Sound power level study 

of a roof bolter

J.S. Peterson, P.G. Kovalchik and R.J. Matetic,

Research engineer, noise control team leader and hearing loss prevention branch chief, respectively, 

National Institute for Occupational Safety and Health, Pittsburgh, Pennsylvania

Abstract

The National Institute for Occupational Safety and Health (NIOSH) has initiated a study of a roof 

bolter to reduce noise exposure to mine roof-bolter operators. An important segment of this research 

entails determining the affect of various drilling configurations on the performance (penetration rate) 

and sound power emissions when drilling into granite. Test conditions included using various combinations of rotational speeds and thrusts, using 2.54- and 3.49-mm- (1- and 1 3/8-in.-) diameter hexagonal 

and round drill steels, using vacuum and wet conditions and using drill media of differing compressive 

strengths. This paper details the affects of each of these variables on the performance and sound power 

level emission. When drilling into granite, it was found that wet drilling generated lower sound power 

levels and performed better than vacuum drilling.

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% dosage (Fig. 1).

This paper presents results from a project that forms part of 

the latest effort by NIOSH’s Pittsburgh Research Laboratory 

(PRL) to control noise exposure in mining environments. Specifically, this paper concentrates on the noise emissions during 

the drilling cycle of a roof-bolting machine. This information 

is available for reference to designers of mining equipment, 

mining companies and MSHA. 

Background

Underground drilling is subject to many variables that can 

affect noise emissions. Some of these variables, such as the 

acoustic environment, the geometry and composition of the 

surfaces, the mine shape and the compressive strength of the 

drill medium, cannot be controlled. Other variables such as 

drill steel shape and bit size are controlled by the operator. 

The acoustic environment that the mining machines operate 

in is a critical factor in influencing the sound pressure levels to 

which workers are exposed. Underground mines are enclosed 

areas with reverberant sound fields. The geometry and the 

composition of the surfaces influence the overall sound level 

by reflecting or absorbing the incident sound energy. The 

compressive strength of the media being drilled affects the 

acoustic absorption properties of the mine environment. Harder 

media, with compressive strengths above 138 MPa (20,000 

psi), reflect more acoustic energy than softer media, i.e., less 

than 83 MPa (12,000 psi) compressive strength.

The operator is responsible for controlling the roof bolter, 

which may directly or indirectly affect the overall noise level 

generated while drilling. Thrust levels and rotation speeds supplied to the drill will also affect the sound power emissions. 

Variations of the thrust and rotation speed in different media will 

affect the performance of the drill, which is indirectly related 

to the sound power level emissions. For instance, stalling or 

jamming of the drill bit because of incorrect thrust levels or 

rotation speeds will create an increased sound power emission. 

Using the manufacturers recommended settings of the roof 

bolter for the drill media would enhance the performance of 

the drill, penetration rate and directly affect the sound pressure 

levels to which workers are exposed.

The roof bolter has several noise sources that may be considered worthy of examination. These include the vacuum 

pump, hydraulic pumps, drill chuck, drill steel and drill bit. 

The vacuum pump, hydraulic pumps and drill chuck will vary 

for different drill manufacturers. Both vacuum and hydraulic 
pumps contributes some to overall noise emissions. However, 

early tests at PRL confirmed that the hydraulic and vacuum 

pumps are not a major noise source for roof bolters.

Drill steels are manufactured either in hexagonal or round 

shapes. The steel can be manufactured at any length, but typical 

lengths are 1.2, 1.8 and 2.4 m (4, 6 and 8 ft). Bits are available 

in several types, usually designed for either wet or vacuum 

drilling. The sharpness of the drill bit will have considerable 

effect on the performance of the bolter, which will influence 

the sound power levels generated. Wet drilling, as opposed to 

vacuum, was expected to generate a lower sound power level 

because water attenuates higher frequency noise. 

These concerns prompted NIOSH to perform research to 

determine the effects of each of these factors on the performance 

and sound power emissions when drilling.

Technical approach

Sound levels to which roof bolter operators are exposed are 

determined by the sound power radiated from the drilling process 

and by the acoustic characteristics of the mine environment. 

The sound power is the quantity of most interest because it is 

a fundamental property of the drilling process and can be used 

for comparative purposes of differing drilling configurations 

independent of acoustical environmental conditions in any 

specific mine environment. Additionally, once the sound power 

level is determined, it is possible to predict the sound pressure 

level that the operator will be exposed to once the acoustic 

characteristics of the mine environment are understood. Sound 

level prediction research of this type is currently in development at PRL and is beyond the scope of this paper.

Test plan

Given the variation in underground mines, both geometrical 

and acoustical, it is difficult to achieve uniform test conditions. 

Therefore, testing was conducted in a reverberation chamber 

at PRL. This allows sound power-level determination in a 

known acoustic environment, and the results are independent 

of the variables associated with the acoustic properties of 

underground mines.

Environmental conditions in the test chamber are controlled 

so as to not influence test results in excess of the uncertainty 

stated below. The key environmental condition to control is 

relative humidity, which is controlled via humidifiers. The 

relative humidity is maintained above 65% at all times, so as 

to not affect the data at higher frequencies, i.e., 1,000 Hz or 

greater.

A survey of acoustics standards detailing accepted procedures for determining sound power levels in hard walled or 

reverberant test chambers led to the selection of ISO 3743-2 

(ISO, 1994) as the industry standard around which the testing was based. The objective is to ensure that sound power 

determinations are sufficiently repeatable to allow detection 

of changes in emissions due to the various test conditions 

and sufficiently reproducible to allow the data to be used 

for meaningful sound pressure level prediction in real mine 

environments. Measurement repeatability is a measure of the 

ability of laboratory to get the same result on the same test 

specimen within the laboratory and is determined through repeatability studies conducted by the laboratory. Measurement 

reproducibility is a measure of the uncertainly associated with 

test results from different laboratories and is stated in the test 

standard based on round robin studies conducted amongst 

acoustics laboratories.

The ISO 3743-2 test method has a stated standard deviation 

of reproducibility of 2.0 dBA, with uncertainty as a function 

of frequency as shown in Table 1. Internal quality-assurance 

studies in the PRL reverberant test facility for well-behaved 

sources indicated that the standard deviation of repeatability 

in the laboratory is on the order of 0.25 dBA. As the 95% 

uncertainly interval associated with both the repeatability 

and reproducibility of the method is estimated at 1.96 times 

the standard deviation, the PRL measurement program is capable of detecting changes to the sound power emissions of 

a specific device on the order of 0.5 dB and to determine the 

true value of the sound power emissions with an uncertainty 

of about 4 dBA.

A Bruel and Kjaer pulse system served as the data-acquisition 

system for the sound power-level determinations associated 

with this research. One-third octave band sound pressure levels 

are collected by the pulse system at 15 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 on the device 

Figure 1 — MSHA coal noise sample data — percentage of 

equipment whose operators exceeded 100% dose.

*Applicable to a relatively flat spectrum from 100 through 10 kHz.

125 5.0

250 3.0

500 through 4,000 2.0

8,000 3.0

A-weighted 2.0*

Octave-band center Standard deviation of

frequency, reproducibility, 

Hz σR

Table 1 — Estimated standard deviation of reproducibility of sound power levels.
under test. These sound pressure-level data are then used to 

calculate the sound power-level emissions for the device using 

the comparison method with the equation

	 LW = LWr + (Lp – Lpr) (1)

where

LW is the sound power of the device under test,

LWr is the sound power of a calibrated reference sound 

source,

Lp is the averaged sound pressure level of the device under 

test and

Lpr is the averaged sound pressure level for the calibrated 

reference sound source.

The sound power level is calculated for each one-thirdoctave band, and these values are logarithmically summed to 

calculate an overall sound power level.

Because roof bolter testing requires a rather large drill media 

(granite), as well as a large support stand, NIOSH constructed a 

steel support apparatus to hold the drilling media (Fig. 2). Care 

was taken to ensure that the stand did not radiate significant 

amounts of sound energy. To prevent this, the support stand, 

with the exception of its diagonal members and the short horizontal members along its minimum direction of the top of the 

structure, are filled with sand, and two layers of urethane are 

bonded to the rock support members below the drill media. 

Finally, an additional layer of urethane is laid between the 

drill media and chain holding the rock in place. All of these 

measures serve to reduce vibration transmissions into the stand 

and noise emissions from the stand surfaces. The device was 

placed near one corner of the chamber in accordance with the 

spacing requirements listed in ISO 3743-2.

Typically, testing of this type in conducted without anyone 

in the test chamber. However, because the roof bolter cannot be 

operated remotely, an operator was allowed in the reverberation 

chamber during testing. The high noise level in the chamber 

necessitated the operator wear double hearing protection, 

consisting of earplugs and earmuffs. 

In formulating a conservative test plan, it was decided to use 

drilling components and parameters that were representative of 

industry usage. These are listed in Table 2. Various combinations 

of these comprise the test configurations. Drill media of three 

compressive-strength ranges were tested: a low compressive 

strength material (concrete), an intermediate compressive 

strength material (sandstone) and a high compressive strength 

material (granite). These providing for a wide range of penetration rates with respect to noise emissions (Table 3).

Granite test results

The data listed in the following discussion are the average 

sound power levels (LwA) expressed in A-weighted decibels 

(dBA) and the penetration rates (in./sec) of two tests at a given 

test configuration. The sound power levels of these two tests 

varied by not more than 1.0 dBA. Further, all like tests (e.g., 

same steel shape and bit size) are averaged for a given thrust 

or rotation speed. This facilitates illustrating trends showing 

the affects of thrusts or rotation speeds on the data. 

Round drill steel compared to hexagonal drill steel. For 

vacuum tests and relative to thrust, the 25.4 mm (1-in.) round 

drill steel results generates a lower sound power than similar 

hex drill steel tests. The reverse is true for wet test results, 

though the differences are negligible (Fig. 3). While an increased thrust does yield an increased sound power level, the 

differences, again, are negligible.

As was the case with thrust, when using rotation speed 

as a comparative basis, the vacuum round drill steel results 

indicate a lower sound power than similar hex drill steel tests. 

For wet testing, there is no difference between the 25.4 mm 

(1-in.) and the 34.9-mm (1 3/8-in.) round and hex results. At 

lower rotation speeds, the 25.4-mm (1-in.) hex results showed 

Figure 2 — Drilling support fixture loaded with granite.

Drilling type vacuum, mist at 3 qt/min, wet at 3 gpm*

Drill steel round, hexagonal

Drill bit size 1 and 1 3/8 in. (25.4 and 34.9 mm)

Rotation speed, rpm 200, 300, 300, 400, 500, 600

Thrust, lbs 2,121, 2,828, 3,535, 4,242, 

 4,949, 5,656** and 6,363**

 *This report limited to vacuum and wet drilling.

**34.9-mm (1 3/8-in.) bit size only.

Item Values

Table 2 — Sound power level testing variables.

*This report limited to granite data.

Concrete 6,000

Sandstone 16,000 to 21,000

Granite* 24,000

Drill media Compressive strength, lbs

Table 3 — Drill media compressive strengths.
sound power levels less than the 25.4-mm (1-in.) round, but 

the advantage disappears at 500 and 600 rpm (Fig. 4). There 

was also the unexpected result where the sound power level 

decreased with increasing rotation speed for the 25.4-mm (1-

in.) vacuum tests.

As for performance, the penetration rate did not vary significantly when comparing round and hex data, whether using thrust 

Figure 3 — Round (solid) and hex (dash) LwA vs. thrust.

Figure 4 — Round (solid) and hex (dash) LwA vs. rotation 

speed.

Figure 5 — Round (solid) and hex (dash) penetration rate 

vs. thrust.

Figure 6 — Round (solid) and hex (dash) penetration rate 

vs. rotation speed.

Figure 7 — 25.4 mm (1 in.) (solid) vs. 3.49 mm (1 3/8 in.) 

(dash) LwA vs. thrust.

or rotation speed as a comparative basis (Figs. 5 and 6).

Recommendations: When drilling into hard materials having 

high compressive strengths, for vacuum drilling use round drill 

steels and use hex drill steels for wet drilling. While increasing 

thrust or rotation speed increases sound power emission, the 

affects are minimal. For performance, there appeared to be little 

difference between the round and hex drill steels.

25.4-mm (1-in.) diameter compared to 34.9-mm (1 3/8-in.) 

diameter. To date, there is little overlap for thrust data when 

comparing 25.4-mm (1-in.) and 34.9 mm (1 3/8-in.) data, 

but what can be seen is that there is a 1 to 2 dBA difference 

between the larger drill bits and the smaller drill bits, with the 

25.4 mm (1-in.) being slightly quieter (Fig. 7). 

Relative to rotation speed, the 34.9-mm (1 3/8-in.) LwA

results were slightly higher than similar results for 25.4-mm 

(1-in.) tests. These varied by up to 2.5 dBA (Fig. 8). 

Analysis showed that thrust has a significant affect on the 

penetration rate for all cases (Fig. 9). The 25.4-mm (1-in.) 

performance was on the order of two to three times that of the 

34.9-mm (1 3/8-in.) data, a significant difference.

Similar results are observed when comparing the data on a 

rotation speed basis. The 25.4-mm (1-in.) drill performed significantly better than the 34.9-mm (1 3/8-in.) drill (Fig. 10). 

Recommendations: When drilling into hard materials having high compressive strengths, 25.4-mm (1-in.) drill bits are 
slightly quieter than the 34.9-mm (1 3/8-in.) drill bits. It was 

shown that 25.4 mm (1-in.) bits drill significantly faster than 

34.9-mm (1 3/8-in.) bits. Increasing thrust will improve performance. As for rotation speed, lower values tend to perform 

better. It is advised to try lowering rotation speeds to the 200 

to 400 rpm range and then evaluate its affect on performance 

and noise exposure.

Vacuum drilling compared to wet drilling. Wet drilling 

proved to emit less noise than similar tests conducted under 

vacuum conditions. Figures 11 and 12 show vacuum and 

wet drilling tests A-weighted sound power levels plotted vs. 

thrust and speed, and in all comparative cases, the wet drilling 

produced sound power levels lower than vacuum drilling. The 

differences between the sound power levels for vacuum and 

wet drilling are shown in Figs. 13 and 14.

Much of this difference is attributable to the lubricating 

affect of the water, which attenuates higher frequency noise. 

An example of this effect is given in Fig. 15. Here, 25.4-mm 

(1-in.) round data tested at a thrust of 2,245 kg (4,949 lbs) and 

a rotation speed of 200 rpm are given for testing under both 

vacuum and wet conditions. For all one-third octave band 

frequencies of 1,000 Hz and greater, it is clearly shown that 

the sound power levels are greater for vacuum drilling. Four 

Figure 8 — 25.4 mm (1 in.) (solid) vs. 3.49 mm (1 3/8 in.) 

(dash) LwA rotation speed.

Figure 9 — 25.4 mm (1 in.) (solid) vs. 3.49 mm (1 3/8 in.) 

(dash) penetration rate vs. thrust.

Figure 10 — 25.4 mm (1 in.) (solid) vs. 3.49 mm (1 3/8 in.) 

(dash) penetration rate vs. rotation speed.

Figure 11 — Vacuum (solid) and wet (dash) LwA vs. 

thrust.

Figure 12 — Vacuum (solid) and wet (dash) LwA vs. rotation speed.

examples of this are listed in Table 4. For each case, the sound 

power level contributions for the one-third octave bands from 

50 through 800 Hz are essentially the same for vacuum and 

wet drilling, e.g., for the 25.4 mm (1-in.) round case they are 

95 and 96 dBA, respectively. There is a significantly larger 
Figure 15 — Round 1-in. (25.4-mm) vacuum and wet LwA, 

2,245 kg (4,949 lbs) thrust, 200 rpm.

1 in. round 4,949 50 through 800 95 96

 400 1 k through 10 k 113 108

1 3/8 in. round 6,363 50 through 800 96 96

 400 1 k through 10 k 115 109

1 in. hex 4,949 50 through 800 93 96

 400 1 k through 10 k 112 105

1 3/8 in. hex 5,656 50 through 800 97 95

 200 1 k through 10 k 114 107

Size and Thrust, lbs Frequency band, Vacuum Wet

shape Speed, rpm Hz LwA LwA

Table 4 — Sound power level contributions of two 

frequency bands.

A-weighted sound power levels are essentially unaffected by 

the sound power generated in the one-third octave bands below 

1 kHz. For each example given, the overall A-weighted sound 

power level is the same as the 1 through 10 kHz contributions, 

given rounding the sound power levels to the nearest dBA.

As for performance, i.e., the penetration rate, vacuum drilling 

did not perform as well as the wet drilling (Figs. 16 and 17). 

For each comparative case, the wet drilling penetration rate 

exceeded the vacuum drilling penetration rate, on occasion 

significantly. Referring back to Figs. 14 and 15, the increase 

in the penetration rate vacuum to wet drilling is given.

As mentioned above, increasing thrust tends to slowly 

increase the sound power level emission (Figs. 11 and 12), 

while the same cannot be said for increasing rotation speed. 

As evident in Figs. 16 and 17, increasing the rotation speed 

increases the sound power level emission but begins to negatively affect the drilling performance.

Recommendations: When drilling into hard materials having high compressive strengths, use wet drilling as opposed to 

vacuum, if possible, to decrease sound power level emissions 

and increase performance. Further, investigate lowering rotation speed to 200 to 400 rpm and then evaluate its affect on 

performance and noise exposure.

Conclusions

Drilling with water has a significant influence on lowering the 

sound power emissions of the roof bolter tested by NIOSH 

when compared to vacuum drilling. The primary affect is 

in attenuating noise at the frequencies above 1 kHz, which 

are the dominant contributors to the overall sound power 

level emissions. Thus, where conditions permit, it would 

be advantageous to explore wet drilling. For vacuum drilling, round drill steel testing resulted in slightly lower sound 

power emissions than hex drill steels, while the reverse was 

true for wet drilling.

In general, experimental results show that drilling performance increased as the thrust increased, while higher rotation 

speeds yielded diminishing results. It is suggested that when 

drilling into hard materials, lower rotation speeds should be 

attempted and their affects on performance should then be 

evaluated. Additionally, water increased the performance of 

the drilling as well. In terms of drilling performance, there 

appeared to be little difference between round and hex drill 

steels and 25.4 mm (1-in.) bits performed significantly better 

than 34.9-mm (1 3/8-in.) bits.

difference in the frequency range 1 through 10 kHz. For the 

25.4 mm (1-in.) round example, there is a 5-dBA difference. 

Similar results are listed for the other three cases.

Further analysis indicated another key point. The overall 

Figure 14 — Changes in LwA (solid) and penetration rate 

(dash), vacuum to wet drilling.

Figure 13 — Changes in LwA (solid) and penetration rate 

(dash), vacuum to wet drilling.
Figure 16 — Vacuum (solid) and wet (dash) penetration 

rate vs. thrust.

Figure 17 — Vacuum (solid) and wet (dash) penetration 

rate vs. rotation speed.

Future work

Future testing will entail completing mist system experiments for all three rock media and wet and vacuum testing 

in sandstone. 

Additional tests will also be conducted to fill in gaps in 

the data.

References

MSHA, 2000-2002, Mine Safety and Health Administration, Title 30 CFR, Part 

62, 2000-2002, U.S. Department of Labor, Information Resource Center, 

Denver, Colorado.

ISO, 1994, International Organization for Standardization, ISO 3743-2, “Acoustics – Determination of sound power levels of noise sources – Engineering 

methods for small, movable sources in reverberant fields – Part 2: Methods 

for special reverberation test rooms.”