 1.52 m was selected because this distance, on 

average, represents the average ear-height of an individual. The three measurement locations 

were chosen based on the operator’s working locations. Finally, research results have shown the 

dominant noise sources of a roof bolting machine are the drill bit, the drill steel, and the drill 

chuck6

. These noise sources correspond to three points on a line source extending from the bit to 

the chuck, with the drill bit/contact surface as one source, the drill steel another and the third 

source being the drill chuck. 

4. LABORATORY TEST PLAN 

Two Fletcher roof bolting machines were tested, a Roof Ranger II and a HDDR. A test 

apparatus for holding the drilling medium was located in the rear of the reverberation chamber 

(Illustration 1). The machine position in the reverberation chamber was chosen, due to the 

length of the machine. The only location requirements for both the test apparatus and the roof 

bolting machine were they be a minimum distance of 1.5 m from any wall or ceiling and at least 

1 m from the microphones. Also, the machine should be placed asymmetrically in the chamber 

according to the ISO 3741 standard, section 7.27

. Since, the machine cannot be operated 

remotely only one operator is allowed in the reverberation chamber during testing. This person 

must wear dual hearing protection, both ear plugs and ear muffs. 

In formulating a conservative test plan for testing, it was decided to use drilling components and 

parameters that were representative of industry usage. The roof bolting machine was tested 

using 2.54 cm and 3.49 cm carbide bits both vacuum and wet, a 1.22 m drill steel either hex or 

round, and under varying loading conditions. These loading conditions were accomplished by 

varying the thrust pressure and rotation speed. Also, three different media for drilling were used, 

which would provide a wide range of penetration rates with respect to noise. 

Given these parameters two different size drill bits, two different type drill bits, two types of drill 

steel, three different drilling media, five rotation speeds for each thrust pressure, and five 

different thrust pressure were considered for each media. Also, each test was conducted using 

three different drilling methods (vacuum, mist, and drilling). This resulted in a total of 900 

sound power tests that were conducted. 

The individual test data collected (900 tests) from the research effort was then compiled and 

summarized into a matrix for drilling into the three compressive strength media tested (soft41.368 MPa, medium-117.211 MPa, or hard-165.473 MPa). This would be used to determine 

overall sound power levels given any type of drilling method (vacuum, wet or mist) and using 

varying types of drilling parameters or configurations related to thrust, rotational speed, bit size 

and shape of drill steel. 

5. COMPARING SOUND PRESSURE LEVELS –UNDERGROUND MEASUREMENT 

RESULTS VS RAYNOISE RESULTS 

Sound pressure level measurements were conducted at the operator position of a Fletcher Roof 

Ranger II roof bolting machine in an underground coal mine. The underground mine height 

varied from approximately 1.22 to 1.52 m and the thickness of the coal seam varied from 81 -102 
C 

XX 

XX 

X 

X 

X X X 

XX 

Noise Measurement 

cm. The mine operator provided the opportunity to measure sound pressure levels relative to two 

bolting machines utilizing a vacuum system and mist system drilling method respectively. The 

roof bolting plan for the section is shown in illustration 2. 

Vacuum Drilling 

Noise Measurement 

X X 

24.4 m. X 24.4 m 

X 

Mist Drilling X X 

24.4 m. X 24.4 m 

5.79 m. 

Illustration 2: Roof bolting plan 

Each roof bolting machine utilized a 2.54 cm drill bit, hex drill steel and drilled to a depth of 

1.52 m into the immediate roof. The roof consisted of a highly fractured shale rock consistent of 

a rock type with a low compressive strength (approximately 41.368 MPa. The drilling 

configurations for both machines were set at a rotational speed of 500 rpm and a thrust of 

approximately 6.5 kip. Sound pressure level measurements were conducted at the operator 

position of each machine. Table 1, illustrated below provides the results of the sound pressure 

level testing. 

Table 1: Underground test results 

X = Drill Holes 

Operation of roof 

bolting machine 

Roof Ranger II 

(vacuum) 

dB(A) 

Roof Ranger II 

(mist system) 

dB(A) 

Idling 82 82 

Mist system on 

No idling 

NA 88 

Drilling 101 96 

The sound pressure levels experienced at the operator position during drilling utilizing the 

vacuum system of drilling was 101 dB(A). Utilizing a mist system type of drilling, the operator 

was exposed to a sound pressure level of 96 dB(A), resulting in a 5 dB(A) difference in sound 

pressure level. 
 

 

The next step was to utilize the Raynoise program to compare the measured underground sound 

pressure level experienced at the operator position to the predicted sound pressure level 

determined thru Raynoise. 

The input parameters used for Raynoise were based on the underground measurement conditions 

and operating parameters. The underground rock media had a compressive strength-41.368 MPa 

and the sound power was based on a Roof Ranger II vacuum drilling with a hex drill steel, 2.54 

CM bit, rotational speed of 500 rpm, and a thrust setting of 6.36 kip. Table 2 gives the sound 

power levels used based on this information. The other input parameters used for the Raynoise 

program are listed in Table 3. 

Table 2: Sound power levels results for a roof bolting machine drilling into a media of 6,000 psi at operating 

parameters of 500 rpm rotation speed and a thrust of 6,363 lbs using a 2.54 cm bit and hex drill steel. 

Octave-band (Hz) Sound Power 

Level dB 

63 85.6 

125 89.4 

250 101.1 

500 96.5 

1000 97.4 

2000 103.3 

4000 103.7 

8000 97.1 

Table 3: Other input parameters used for the Raynoise program 

Entry absorption coefficients (octave band): 

.03 .04 .2 .14 .15 .19 .28 .45 

Entry dimensions: 1.83 m H x 5.79 m W x 24.38 m L 

Measurement points 1.52 m above floor. (Operator’s 

position locations designated by the numbers 1, 2, & 3. 

Illustration 4) 

1 Line source on LEFT boom approx. 1 m long from 

chuck to roof entry point. 

Illustration 3 displays the Raynoise sound pressure level contours, along with the A-weighted 

sound pressure levels based on the input parameters of tables 2 and 3. 
 

�

100 dB(A) 103dB(A) 

101dB(A) 

Illustration 3: A Raynoise sound pressure level contour for a roof bolting machine. 

Table 1 data provided a sound pressure level of 101 dB(A) at the operator position of the roof 

bolting machine from underground measurements. The location of the measurement 

underground was the same location as measuring point number 2 utilized for Raynoise, 

Illustration 3. The determined or predicted sound pressure level using Raynoise was 101 dB(A), 

the same sound pressure level as measured underground. It should be noted, in order for 

Raynoise validation, data was collected in an actual underground mine environment. Due to 

mine accessibility, characterization of the acoustic environment, along with measuring sound 

pressure levels associated with operators of roof bolting machines occurred in a mine with low 

compressive strength rock media during drilling operations. However, the research effort has 

proven that by measuring laboratory sound power level results of the roof bolting machine given 

any compressive strength rock media, along with proper characterization of the acoustic and 

geological environment underground, one could predict the sound pressure level experienced by 

a roof bolting machine operator with confidence. 

This research would provide the mining community with an approach, which utilizes sound 

power levels acquired from laboratory tests and measured absorption coefficients from 

underground testing to accurately predict or determine sound pressure levels, at any location in a 

mine section. The next section will demonstrate and provide an approach for determining the 

noise dosage of a roof bolting machine operator utilizing the predicted or determined sound 

pressure levels obtained from Raynoise. 

6. DETERMINING NOISE DOSAGE OF A ROOF BOLTER OPERATOR FROM 

PREDICTED SOUND PRESSURE 

The noise dosage a worker receives can be expressed by the following equation: 

RTime � (LS �CL) / ER Dose% � (100 / TC)[ 2 dt] , (2) 0 

and 

RTime ( LS �CL) / ER Dose% � (100 / TC)(2 )[ dt], (3) 0 

Therefore 

( LS �CL) / ER Dose% � (100 / TC)(2 )(RTime) , (4) 

where 

Dose% = workers’ noise dosage, percent 

TC = criterion time, 8 hours or 28,800 seconds 
LS = Sound pressure level, dB(A) 

CL = Criterion level, dB(A) 

ER = Exchange rate, dB(A) 

RTime = Run time, seconds 

Using the equation above, with a predicted or determined sound pressure level at the operator 

position of a roof bolting machine, the machine operators’ noise dosage could then be 

determined. For instance, if we use the example illustrated in section 5, where the compressive 

strength of the rock media was 41.368 MPa and the roof bolting machine operator utilized the 

vacuum drilling method, with a 1-inch bit, hex drill steel, a rotational speed of 500 rpm and a 

thrust setting of 6.36 kip, the measured and validated predicted sound pressure level was 

determined to be 101 dB(A) at the operator position of the machine. Using the above equation, 

the mining community can then determine the operator’s noise dosage, relative to the MSHAPermissible Exposure Limit (MSHA-PEL) (90 decibels, A-weighted, as an 8-hour time-weighted 

average [90 dBA as an 8-hr TWA]), with a 5 dBA exchange rate) or the NIOSH-Recommended 

Exposure Limit (NIOSH-REL) (85 decibels, A-weighted, as an 8-hour time-weighted average 

[85 dBA as an 8-hr TWA]), with a 3 dBA exchange rate received per drilling an individual hole 

during the drilling cycle of the roof bolting machine as shown below: 

( LS �CL) / ER Dose% � (100 / TC)(2 )(RTime) (5) 

where 

TC = 28,800 seconds 

LS = 101 dBA 

CL = 90 dBA 

ER = 5 dBA 

RTime = 50 seconds 

(based on a penetration rate of 3.05 cm/sec and a hole depth of 1.52 m) 

Therefore, the dose percentage of the roof bolter operator for this particular example would be 

80%. Additionally, assuming similar rock media (compressive strength of 41.368 MPa) and the 

operator of the roof bolting machine drilling 78 drill holes per shift for installation of roof bolts, 

the operator’s noise dosage relative to only utilizing the roof bolting machine for drilling would 

be:0.8% x (78 drill holes) = 62.4 % of the MSHA-PEL of 100%. In comparison, the NIOSHREL noise dosage for the same situation mentioned above would be 546%, based on a criterion 

level of 85 dB(A) and an exchange rate of 3 dB(A). Table 4 provides the noise dosage of a roof 

bolter operator (per hole and per shift), relative to a respective sound pressure level and based on 

a run time of each hole consisting of 50 seconds and assuming the operator will drill 78 holes per 

his working shift. 
 

Table 4: Noise Dosage (MSHA and NIOSH) of Roof Bolting Machine Operator 

Sound 

Pressure Level 

dB(A) 

Run Time 

(sec/hole) 

MSHADose 

(%/hole) 

NIOSHDose 

(%/hole) 

Holes 

Drilled per 

Shift 

MSHADose 

(%/shift) 

NIOSHDose 

(%/shift) 

85 50 0.0 0.2 78 0.0 13.5 

86 50 0.0 0.2 78 0.0 17.1 

87 50 0.0 0.3 78 0.0 21.5 

88 50 0.0 0.3 78 0.0 27.1 

89 50 0.0 0.4 78 0.0 34.1 

90 50 0.2 0.6 78 13.5 43.0 

91 50 0.2 0.7 78 15.6 54.2 

92 50 0.2 0.9 78 17.9 68.2 

93 50 0.3 1.1 78 20.5 86.0 

94 50 0.3 1.4 78 23.6 108.3 

95 50 0.3 1.7 78 27.1 136.5 

96 50 0.4 2.2 78 31.1 172.0 

97 50 0.5 2.8 78 35.7 216.7 

98 50 0.5 3.5 78 41.1 273.0 

99 50 0.6 4.4 78 47.2 343.9 

100 50 0.7 5.6 78 54.2 433.3 

101 50 0.8 7.0 78 62.2 546.0 

102 50 0.9 8.8 78 71.5 687.9 

103 50 1.1 11.1 78 82.1 866.7 

104 50 1.2 14.0 78 94.3 1091.9 

105 50 1.4 17.6 78 108.3 1375.7 

106 50 1.6 22.2 78 124.4 1733.3 

This approach has provided the mining community with a way to predict sound pressure levels, 

with relative certainty, at the operator position of a roof bolting machine and at differing 

locations within a mine section, utilizing laboratory testing results relative to the roof bolting 

machine. Additionally, the mining community was presented with an approach to characterize 

the noise dosage a roof bolting machine operator will receive, based on laboratory results. This 

will provide the mining industry with the opportunity to predict sound pressure levels and noise 

dosage to machine operators without the laborious effort of conducting numerous underground 

measurements. 

7. CONCLUSIONS 

NIOSH researchers conducted sound pressure level measurements at the operator position of a 

Fletcher Roof Ranger II roof bolting machine in an underground coal mine and the results 

provided a sound pressure level of 101 dB(A). When comparing the results to the Raynoise 

program the results were the same as the operator’s position. The research effort has proven that 

laboratory sound power level results could predict with confidence the sound pressure level 

experienced by a roof bolting machine operator. 

In general, given any type of drilling method (vacuum, wet or mist), rock media (compressive 

strength), drilling parameters or configurations related to thrust, rotational speed, bit size and 
 

shape of drill steel for roof bolting machines, sound power levels can be determine from the 

laboratory data. Utilizing the sound power data with Raynoise the sound level of the operator 

could be determined, which could be used to determine the noise dosage to machine operators 

from drilling. This research will provide the mining industry with a valid method for predicting 

a roof bolting machine operator’s noise dosage underground given any type of drilling 

configuration or drilling method utilized from laboratory testing. 

REFERENCES 

1

SHA [2006]. Accident, illness and injury and employment self-extracting files (part 50 data), 2000-2005. Denver, 

CO: U.S. Department of Labor, Mine Safety and Health Administration, Office of Injury and Employment 

Information. [http://www.msha.gov/STATS/PART50/p50y2k/p50y2k.HTM]. Date accessed: July 2006. 2

Peterson JS, Kovalchik PG, Matetic RJ [2006]. A Sound Power Level Study of a Roof Bolter. In: Yernberg WR, 

ed. Transactions of Society for Mining, Metallurgy, and Exploration, Inc. Vol. 320. Littleton CO: Society for 

Mining, Metallurgy, and Exploration, Inc., pp 171-177. 3

Raynoise Revision 3.0, Building Acoustics and Industrial Noise Simulation, User Manual, LMS International 4

ISO 3741 entitled “Acoustics --- Determination of sound power levels of noise sources using sound pressure --- 

Engineering method in an essentially free field over a reflecting plane”, 31 pages, Second edition 1994-05-001. 5

Kovalchik PG, Matetic RJ, Cole GP, Smith AK, [2006], A Measurement Method for Determining Absorption 

Coefficients for Underground Mines. Proceedings of INTER-NOISE December 2006.,” pp. 1-9. 6

Peterson JS, Kovalchik PG, Matetic RJ [2006]. A Sound Power Level Study of a Roof Bolter. In: Yernberg WR, 

ed. Transactions of Society for Mining, Metallurgy, and Exploration, Inc. Vol. 320. Littleton CO: Society for 

Mining, Metallurgy, and Exploration, Inc., pp 171-177. 7

ISO 3741 entitled “Acoustics --- Determination of sound power levels of noise sources using sound pressure --- 

Engineering method in an essentially free field over a reflecting plane”, 31 pages, Second edition 1994-05-001.