Application of a microphone phased array to identify noise 

sources on a roof bolting machine 

David S. Yanteka 

J. Shawn Petersonb 

Adam K. Smithc 

NIOSH/PRL 

626 Cochrans Mill Road 

Pittsburgh, PA 15236 

The findings and conclusions in this report have not been formally disseminated by the National Institute for 

Occupational Safety and Health and should not be construed to represent any agency determination or policy. 

Reference to specific brand names does not imply endorsement by NIOSH. 

ABSTRACT 

Exposure to excessive noise over time can cause permanent hearing loss. Workers in the mining 

industry are frequently exposed to A-weighted sound levels in excess of 90 dB. The A-weighted 

sound level at the roof bolter operator’s location can exceed 100 dB while drilling. The National 

Institute for Occupational Safety and Health (NIOSH) measured the sound pressure at the roof 

bolter operator’s position and utilized a phased array of microphones with beamforming software 

to identify noise sources on a roof bolting machine while drilling. The test data indicates the 

sound level at the operator’s position is dominated by the 2000 Hz through 5000 Hz one-thirdoctave-band sound levels. The beamforming results indicate that the drilling noise is primarily 

from two areas: the portion of the drill steel just below the rock and the drill steel-chuck 

interface. This paper will discuss the methods used to identify the primary noise sources. 

1. INTRODUCTION 

In 1996, NIOSH published the National Occupational Research Agenda, which identified 

hearing loss as the most common job-related disease in the United States.1

 Approximately 30 

million workers are exposed to hazardous sound levels alone or to hazardous sound levels in 

conjunction with ototoxic agents.2

 Despite more than 30 years of noise regulation in the mining 

industry, mine workers develop hearing loss at a significantly higher rate compared to the nonnoise exposed population. An analysis of audiograms conducted by NIOSH in 1996 shows that 

by the age of 50, nearly 90% of coal miners had a hearing impairment.3

 In contrast, only 10% of 

those who are not exposed to occupational noise experienced a hearing loss by the same age. 

In 1999, the Mine Safety and Health Administration (MSHA) modified its rules regarding 

noise exposure in an effort to reduce the occurrence of noise-induced hearing loss.4 Rather than 

relying solely on hearing protection devices, MSHA’s new rule requires mine operators to use all 

feasible engineering and/or administrative controls to reduce the noise exposures of overexposed 

miners’. However, for many machines, such as a roof bolter, noise controls that reduce the 

a

 Email address: DYantek@cdc.gov 

b

 Email address: JPeterson@cdc.gov 

c

 Email address: ASmith9@cdc.gov 
 

operator’s noise exposure below the MSHA Permissible Exposure Level (PEL) are not currently 

available. 

When measuring noise exposure, NIOSH recommends using a criterion level of 85 dB(A) 

with a 3-dB exchange rate5

 whereas the MSHA PEL uses a criterion level of 90 dB(A) with a 5­

dB exchange rate.4 Between 2000 and 2005, MSHA made 6215 measurements of roof bolter 

operators’ noise exposures. Of these measurements, 1086 exceeded the MSHA PEL.6 

Furthermore, in the coal industry, roof bolter operators accounted for 17% of all the noise 

exposures that exceeded the PEL. The overall A-weighted sound level at the roof bolter 

operator’s location often exceeds 100 dB when drilling. At this sound level, a roof bolter 

operator would reach MSHA’s PEL after 2 hours and NIOSH’s Recommended Exposure Level 

(REL) after 15 minutes. To reduce roof bolter operators’ noise exposures, noise controls must be 

developed that target the dominant noise sources during drilling. Determining the dominant 

source(s) of roof bolter drilling noise is the focus of this paper. 

A roof bolter is a large electrically-powered machine used to stabilize the mine roof after 

coal has been extracted. The roof bolter is used to drill holes into the mine roof using 25 to 35 

mm diameter drill bits attached to drill steels with either hexagonal or round cross-sections. The 

lengths of the drill steels vary from approximately 0.3 meters to 1.5 meters, or longer, depending 

on the mine and its roof conditions. After a hole is drilled, a roof bolt is inserted into the drilled 

hole to connect the overlying strata, thereby supporting the roof. To spread the load across the 

roof, roof bolts are sometimes inserted through a steel plate prior to inserting the bolt into the 

roof. Figure 1 shows a roof bolter in an underground coal mine. Figure 2 shows a hexagonal 

drill steel, a round drill steel, and two drill bits. 

Figure 2: hex and round drill steels for a 35-mm diameter 

Figure 1: A roof bolter in an underground coal mine. drill bit (upper) and two examples of drill bits (lower). 

2. TEST SETUP AND INSTRUMENTATION 

All testing was performed using a Fletcher model HDDR roof bolter in the NIOSH Pittsburgh 

Research Laboratory (PRL) hemi-anechoic chamber (see Figure 3). An overhead view of the 

PRL hemi-anechoic chamber is shown in Figure 4. A test fixture constructed of welded steel 

tubes was used to support the material to be drilled. Some of the tubes of the test fixture were 

filled with sand to reduce vibration of the test fixture to minimize fixture-radiated noise. The 

fixture was positioned approximately 3.7 meters from the end and 2.4 meters from the left side of 

the hemi-anechoic chamber walls. C-channels welded to the fixture enable the material to be 

cantilevered over the fixture which allows a clear view of the interface between the material and 

the drill steel. Granite blocks were used as the material for all tests because prior experience has 

shown that using granite reduces test-to-test variability. The granite block was held in place 

using chains that are tensioned with ratcheting chain binders. Rubber strips were used between 
 

 

the block and the fixture and a urethane sheet was placed between the chains and the block to 

minimize noise radiated by the chains and fixture. 

Figure 3: Fletcher HDDR in the PRL hemi-anechoic chamber. 


Figure 4: Overhead view of the PRL hemianechoic chamber. 

The HDDR, which is approximately 8 meters long and 3 meters wide, uses two 37.3-kW AC 

motors to propel the machine and to operate the drilling apparatus which is powered by hydraulic 

pumps and motors. The HDDR has two drilling stations so two operators can drill holes and 

install roof bolts simultaneously. To drill a hole, a drill steel is inserted into the chuck on the 

drill head and a bit is attached to the top of the drill steel. The operator then positions the drilling 

apparatus and sets the thrust and rotation rate. For this machine, the cuttings from drilling are 

collected by a vacuum system through vent openings in the bit (refer to Figure 2), down the drill 

steel, which is hollow, and into the dust collection box located at the rear of the machine. The 

potential sources of noise during drilling include the electric motors, hydraulics, vacuum system, 

and drilling apparatus. Figure 5a shows the operator’s platform and drilling apparatus and 

Figure 5b shows the operator at the controls. While drilling, the operator is approximately 1 m 

from the drill steel. 

(a) (b)
 

Figure 5: Operator station, drilling apparatus, and controls (a) and operator at controls under text fixture (b). 

z = 1.425 m (75% D)

z = 1.9 m (100% D)

z = 2.375 m (125% D)

Two data acquisition systems were used for data collection: an LMS Pimento and a Bruel & 

Kjaer Pulse. The LMS Pimento data acquisition system was used to measure the sound pressure 

at the operator’s ear using a Bruel & Kjaer 4188 microphone and the drill steel rotational speed 

using an optical sensor. The recorded sound pressure was post-processed to calculate the Aweighted one-third-octave-band sound level spectra. Noise source identification was performed 

using beamforming.7 For this application, beamforming is more appropriate than sound 

intensity8 and near field acoustic holography9 because drilling noise contains significant content 

up to about 5000 Hz and the data must be acquired simultaneously due to movement of the drill 

and test-to-test variability. To collect the beamforming data, the Pulse data acquisition system 

was used to simultaneously record the sound pressures using a 42-microphone wheel array (see 

Figure 6). The on-axis spatial resolution for this array for several measurement distances is 

shown in Figure 7. The data from the array was post-processed using Bruel & Kjaer’s 

Beamforming application. 

1.2

0.0

0.2

0.4

0.6

0.8 

z = 1.425 m (75% D) = 25 75% D)

z = 1 .9 m ( 100% D ) = 1 m ( % D)

z = 2. 375 m (125% D) 2. m 5% D) 

1.0 

Figure 6: NIOSH 42-microphone 

beamforming array. 

Resolution (m)tion (m)


0 1000 2000 3000 4000 5000 

Frequency (Hz) 

Figure 7: Spatial resolution of the NIOSH beamforming array for 

measurement distances of 1.425 m, 1.9 m, and 2.375 m. 

3. TEST PROCEDURES 

The HDDR was positioned so the left drilling apparatus was beneath the test fixture (refer to
 

Figure 3). The desired rotational speed and thrust were set using an automatic control system. 


To eliminate the effects of drilling with a dull bit, a new drill bit was used for each hole. 


The sound pressure at the operator’s ear was recorded (refer to Figure 5b) for the following 


conditions: 


� Electric motors and hydraulics operating without drilling 

� Electric motors, hydraulics, and vacuum system operating without drilling 

� Electric motors, hydraulics, and vacuum system operating with drilling. 

The recordings without drilling were used to determine the contributions of the electric 

motors, hydraulics, and vacuum system to the overall A-weighted sound level at the operator’s 

ear while drilling, which requires the operation of these components. A 35-mm diameter drill bit 

and a 1.2-meter long hexagonal drill steel were used for these tests. Each recording was a 

minimum of 15 seconds in length. The resulting drilling data were examined to determine when 
the drill steel rotational speed stabilized. The sound pressures for each measurement were postprocessed to calculate the A-weighted sound levels in one-third-octave bands. For the sound 

pressure recorded while drilling, only the data after the rotational speed stabilized were postprocessed to maintain consistency. The post-processed data for each drilling test was 

approximately 10 seconds in length due to the time required for the rotational speed to stabilize. 

To identify noise sources, sound pressures were recorded and post-processed using the 

NIOSH beamforming array with the Bruel & Kjaer Pulse. The array was positioned 2.4 meters 

in front of the drill head. Holes were drilled using a 35-mm diameter drill bit and either a 1.2­

meter long hexagonal drill steel or a 1.5-meter long round drill steel. The rotational speed and 

thrust were set at 200 RPM and 28.3 kN, respectively. These values were used because they are 

representative of commonly used settings. Once again, the rotational speed of the drill steel was 

measured with an optical tachometer. After the rotational speed stabilized, which took only a 

few seconds, the sound pressures were recorded for 2.5 seconds. The data was then postprocessed with the Bruel & Kjaer beamforming software using 1-second long clips to compute 

the one-third-octave-band sound pressure levels across multiple grid points located on a plane 

which encompassed the drilling apparatus. In the beamforming software, the source plane 

distance was set to 2.4 meters and free field was selected as the measurement condition. Each 

set of data was processed immediately after collecting the data using a 2.2 meter wide by 1.6 

meter high grid with a spacing of 0.10 meters. After reviewing the results, the data was 

reprocessed to improve the spatial resolution using a 0.3 to 0.4 meters wide by 1.6 meters high 

grid with a spacing of 0.05 meters. 

4. TEST RESULTS AND DISCUSSION 

A. Operator Ear Measurements with Hexagonal Drill Steel 

Prior to examining the results of the beamforming calculations, it is important to gain an 

understanding of the frequency content and sound level at the operator’s ear. The beamforming 

results provide good information on the noise radiated toward the array. However, the noise 

radiated toward the operator may have different frequency content. Therefore, an analysis of the 

sound pressure at the operator’s station was performed. 

Figure 8 shows the A-weighted one-third-octave-band sound levels at the operator’s ear with 

the electric motors and hydraulics operating and also with the electric motors, hydraulics, and 

vacuum system operating. The data indicate that the overall A-weighted sound level is below 75 

dB without the vacuum system operating and 90 dB with the vacuum system operating. Since 

the levels are more than 10 dB different, the electric motors and hydraulics are insignificant in 

terms of the overall A-weighted sound level for the second measurement. Figure 9 shows the Aweighted one-third-octave-band sound levels at the operator’s ear while drilling a hole into the 

granite block with a 35-mm diameter bit and a 1.2-meter long hexagonal drill steel with the 

rotational speed and thrust set to 200 RPM and 9.4 kN, 400 RPM and 9.4 kN, and 200 RPM and 

28.3 kN. The overall A-weighted sound levels of these measurements were 102.6 dB, 107.8 dB, 

and 109.1 dB, respectively. Each set of drilling data shows a hump in the one-third-octave-band 

spectra from about 1250 Hz to 10 kHz. Furthermore, the 2000 Hz through 5000 Hz one-thirdoctave bands have the highest in-band levels for each case. An important observation is that the 

frequency content does not change much with increasing rotational speed or thrust while the 

levels increase significantly with increasing rotational speed or thrust. 

Figure 10 shows the A-weighted one-third-octave-band spectra with the electric motors, 

hydraulics, and vacuum system without drilling overlaid with the data measured while drilling 

with a rotational speed of 200 RPM and a thrust of 28.3 kN. The most significant changes occur 
Electric & Hydraulic

Electric, Hydraulic, & Vacuum

 

200 RPM, 9.4 kN

400 RPM, 9.4 kN

200 RPM, 28.3 kN

 

80 

85 

90 

110 

115 

120 

at and above the 1250 Hz band. The in-band A-weighted sound levels increase by more than 10 

dB for each of these bands. The largest increase, about 25 dB, occurs in the 4000 Hz band. 

120 

Figure 8: Operator ear A-wtd 1/3-octave-band sound Figure 9: Operator ear A-wtd 1/3-octave-band sound 

levels with the electric motors and hydraulics and levels while drilling into granite with a 35-mm 

with the electric motors, hydraulics, and vacuum diameter bit and a 1.2-meter long hexagonal drill 

system. steel. 

1/3-Octave-Band Center Frequency (Hz) 

Figure 10: Operator ear A-weighted one-third-octave-band spectra with the electric motors, hydraulics, and 

vacuum system without drilling and for drilling with a rotational speed of 200 RPM and a thrust of 28.3 kN. 

To reduce the sound level at the operator’s ear while drilling, noise controls must be 

developed that target the noise generating mechanisms that are responsible for the high 

frequency drilling noise. Prior to designing these controls, it is necessary to determine the 

components that are the most significant sources of drilling noise. One possible source is 

fracturing of the rock at the bit-rock interface. In addition, the drill steel may radiate noise due to 

vibration from the cutting forces. However, due to the slenderness of the drill steel, it may not 

be an efficient radiator of noise. The drill head is another possible noise source. Forces at the 

bit-rock interface may be carried down the drill steel and transmitted into the drill head. 

1/3-Octave-Band Center Frequency (Hz) 

115 

110 

105 

100 

95 

A-wtd Sound Level (dB) 90 

85 

80 

75 

70 

65 

60 

55 

50 

45 45 

40 40 

1/3-Octave-Band Center Frequency (Hz) 

A-wtd Sound Level (dB)105 

100 

95 

75 

70 

65 

60 

55 

50 

200 RPM, 9.4 kN 

400 RPM, 9.4 kN

200 RPM, 28.3 kN 

OV (A) 

10000

80006300

5000

4000315025002000160012501000

63800

0

40500

0

31520250

0

16010125

0

A-wtd Sound Level (dB)

40 

50 

60 

70 

80 

90 

100 

110 

120 

Drilling at 200 RPM, 28.3 kN

Electric, Hydraulic, & Vacuum 1000
 1250


1600


2000


2500
3150


4000


5000


6300


8000


10000


OV (A)


OV (A)

Electric & Hydraulic

Electric, Hydraulic, & Vacuum 

OV (A) 800


10000 630


80006300 500


5000

400031502500 315


400


200016001250

250


1000

63800

0

500 125


160
200


40031520250

0

16010125

0

100

 

 

If the main source of noise is the bit-rock interface, noise controls would be limited to bit 

design, changes in drilling parameters, and barrier-type controls. If the noise is radiated by the 

drill steel and/or the drill head, application of isolation and damping techniques may reduce the 

radiated noise. Barriers or partial enclosures around the drill steel and drill head may also be 

effective at reducing noise. However, barrier-type controls for the drill steel may be 

unacceptable to the bolter operator. 

B. Beamforming Results for Small Drilling Depth using Hexagonal Drill Steel 

The first recordings using the beamforming array were collected using a 35-mm diameter drill bit 

and a 1.2-meter long hexagonal drill steel. The recording started as soon as the rotational speed 

became stable. This required only a few seconds. With drill settings of 200 RPM and 28.3 kN, 

the penetration rate was roughly 10 mm/sec. So, the rotational speed would stabilize prior to 

reaching a depth of 50 mm. When collecting data with the beamforming application, the picture 

is automatically taken immediately prior to recording the sound pressures. Only the first second 

of data was processed to minimize smearing of the contour plots due to movement of the drill. 

Since the drill moved roughly 10 mm during 1 second, this effect is insignificant. 

Figure 11 shows the contour plot for the 2500 Hz through 5000 Hz one-third-octave bands 

for the first test. This figure is from the results of processing the data at the beginning of a hole 

with a depth of a few centimeters immediately after the test using a grid spacing of 0.1 meters. 

The data processed was collected at the beginning of a hole with a depth of a few centimeters. 

The box around the perimeter of the figure indicates the boundary of the calculation grid and the 

dots represent the calculation locations. The red dot corresponds to the grid point yielding the 

highest level in the 4000 Hz one-third-octave band. The one-third-octave-band spectrum for this 

grid location is shown in the lower left corner of the figure and the average one-third-octaveband spectrum for all grid locations is shown in the lower right corner. The frequency content of 

the noise radiated toward the array is dominated by the 4000 Hz one-third-octave band, which is 

the largest contributor to the operator ear sound level with the same rotational speed and thrust 

(refer to Figure 10). In addition, there appears to be a hump in the spectrum at and above 1600 

Hz, similar to the spectrum of the sound pressure at the operator’s ear. The figure shows the 

largest contribution to the noise radiated toward the array originates near the top of the drill steel. 

In addition, there appears to be a secondary source near the bottom of the drill steel. The top 

source could be from the drill steel, the bit-rock interface, or both. The bottom source could be 

from the drill steel, the drill head, or both. 

Several additional measurements were performed with the same rotational speed and thrust 

to gain a better understanding of the noise sources. Near the top of the drill steel, the contour 

appears to be shifted slightly toward the C-channel used to support the rock. This effect could be 

due to sound reflecting off the C-channel. In addition, the cross-tube behind the drill steel is 

another surface that could reflect sound and alter the contour map. Neither of these reflective 

surfaces would be present under real-world conditions in a mine. To reduce the effects of these 

reflective surfaces, 25-mm thick acoustic foam was affixed to the C-channels and cross-tube and 

the measurements were repeated. The bottom surface of the block may also be a significant 

reflector of noise. Reflection of sound off the block would tend to shift the contour toward the 

block which makes it difficult to determine if the noise is generated by the drill steel or the bitrock interface. Therefore, a layer of 25-mm thick acoustic foam was attached to the bottom of 

the block and a third measurement was performed. Even though attaching foam to the bottom of 

the block would reduce the reflection from the block, noise generated from the bit-rock interface 

would still be able to emanate from within the hole. To determine if significant noise is 
generated at the bit-rock interface, a fourth test was performed with a layer of barrier-foam on 

the bottom of the block instead of the layer of acoustic foam. If the bit-rock interface is a 

significant source of noise, the results would show a change in the levels and, possibly, the 

source location near the top of the drill steel. 

Figure 11: Beamforming results with 35-mm dia. drill bit and 1.2-m long hex drill steel for RPM and 28.3 kN. 

Figure 12 shows the contour map for the first measurement with (a) no treatments, (b) with 

the C-channels and cross-tube treated with acoustic foam, (c) with the C-channels, cross-tube, 

and bottom of the block treated with acoustic foam, and (d) with the C-channels and cross-tube 

covered with acoustic foam and the bottom of the block covered with the barrier-foam. Each of 

these images show the results computed for the 2500 Hz through 6300 Hz one-third-octave 

bands using a grid spacing of 0.05 meters with a 0.3 to 0.4 meters wide by 1.6 meters high grid 

centered on the drill steel. Adding the foam to the C-channels and cross-tube appears to shift the 

high noise area at the top of the drill steel slightly so it is more centered on the drill steel (refer to 

Figures 12a and 12b). In addition, the portion of the drill steel directly in front of the cross-tube 

appears to radiate less noise due to reducing the reflection from the cross-tube. The high 

radiation area near the bottom of the drill steel also seems to be more significant since the noise 

radiated near the top of the drill steel was reduced. Adding foam to the bottom of the block 

slightly shifts the source at the top of the drill steel toward the chuck (see Figure 12c). The 

source at the top of the drill steel also appears to begin several centimeters down from the bottom 

of the block. Changing to barrier-foam on the bottom of the block (Figure 12d) appears to have 

no effect on the resulting contour plot. This result seems to prove that the noise near the top of 

the drill steel is due to radiation from the drill steel rather than from the bit-rock interface. The 

noise near the bottom of the drill steel may be radiated by the drill steel, the chuck, or both. 
(a) (b) 

(c) (d) 

Figure 12: Beamforming results with 35-mm dia. bit and 1.2-meter long hexagonal drill steel for 200 RPM and 

28.3 kN (a) std fixture, (b) 25-mm thick foam on C-channels and cross-tube, (c) 25-mm thick foam on C-channels, 

cross-tube, and bottom of block, and (d) 25-mm thick foam on C-channels and cross-tube and barrier-foam on 

bottom of block. 

C. Beamforming Results for Increasing Drilling Depth 

The previous measurements examined the radiation for only the first few centimeters of 

penetration into the rock. Two series of measurements were performed as a hole was drilled 

with a 35-mm diameter drill bit. Data were collected for depths of 0 cm, 0.15 cm, 0.30 cm, and 

0.45 cm. A 1.2-meter long hexagonal drill steel was used for the first series of measurements 

and a 1.5-meter long round drill steel was used for the second series of measurements. All 

measurements were performed with 25-mm thick acoustic foam on the C-channels and crosstube to reduce the influence of the test fixture. The foam was removed from the bottom of the 

block because under real-world conditions the reflection of sound from the bottom of the block is 

present. The foam may reduce the importance of the noise radiated from the top of the drill steel 

relative to the total noise. All measurements were performed with a rotational speed of 200 

RPM and a thrust of 28.3 kN. 

 Figures 13 and 14 show the resulting contour plots for the 2500 Hz through 6300 Hz onethird-octave bands with a grid spacing of 0.05 meters for the 1.2-meter long hexagonal drill steel 

and the 1.5-meter long round drill steel, respectively. Each figure shows that as the penetration 

depth increases, the noise source near the top of the drill steel remains in nearly the same 
 

 

 

 

 

 

location. This may be due to the cancellation effect along the length of the drill steel as adjacent 

portions of the drill steel vibrate out of phase. At the onset of drilling the hole, there is no 

cancellation at the top of the drill steel. However, adjacent sections down the length of the drill 

steel tend to cancel one another. As the depth is increased, the portion of the steel just beneath 

the block is not cancelled but portions of the drill steel away from the block cancel the radiation 

from adjacent sections. The source near the bottom of the drill steel travels with the drill head. 

These results seem to indicate that the mechanisms of noise radiation are independent of drilling 

depth. 

(a) (b) (c) (d) 

Figure 13: Beam forming results for 1.2-m hex drill steel (a) start of ho le, (b) 0.15 m, (c) 0.30 m, an d (d) 0.45 m. 

(a) (b) (c) (d) 

Figure 14: Beamforming results for 1.5-m round drill steel (a) start ofhole , (b) 0.15 m, (c) 0.30 m, and (d) 0.45 m. 

D. Beamforming Results with Trial Noise Treatments 

Based on the results from prior measurements, it appears that tw o regions are responsible for the 

noise generated during drilling. Each contour plot shows the top portion of the drill steel as a 

region of high noise radiation. In addition, the interface between the bottom of the drill steel and 

the chuck seems to be a significant contributor to radiated noise. Noise controls that target these 

areas could significantly reduce the radiated noise. 

To validate this presumption, a series of tests were perform ed with trial noise treatments 

applied to the top of the drill steel and the interface between the bottom of the drill steel and the 

chuck. A barrier with a foam decoupler was wrapped around the top of the drill steel and 
 

secured to the drill steel using nylon cable ties. To treat the drill steel-chuck interface, a barrierabsorber lined enclosure was used. A hole was placed in the top of the enclosure to enable the 

enclosure to be slid over the drill steel. The through-hole was sized to minimize the gap between 

the drill steel and the enclosure. A round drill steel was used to reduce the noise leaking through 

the gap between the drill steel and the enclosure. 

Three tests were performed: (1) no treatments, (2) barrier-decoupler wrap at the top of the 

drill steel, and (3) barrier-decoupler wrap at the top of the drill steel and the barrier-absorber 

lined enclosure at the bottom of the drill steel. The C-channels and cross-tube on the fixture and 

the bottom of the granite block were treated with 25-mm thick acoustic foam for these 

measurements to reduce reflections from these surfaces. This makes the two areas of high noise 

radiation more obvious on the contour maps. Each test was performed with a rotational speed of 

200 RPM and a thrust of 28.3 kN. Both beamforming and operator ear data was collected and 

post-processed for each test. 

Figure 15 shows the contour maps for the results of the aforementioned series of tests in the 

2500 Hz through 6300 Hz one-third-octave bands using a grid spacing of 0.05 meters. Figures 

15 a, b, and c have the same range for the color bar to show the effects of the trial noise 

treatments whereas Figures 15 d and e are scaled to show the top 10 dB of noise sources to 

highlight the spots of high noise radiation with the treatments. Figures 15 b and c show the trial 

noise treatments substantially reduced the noise radiated at the top and bottom of the drill steel. 

Figures 15 d and e show that with the trial noise treatments the area of the drill steel just below 

the barrier-decoupler wrap is an area of high noise radiation. Moreover, Figure 15 e shows the 

leaks through the gap between the enclosure and the drill steel and at the interface between the 

enclosure and the chuck. The A-weighted sound levels at the operator’s position corresponding 

to Figures 15 a, 15 b, and 15 c were 108.3 dB, 107.1 dB, and 102.9 dB, respectively. 

(a) (b) (c) (d) (e) 

Figure 15: Trial noise control tests - (a) baseline, (b) ba rrier-decoupler wrap attop o f drill steel, (c) ba rrierdecoupler wrap at top of drill steel and enclosure at the bottom of the drill steel, (d) same as (b) with adjust ed 


scale for color bar, and (e) same as (c) with adjusted scale for color bar. 


5. CONCLUSIONS 

The A-weighted sound level at the operator’s station on a roof bolter used in coal mining 

typically exceeds 100 dB while drilling. When drilling into granite with a 35-mm diameter drill 

bit attached to a 1.2-meter long hexagonal drill steel in the laboratory, the A-weighted operator 

ear one-third-octave-band sound level spectrum is dominated by the 1250 Hz through 10 kHz 

bands. The 2000 Hz through 5000 Hz one-third-octave band have the highest levels in this 

instance. The results of beamforming measurements indicate that the majority of drilling noise is 

radiated by two areas: the portion of the drill steel just below the rock and the drill steel-chuck 
interface. This result is independent of drill steel type (e.g. hexagonal or round) and drilling 

depth. The beamforming results from tests with trial barrier-type noise treatments applied to the 

top of the drill steel and to the drill steel-chuck interface indicate that barriers may be a viable 

means of reducing roof bolter operators’ noise exposures. Noise controls that isolate cutting 

forces from the drill steel or damp drill steel vibrations could also greatly reduce sound levels 

generated during drilling. 

ACKNOWLEDGEMENTS 

The authors would like to acknowledge Paul Jurovcik and Patrick McElhinney for their help in 

planning and conducting this research. 

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National Institute for Occupational Safety and Health, “National Occu pational Research Agenda,” DHHS (NIOSH) 

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Franks JR, Analysis of Audiograms for a Large Cohort of Noise-Exposed Miners. Unpublished technical report. 

NIOSH, Cincinnati, OH: (1996) 4

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National Institute for Occupational Safety and Health, “Criteria for a Recommended Standard: Occupation al Noise 

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[http://www.msha.gov/STATS/PART50/p50y2k/p50y2k.HTM]. Date accessed: July 2006. 7

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Vibration Measurment A/S, Naerum, Denmark, 2004). 

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