A NOISE CONTROL FOR A ROOF BOLTING MACHINE: 

COLLAPSIBLE DRILL STEEL ENCLOSURE 

Rudy J. Matetic1

Peter G. Kovalchik2

J Shawn Peterson3

Lynn A Alcorn 4

National Institute for Occupational Safety and Health 

Pittsburgh Research Laboratory 

626 Cochrans Mill Rd. 

Pittsburgh, PA 15236 

1

 Email address: RMatetic@cdc.gov 

2

 Email address: PKovalchik@cdc.gov 

3

 Email address: JPeterson@cdc.gov 

4

 Email address: LAlcorn@cdc.gov 

ABSTRACT 

The National Institute for Occupational Safety and Health (NIOSH) at the Pittsburgh 

Research Laboratory (PRL) is conducting research to reduce the noise exposures of roof 

bolting machine operators and to prevent additional cases of Noise Induced Hearing Loss 

(NIHL) thru the development of engineering noise controls. This paper describes and 

evaluates an engineering noise control for the roof bolting machine, namely the 

collapsible drill steel enclosure. The results of the evaluation demonstrated that using the 

collapsible drill steel enclosure provided a 7 dB(A) reduction in sound pressure level at 

the operator’s position of a roof bolting machine. 

1. INTRODUCTION 

The drilling of rock in a confined work environment contributes to high levels of noise 

exposure in the mining industry. The Mine Safety and Health Administration (MSHA) 

coal data from 2000 to 2005 has shown that roof bolting machine operators were second 

among all underground coal mining equipment operators who exceeded 100% dosage, 

per the MSHA Permissible Exposure Limit (PEL) [1]. According to the Bureau of Labor 

Statistics (BLS) approximately 3,930 roof bolting machine operators are employed in 

coal mining [2]. 

Roof bolting machines are underground drilling machines used to implement a mine roof 

control plan. All underground coal mine operators are required to develop and submit 

roof control plans to MSHA for evaluation and approval. These plans provide the 

necessary requirements and placement of roof supports to eliminate any potential ground 

control issues [3]. A common method used in a roof control plan for attaching lower 

level rock strata to upper layers is to use a roof bolt and epoxy resin to seal the layers of 

rock strata. A roof bolting machine trams to the required location and drills a hole 

several feet into the mine roof, as described by the roof control plan. After a hole is 

 
placed in the roof, a plastic tube with epoxy resin is inserted into the hole. A roof bolt is 

then inserted in the hole and rotated by the roof bolting machine tearing the plastic tube 

of epoxy resin and mixing the resin to the bolt and the surrounding rock layers. The 

epoxy resin typically "sets up" or hardens within a couple of seconds; attaching the bolt 

and rock layers to each other. In most underground mining situations, a roof bolt is 

placed approximately every four feet in the mine. The roof bolt and epoxy combination 

are a conventional mean to provide primary roof support. The roof bolting machine 

operator will repeat this process as needed to comply with the requirements of the 

underground roof control plan for a respective mine. 

The roof bolting machine operator is exposed to several noise sources from two 

processes, one is drilling and the other is bolting. The dominant noise sources are a result 

of the drilling process [4]. 

To reduce noise exposure caused by roof bolting machines generated during the drilling 

process a collapsible drill steel enclosure has been developed. Specifically, this paper 

describes the collapsible drill steel enclosure, the measurement method used to evaluate 

the enclosure for noise reduction, and the results. 

2. COLLAPSIBLE DRILL STEEL ENCLOSURE 

The collapsible drill steel enclosure, Figure 1, consists of a round aluminum coated 

fiberglass bellows with a spring enclosed inside the bellows. 

Figure 1: The collapsible drill steel enclosure installed on a roof bolting machine 

Fiberglass was chosen because it provides good acoustical absorptive properties, has 

excellent heat resistance and is incombustible. In addition the aluminum coated 

fiberglass can not absorb moisture so it will not decay and is resistant to acids, oils, many 

solvents, and corrosive vapors. Fiberglass with an aluminum cover allows the material a 

95% reflective heat property that makes it a very good material in high heat use. The 

fiberglass bellow is 1.905 cm thick by 19.685 cm outside diameter and the extended 
length is 1.2192 m; however, diameters up to 1.8288 m and extended lengths up to 6.096 

m can be procured (Figure 2). 

Figure 2: Fiberglass bellows used in the collapsible drill steel enclosure 

The spring is a straight cylindrical compression spring made of .635 cm of round wire 

with a load of .303 kN. Also, the spring is 1.0668 m in length with a total of 16 coils. 

The outside diameter is 15.8242 cm for each coil (Figure 3). 

Figure 3: Cylindrical compression spring used in the collapsible drill steel enclosure 

3. MEASUREMENT METHOD 

To effectively develop and evaluate engineering noise controls for large mining equipment, 

such as a roof bolting machine, the NIOSH / PRL has a large reverberation chamber that is 

designed and equipped to determine sound power level emissions (Figure 4). The Acoustic 

Test Chamber (ATC) is at the PRL and yields scientifically valid and reliable testing of 

engineering controls because the influences of varied equipment and operator underground 

environments can be controlled in the chamber. The ATC has been NVLAP accredited for 

sound power level testing per the ISO 3741 / ANSI S12.51 standard for broadband precision 

grade measurements [5]. 
Figure 4: The ATC facility was designed to determine the sound power level emissions 

of large mining equipment. 

The nominal room dimensions are 18.3 meters long by 10.3 meters wide with a height of 

6.7 meters. The chamber volume is 1,286 cubic meters. These dimensions are important, 

as few reverberation facilities are not large enough to test large mining equipment. 

A Bruel & Kjaer Pulse system serves as the data acquisition system for the sound power 

level determinations. One-third octave band sound pressure levels are collected by the 

Pulse system at fifteen microphone locations within the reverberation chamber and these 

data are dynamically linked to an Excel spreadsheet in real time. In Excel, these data are 

logarithmically averaged to calculate an average sound pressure level for the test. These 

measurements are conducted for a calibrated reference sound source and then for the 

device under test. These sound pressure level data are then used to calculate the sound 

power level emissions for the device using the comparison method [5]. 

4. MEASUREMENTS 

Testing was conducted in the ATC with a Fletcher HDDR roof bolting machine, shown 

in Figure 4. Since the dominate noise sources are a result of the drilling process, the 

measurements were conducted while drilling. Granite blocks were used as the drilling 

media for the tests because prior experience has shown that using granite reduces test-totest variability [4]. Also, a low rotation speed of 200 rpm and a low thrust of 9.4 kN. was 

used. The lower thrust was used so a longer drill time could be obtained. A new drill bit 

was used for each hole also to reduce test-to-test variability. 

Drill bits and drill steels that were representative of industry usage were used for testing. 

Commonly used drill steels are either round or hexagonal in shape and are available to be 

used with either 2.54 cm or 3.49 cm drill bits (Figure 5). 

Figure 5: Hex and round drill steels for a 3.49 cm diameter drill bit (upper) and two 

examples of drill bits (lower). 

Because the roof bolting machine measurements require a rather large drill media 

(granite), a steel support apparatus had to be constructed to hold the drilling media. This 

large steel support stand is comprised of rectangular tubes to hold the drilling media, 

shown in Figure 6. To prevent the support stand from radiating 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 performed 

to create a vibration impedance mismatch in the structure to reduce vibration 
transmission. 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 layer of urethane was 

placed between the rock and the chain used to support the rock media. 

Figure 6: A large steel support stand comprised of rectangular tubes to hold the drilling 

media. 

The test plan was to compare the sound power results generated by drilling into granite 

and comparing the sound power results when using the collapsible drill steel enclosure 

under the same conditions. In addition, tests were conducted with varying gaps between 

the top of the collapsible drill steel enclosure and the bottom of the drill media. The 

varying gaps would determine how tight of a seal was needed between the collapsible 

drill steel and the rock media in reducing sound power levels. Gap lengths of 15.24 cm, 

10.16 cm, 7.62 cm, 5.08 cm and no gap were tested. This resulted in 6 different 

combinations of test parameters, including the baseline test, with 3 measurements for 

each combination totaling 18 measurements. 

5. RESULTS 

Sound power gives a direct comparison of noise generated by any device tested under the 

same conditions. Therefore, determining a sound power level is an effective way of 

evaluating the collapsible drill steel enclosure as a noise control. 

Baseline results for drilling in granite demonstrated that the sound power was at 111 

dB(A). Figure 7 illustrates the one-third-octave-band spectrum for drilling in granite at 

200 rpm and a thrust of 9.4 kN. The frequency content of the noise radiated is dominated 

by the 1250 Hz through 8 kHz bands. 
Granite sound power level

40

50

60

70

80

90

100

110

120

5080

125200315500800

12502000315050008000

Overall

One-third octave band center frequency (Hz)

LwA (dBA)

Baseline

Figure 7: One-third-octave-band spectrum for drilling in granite at 200 rpm and a thrust 

of 9.4 kN. 

Results when using the collapsible drill steel enclosure indicate that the sound power 

level was reduced by 6 dB(A) with gaps of 10.16 cm, 7.62 cm, 5.08 cm, and no gap. 

However, with a 15.24 cm gap the sound power level was only reduced by 5 dB(A). 

Figure 8 illustrates the one-third-octave-band spectrum for drilling in granite at 200rpm at 

a thrust of 9.4 kN for all configurations. It can be seen from Figure 8 that all 

configurations were effective in reducing the dominant frequencies bands from 1250 Hz 

through 8 kHz. 

Granite sound power level

40

50

60

70

80

90

100

110

120

5080

125200315500800

12502000315050008000

Overall

One-third octave band center frequency (Hz)

LwA (dBA)

Baseline 15.24 cm gap 10.16 cmgap 7.62 cm gap 5.08 cm gap no gap
Figure 8: One-third-octave-band spectrum for all configurations while drilling in granite 

at 200rpm and a thrust of 9.4kN. 

Since the collapsible drill steel enclosure displayed promise in the laboratory, the next 

step would be to conduct an underground field evaluation for durability and noise 

exposure reduction. Recommendations for the underground field evaluation would be to 

use dosimeters to determine the amount of acoustic noise that a roof bolting machine 

operator is exposed to during a work shift with and without the collapsible drill steel 

enclosure. 

6. CONCLUSION AND RECOMMENDATIONS

NIOSH laboratory results showed that the collapsible drill steel enclosure had a 

significant influence on lowering the sound power level of the roof bolting machine. The 

one-third-octave-band spectrum for drilling is dominated by the 1250 Hz through 8kHz 

bands and the collapsible drill steel enclosure was effective in reducing these frequencies 

bands. In addition, the gap from the top of the collapsible drill steel enclosure to the 

bottom of the rock media did not have to be a good seal. With a gap of 15.24 cm, the 

collapsible drill steel enclosure was still an effective noise control. The sound power 

level for the collapsible drill steel enclosure was consistently 5 to 7 dB(A) below that of 

baseline levels of the roof bolting machine without any noise controls and the same 

operating conditions. It appears that the collapsible drill steel enclosure is potentially a 

viable engineering noise control for roof bolting machines. 

REFERENCES 

[1] Title 30 CFR Part 62, 2000-2005, U.S. Department of Labor, Mine Safety and Health 

Administration, Information Resource Center, Denver Co. 

[2] U.S. Department of Labor Bureau of Labor Statistics, May 2006, web site 

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

[3] U.S. Department of Labor, Mine Safety and Health Administration, web site 

http://www.msha.gov/regs/fedreg/informat/2006icr/e6-97.asp

[4] 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. 

[5] Peterson JS, Bartholomae RC [2003]. Design and instrumentation of a large 

reverberation chamber. In: Proceedings of NOISE-CON 2003. Ames, IA: Institute of 

Noise Control Engineering of the USA, pp. 8.