Noise controls for roof bolting machines
 

p. kovalchik, a. Smith, R. matetic 

and J. peteRSon

P. Kovalchik, A. Smith, R. Matetic, member Sme, and J. 

Peterson are manager, research engineer, branch chief and 

research engineer, respectively, with the pittsburgh Research 

laboratory, national institute for occupational Safety and health 

(nioSh), pittsburgh, pa. 

Abstract 

Prior to 2004, the Bureau of Labor Statistics (BLS) 

classified hearing loss in the “all other illnesses” category. However, in 2004 hearing loss was categorized 

as a separate illness that accounted for 11% of work 

related illnesses. Most categories of illnesses and injuries 

associated with mining have improved, with the exception of hearing loss. The drilling of rock in a confined 

work environment contributes to high levels of noise 

exposure. Information gathered from the Mine Safety 

and Health Administration (MSHA) coal database from 

2000 to 2005 has shown that roof bolting machines were 

second among all equipment whose operators exceed 

the MSHA Permissible Exposure Limit (PEL). In response, the National Institute for Occupational Safety 

and Health (NIOSH) at the Pittsburgh Research Laboratory (PRL) has been conducting research to reduce 

noise overexposure caused by roof bolting machines and 

to prevent additional cases of noise-induced hearing loss 

(NIHL). This is achieved through the development and 

application of engineering noise controls. This paper 

describes the procedure used to evaluate noise generated by a roof bolting machine and its components. Two 

engineering noise controls for the roof bolting machine 

were evaluated: a bit isolator and a drill chuck isolator. 

Acoustic beamforming measurements were performed 

at the PRL hemi-anechoic chamber to assess the noise 

controls developed for the roof bolting machine. Results 

showed that in combination, the bit isolator and the drill 

chuck isolator provided a 7 dB(A) reduction in sound 

pressure level at the operator position. 

Introduction	 

The Bureau of Labor Statistics 	

(BLS) classifies hearing loss as a 	

separate illness category. Hearing 

loss accounted for 11% of all illnesses in 2005 (U.S. Department of 

Labor Bureau of Labor Statistics, 

2006a). According to occupational 

data for coal mining provided by 

the Mine Safety and Health Administration (MSHA) from 2000 to 

2005, operators that exceeded 100% 

noise dosage were from only seven types of machines: 

auger miners, bulldozers, continuous mining machines, 

front end loaders, roof bolters, shuttle cars (electric) and 

trucks (Title 30 CFR Part 62, 2000-2005). Roof bolting 

machine operators rank second among all underground 

machine operators that exceed 100% noise dose. There 

are approximately 600 underground coal mines with 

3,930 roof bolting machine operators (U.S. Department 

of Labor, Mine Safety and Health Administration, 2005; 

U.S. Department of Labor Bureau of Labor Statistics, 

2006a). Figure 1 shows the percentage of roof bolting machine operators that exceeded 100% noise dose 

from 2000 to 2005 according to the 

MSHA database (Title 30 CFR Part 

62, 2000-2005). 

Figure 1 

MSHA coal noise sample data: roof bolting machine 

operator’s noise dose samples that exceeded 100% dose 

from 2000 to 2005. 

The roof bolting machine operator is exposed to several noise sources. These include vacuum pumps, 

hydraulic pumps, drill chucks, drill 

steel and drill bits. Both vacuum and 

hydraulic pumps contribute some to 

overall noise emissions. However, tests at PRL confirmed 

that the hydraulic and vacuum pumps are not a major 

noise source for the roof bolting machine (Peterson et 

al., 2006). The dominant noise source on a roof bolting 

machine is the result of the drilling process (Peterson 

and Alcorn, 2007). 

To reduce noise exposure caused by roof bolting machines generated during the drilling process, two engineering noise controls were developed. Specifically, this 

paper concentrates on the noise emitted from the drill 

chuck, the drill steel and the drill bit while drilling with 

a J.H. Fletcher & Co. HDDR roof bolting machine. This 

type of roof bolting machine is representative of industry usage. This research is providing the mining community with a bit isolator and a drill chuck isolator as noise 

controls to be utilized in the drilling process of the roof 

bolting machine. This could provide operators of these 

machines an opportunity to be within the MSHA-Permissible Exposure Limit (MSHA-PEL). 

Test procedure 

Testing was conducted in the hemi-anechoic chamber 

at PRL with a Fletcher HDDR roof bolting machine, 

shown in Fig. 2. The interior dimensions of the room are 

approximately 17.7 m long x 10.4 m wide x 7 m high (58 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

x 34 x 23 ft), with a volume of approximately 1,300 m3 (45,900 cu ft). This 

facility utilizes Eckel Industries Su

persoft Panels on the walls and ceiling 

to yield a free-field over a reflecting 

plane, which meets the requirements 

of ISO 3744 down to approximately 

100 Hz. The chamber was utilized pri

marily for noise source identification 

testing on the Fletcher HDDR roof 

bolting machine. Also, sound pressure 

level measurements were taken at the 

operator’s position to determine the 

overall A-weighted sound level at the 

operator’s ear while drilling. 

Baseline data was collected first 

using beamforming along with sound 

pressure level data at the position of 

the roof bolting machine operator.The 

beamforming and sound pressure level 

data were collected simultaneously. 

The beamforming data identified the 

noise sources from the drilling process 

of the machine. The sound pressure 

level data at the position of the roof bolting machine 

operator quantified the noise at the operator position. 

This was important because the frequency content of the 

two may be different. The baseline data was used for the 

evaluation of the engineering noise controls for the roof 

bolting machine. 

The noise source identification measurements were 

conducted using the beamforming technique. Beamforming is a method of mapping noise sources by differentiating sound levels based on the direction from which they 

originate. The method is very quick, allowing a full map 

to be calculated from a single-shot measurement. The 

technique is also well suited for high-frequency noise 

sources (Brüel & Kjaer, 1995). A Brüel & Kjaer pulse 

data acquisition system with 46 input and two output 

channels served as the data acquisition system for the 

beamforming technique. Figure 3 shows the 42-channel 

beamforming wheel array used for the noise source identification at PRL. 

Figure 2 

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PRL’s hemi-anechoic chamber and support stand for rock media. 

A LMS Pimento system served as the data-acquisition 

system for collecting the sound pressure level data at the 

roof bolting machine operator position and the thrust 

and speed of the roof bolting machine. The recorded 

sound pressure was post-processed to calculate the Aweighted, one-third-octave-band sound level spectra. 

Figure 4 shows the microphone placement used for the 

sound pressure level data at the position of the roof bolting machine operator. 

Table 1 

Drilling parameters. 

Parameter Values 

Drilling type vacuum
 

Drill steel round, hexagonal
 

Drill bit size 34.9 mm
 

Figure 3 

Forty-two channel acoustic beamforming wheel array. 

Figure 4 

Microphone location at the operator’s ear. 

A large steel support stand comprised of rectangular 

tubes was fabricated by PRL to hold the drilling media, 

shown in Fig. 2. To prevent the support stand from radi-
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ating significant amounts of sound, sand was used to fill 

the hollow tubes except for the diagonal tubes and the 

horizontal tubes along the short direction at the top of 

the structure. This was done for convenience and to create a vibration impedance mismatch in the structure to 

reduce vibration transmission. In addition to filling the 

tubes, two layers of a urethane material were bonded to 

the rock support tubes to break direct contact between 

the rock and the structure. Finally, a 12.7-mm- (0.5-in.-) 

thick layer of urethane was placed between the rock and 

the chain used to support the rock media. 

In formulating a test plan, researchers decided to use 

drill bits and drill steels that were representative of industry usage.The drilling parameter configuration is listed in 

Table 1. Granite was chosen as the drilling media because 

it is a high compressive strength material. Past NIOSH 

research shows that higher compressive strength material 

generates more noise then lower compressive strength 

materials (Peterson and Alcorn, 2007). This would provide the worst-case scenario for noise 

emission. Also, a low rotation speed 

of 200 rpm and a low thrust of 962 kg 

(2,121 lbs) were used. Past NIOSH 

research has shown that when drilling into hard materials, lower rotation 

speeds should be used (Peterson and 

Alcorn, 2007). The lower thrust was 

used so a longer drill time could be 

obtained. 

Figure 5 

Beamforming data from 1 to 6.3 kHz with vacuum drilling, 

round drill steel and a 34.9-mm (1.37-in.) drill bit. 

Results 

A baseline drilling measurement 

identified where noise was coming from 

and the frequency content. Beamforming results, shown in Fig. 5, indicated 

that the majority of drilling noise is radiated by two areas: at the drill bit and 

rock interface and at the drill steel and 

drill chuck interface. Figure 6 shows 

the one-third-octave band spectrum 

at the position of the roof bolting machine operator ear. The A-weighted 

sound level at the operator’s position 

was 99.7 dB. The frequency content of 

the noise radiated toward the operator 

was dominated by the 1,250 Hz through 

8 kHz bands. Also, the drill steel type 

and drilling depth had no effect on the 

beamforming results, as was expected 

based on past research at PRL (Yantek 

et al., 2007). 

Figure 6 

Baseline operator ear sound pressure level. 

Figure 7 

Bit isolator and chuck isolator. 

To reduce the sound level at the 

operator’s ear while drilling, noise 

controls must be developed that target 

the noise generated at the drill bit and 

rock interface and the drill steel and 

drill chuck interface. In addition, the 

controls must address the mid- to highfrequency components of the drilling 

noise. To reduce the radiated noise at 

both the bit and chuck interface, isolation techniques were used. A bit isola

tor was developed to reduce the noise 

radiated at the bit and rock interface, 

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shown in Fig. 7. A chuck isolator was 

developed to reduce the noise radiated at the drill steel bit and drill chuck 

interface, also shown in Fig. 7. Both 

isolators were designed to reduce the 

noise emitted during drilling by limiting the vibration transmitted down 

the drill steel from the drill bit/media 

interface. A urethane material with 

a durometer of 58 Shore D was chosen for both isolators in reducing the 

dominant frequency bands from 1,250 

Hz through 8 kHz. The chuck isolator 

used a LoveJoy coupler with the 58 

Shore D durometer urethane. 

Noise controls were examined 

alone and in combination in the hemianechoic chamber using the same test 

parameters as Table 1. Again, granite 

was used as the drilling media and the 

rotation speed of 200 rpm and a thrust 

of 962 kg (2,121 lbs) were used, as in 

the baseline test.The results at the operator’s ear showed noise reductions of 2 dB(A) with 

the bit isolator, 3.5 dB(A) with the chuck isolator and 7 

dB(A) in combination (Fig. 8). ­

Figure 8 

Operator ear sound pressure level.

Figure 9 

Beamforming results from 1 to 6.3 kHz at 200 rpm and 962 kg (2,121 lb) thrust in Granite. 

 Figure 9 shows the beamforming contour plots of the four scenarios from 1,000 to 

6,300 Hz. As expected, the bit isolator showed reduction 

at the higher frequencies, frequencies from 1,200 to 10,000 

Hz. However, the chuck isolator showed most of the re

duction from 1,200 to 3,150 Hz.When using the combina-
 

 

 

 

 

 

 

 

tion of the bit isolator and the chuck isolator the results 

were promising and showed a reduction in the dominant 

frequency bands, 1,250 Hz through 8 kHz, thus reducing 

the noise radiated toward the operator by 7 dB(A). 

Conclusion 

The drilling noise sources on a Fletcher HDDR roof 

bolter were identified by using the beamforming technique.Two areas radiate the majority of the drilling noise: 

the drill-bit/rock-interface and the drill-steel/drill-chuck 

interface. It was also determined that the drill-steel-type 

and drilling depth had no effect on the beamforming 

results. NIOSH laboratory results showed that the chuck 

isolator and the bit isolator had a significant influence on 

lowering the sound pressure level at the position of the 

roof bolting machine operator. In general, experimental 

results showed the chuck isolator to be more effective as 

a noise control, but the combination of both controls had 

the greatest effect on reducing the sound pressure level 

at the operator’s position. Noise controls demonstrated 

in this research are providing the mining community with 

an opportunity to reduce roof bolting machine operator 

noise overexposures. 

References 

Brüel & Kjaer, 1995, product data, website: http://www.bksv.com/pdf/ 

Bp1995_highres.pdf. 

Peterson, J.S., and Alcorn,L.A., 2007,“Results of noise measurements 

from underground testing of a roof bolting machine duty cycle,” Noise 

Con 2007. 

Peterson, J.S., Kovalchik, P.G., and Matetic, R.J., 2006, “Sound power 

level study of a roof bolter,” Transactions of Society for Mining, Metallurgy, 

and Exploration, Inc., Yernberg, ed., Vol. 320, Society for Mining, Metal

lurgy, and Exploration, Inc., Littleton, CO, pp. 171-177. 

Title 30 CFR Part 62, 2000-2005, U.S. Department of Labor, Mine 

Safety and Health Administration, Information Resource Center, Denver 

CO. 

U.S. Department of Labor Bureau of Labor Statistics, 2006a, website: 

http://www.bls.gov/iif/oshwc/osh/os/osnr0025.txt 

U.S. Department of Labor Bureau of Labor Statistics, 2006b, website: 

http://www.bls.gov/oes/current/oes475061.htm 

U.S. Department of Labor, Mine Safety and Health Administration, 

2005, Form 7000-2, “Quarterly Mine Employment and Coal Production 

Report,” website: http://www.eia.doe.gov/cneaf/coal/page/acr/table1.html 

Yantek, D.S., Peterson, J.S., and Smith, A.K., 2007, “Application of 

a microphone phased array to identify noise sources on a roof bolting 

machine,” Noise Con 2007. 

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