A technique for estimating the sound power 

level radiated by pneumatic rock drills and 

the evaluation of a CSIR prototype rock drill 

with engineering noise controls 

by P.G. Kovalchik*, F.T. Duda*

* NIOSH, Pittsburgh, Pa., USA. 

, and G.S. Harper† 

† CSIR Miningtek, Johannesburg, South Africa. 

Synopsis 

Overexposure to noise remains a widespread, serious health hazard 

in the US mining and other industries despite 25 years of regulation. 

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 in mining. 

The National Institute for Occupational Safety and Health 

(NIOSH) is conducting research to reduce the noise exposures of 

jackleg drill operators and to prevent additional cases of noise 

induced hearing loss (NIHL) by developing and evaluating low-cost 

retrofit noise controls for equipment. This report describes the 

procedure for the measurement and reporting of noise from portable 

pneumatic tools such as jackleg drills. The technique used in this 

research allows for the determination of the source A-weighted 

sound power levels and the radiation patterns in octave and 1/3 

octave bands. Overall sound power level is also determined. This 

paper also reports the results obtained by using this procedure to 

evaluate a SECO S215 standard production drill and a CSIR 

Miningtek prototype rock drill incorporating engineering noise 

control measures. 

It was found that by using the manufacture’s recommended 

operating pressure of 496 kPa (72 psi) that the CSIR prototype’s 

sound power was 10 dB(A) less then that of the SECO S215. 

Introduction 

The pneumatic rock drill is one of the most 

severe noise sources in mining operations. The 

operation of these drills produces A-weighted 

noise levels in the range of 100 to 120 dB at 

the operator location. These sound levels are 

unacceptable since they are high enough to 

cause NIHL. This type of rock drill is powered 

by compressed air which is discharged at 1500 

to 2500 cycles per minute causing a series of 

noise pulses. 

Several investigations1–3 have determined 

that the noise sources of pneumatic rock drills 

can be classified into three areas: exhaust 

noise, drill steel noise and mechanical noise 

from the drill components. The exhaust noise 

is generated by spent air passing from the 

exhaust opening at high velocity and mixing 

with the ambient air. Also, the exhaust air can 

create mechanical noise from within the drill. 

The drill steel noise is caused by the vibration 

generated by the impact between the drill 

piston and the drill steel. Also, the noise of the 

drill bit impacting against the rock causes the 

drill steel to ring. Further mechanical noise is 

generated by the rotation mechanism and the 

interaction of internal parts. 

While each of the significant noise sources 

within a conventional rock drilling system has 

been addressed in previous work by many 

researchers, few of the solutions have been 

successfully implemented. However, previous 

work has shown that the problems that 

restricted implementation of these solutions 

can be resolved if the thrust is directed along 

the axis of the drill, i.e. ‘in-line’ thrusting. 

According to experiment and underground 

measurements ‘in-line’ thrust can provide for 

performance increases of up to 100 per cent, as 

well as facilitate the drilling of straight holes, 

thereby minimizing contact between the drill 

steel and the hole. In order to provide ‘in-line’ 

thrusting a concept was developed by which a 

rock drill with minor modifications became a 

piston within what is effectively an enlarged 

thrust-leg tube. 

Noise control of existing pneumatic drills is 

difficult to accomplish. Factors that have to be 

considered are cost, performance, and weight. 

Muffling of exhaust has been done to varying 

degrees for noise attenuation but is only 

marginally successful. Because of the exposure 

of the operator to high noise levels, the need 

exists for quieter pneumatic drills. 

This report documents the evaluation 

procedure and the evaluation performed by 

NIOSH on a SECO S215 rock drill and a SECO 

S215 rock drill with noise controls developed 

by CSIR Miningtek3. This work is in support of 
the NIOSH mission to reduce NIHL among mineworkers. 

NIOSH has recognized NIHL as one of the 10 leading workrelated diseases and injuries in the United States, and has 

emphasized its importance as one of the critical areas 

expressed in the National Occupational Research Agenda. 

Measurement method 

The method used in this evaluation consisted of sound 

pressure level measurements on a surface enveloping the 

noise source, i.e. the enveloping surface method. The acoustic 

environment for the measurements of the source was a freefield over a reflecting plane. The measurements were then 

used to estimate the sound power level at each frequency 

band of interest. The overall A-weighted sound power level 

was then determined by logarithmically adding the sound 

power levels in each frequency band. The surface enveloping 

method allows for three grades of accuracy: precision grade, 

engineering grade, and survey grade. Engineering grade 

accuracy was implemented in this evaluation following ISO 

37444 (see Appendix A). For the sake of completeness, Table 

I presents the general characteristics of the engineering grade 

accuracy used in this evaluation. 

As specified in ISO 3744, a hypothetical rectangular 

parallelepiped reference box was first defined. The reference 

box used was the smallest one that completely enclosed the 

drill ignoring protruding elements that are not significant 

noise radiators. The reference box terminated on the 

reflecting floor surface. 

Since the noise measurements were performed while 

drilling, the rock or block tested was considered as part of the 

source when defining the reference box. On the other hand, 

air and water hoses were not included as part of the source. 

The measurement surface was also a hypothetical 

rectangular parallelepiped whose sides were parallel to the 

reference box. The measurement surface was defined by the 

Table I 

Characteristics of engineering Grade 2 accuracy 

enveloping surface method to estimate sound 

power level 

Parameter Engineering Grade 2 accuracy 

Volume of noise source No restrictions—limited by available 

test environment 

Test environment Outdoors or indoors 

Criterion for suitability of 

test environment 

K2 ≤ 2 dB in each frequency band 

Characteristics of noise Any (broadband, narrow-band, discrete 

frequency, steady, non-steady, impulsive) 

Limitation of background 

noise 

ΔL ≥ 6 dB in each frequency 

band K1≤1.3 dB 

Number of measuring points ≥ 9 

Instrumentation Complying with type 1 

Precision of method for 

determining LWA expressed 

as standard deviation of 

reproducibility 

σ ≤ 1.5 dB 

K1 = correction factor for background noise 

K2 = test environment correction factor based on room absorption 

measurement distance d, which is the normal distance 

between the reference and measurement box surfaces. The 

measurement surface also terminated on the reflecting floor 

surface. The measurement distance d can be 0.25, 0.5, 1, 2, 

4, or 8 metres with a preferred distance of 1 metre. Figure 1 

shows an example of the reference and measurement 

surfaces. The dimensions of the measurement surface are 

length 2a, width 2b, and height c. 

The microphones were positioned on the measurement 

surface. Nine microphones were used which satisfied the 

minimum number of microphones requirement for 

engineering grade. 

The position of the microphones is a function of the size 

and shape of the reference box. The procedure for the 

placement of the microphones was as follows: 

Path 2 Path 3 Reference box 

Path 1 

Reflecting plane 

Figure 1—Example of reference and measurement parallelepiped 

surfaces. • Microphones 

➤	 Each side of the surface measurement box was 

subdivided by the minimum number of equal-sized 

rectangular areas with a maximum length of side ≤ 3d. 

➤	 Microphones were then placed at the centre and each 

corner of each of the equal-sized rectangular areas. 

Microphones were not placed at the corners on the 

reflecting plane. 

Microphone locations that are in the direct path of the air 

exhaust outlet of the pneumatic machine were avoided since 

they can lead to erroneous measurements. Figure 2 shows 

the nine microphone locations on the measurement surface 

used in this study. Microphone 10 is shown in Figure 2, 

located near the operator’s ear, but this microphone was not 

used in this study. This paper documents only the evaluation 

and the estimated sound power levels radiated by a Boart 

Longyear SECO S215 rock drill and a Boart Longyear SECO 

S215 rock drill with noise controls developed by CSIR 

Miningtek. 

Measurements 

For pneumatic rock drills, the major noise sources are the 

drill itself and the drill steel. Therefore, the measurements 

were conducted while drilling into rock or a concrete block5. 

In order to assure repeatable measurements, the starting time 

of the noise measurement was taken as the time when the 
drill bit was at least 0.1 m (3.9 inches) into the rock or 

concrete. The measurement time was at least 15 seconds 

while the drill bit was penetrating the rock or concrete block5. 

If feasible, the measurement time for frequency bands ≤ 160 

Hz were at least 30 seconds. 

The time-average sound pressure level measurements 

were performed in octave or 1/3 octave bands. The octave 

measurements at least included all the bands between 63 and 

8000 Hz, i.e. bands numbers 18, 21, 24... and 39. The 1/3­

octave measurements at least included all the bands between 

50 and 10000 Hz, i.e. band numbers 17 through 40. The 

overall time-average sound pressure level (full frequency 

range) was also measured. 

The time-average background noise level was recorded in 

all the frequency bands selected for measurement at all 

measurement locations. The overall background sound level 

noise was also recorded. 

All sound pressure level measurements were A-weighted. 

The recorded signals were post processed and the sound 

power levels were calculated for each test. 

Test procedure 

Two SECO S215 rock drills were tested at NIOSH’s Pittsburgh 

Research Laboratory (see Figures 2, 3, and 4). The testing 

facility used at Pittsburgh Research Laboratory was 

essentially a free-field environment over a reflecting plane in 

a large bay area. Drill one was a SECO S215 standard 

production rock drill with a SECO air leg. Figure 3 shows the 

drill being evaluated. 

Figure 2—The microphone array setup for the parallelepiped measurement surface 

Figure 3—Drilling with the SECO S215 

Figure 4—CSIR modified drill 

Drill two was the CSIR prototype drill developed by CSIR 

Miningtek. The drill was a SECO S215 with engineering noise 

control modifications resulting in an enlarged thrust-leg tube. 

Details of the engineering noise control modifications are not 

available at this time. Figure 4 shows the prototype drill 

ready for evaluation. 
Both drills were tested using a button bit with a 1.22 

metres (4 ft.) drill steel under normal loading. A controlled 

operation was used in order that repeatability could be 

achieved. Because rocks are a natural material, their 

compression strengths may vary. This makes test repeatability difficult to achieve. Granite was procured with a 

consistent compressive strength of 165.473 MPa (24000 psi). 

Also, a concrete block having a consistent compressive 

strength of 41.368 MPa (6000 psi) was manufactured. It 

should be noted that the button bit is not typically used for 

concrete drilling, but was used in testing to maintain bit 

consistency during drilling6. Also, having two different media 

for drilling provided additional information on the 

penetration rate with respect to noise. The drill being tested 

should always be tested in the medium it was designed for in 

order to validate the drill penetration rate. Testing in soft and 

hard media allows for comparison with other drills at a later 

time. The SECO S215 is designed for a hard medium, such as 

granite. 

The measuring system consisted of 9 microphones and a 

Racal A480 Digital Tape Recording System, which is capable 

of performing measurements in 1/3 octave bands. The 

recorded sound pressures were post-processed using 

commercially available software to determine the sound 

pressure levels in 1/3 octave bands. The tests were set-up 

according to ISO 3744 as outlined in the previous sections. 

Figure 2 shows an example of a typical set-up. For any given 

test, all measurements were made simultaneously. 

The US Department of Trade and Industry’s ‘A Guide for 

Manufacturers to the Evaluation of Uncertainties’7 was used 

as a guideline when formulating the test plan for determining 

the sound power levels of both drills to ensure that repeatable 

results would be obtained. Therefore, the test plan consisted 

of 5 measurements for each test parametre unless 2 out of 3 

measurements were within 1 dB of each other at each 

microphone location. In that case, the remaining two 

measurements were not conducted. The parametres that were 

varied for the jack leg drill consisted of thrust pressure and 

water flow rate. The factory recommended thrust pressure 

and water flow rate were 496 kPa (72 psi) and 7.57 

litres/minute (2 gallons / minute). In order to evaluate how 

the drill would perform in overload and underload conditions, 

the drill was tested at ± 69 kPa (10 psi) and ± 3.79 

litres/minute (1 gallon/minute). Therefore, 3 thrust pressure 

levels and 3 water pressure levels were used. This resulted in 

9 different combinations of test parametres with 5 

measurements for each combination totaling 45 

measurements for each drill and for each drilling material. In 

addition to the drilling tests, non-drilling tests were also 

conducted. These tests consisted of running the drill without 

drilling at the different thrust pressures without water. 

Test results 

Sound levels heard by a drill operator are determined both by 

the sound power radiated by the drill and by the acoustic 

characteristics of the mine environment. The sound power is 

the quantity of most interest. Once the sound power is 

known, one can predict the sound level that the operator is 

exposed to based on the acoustic characteristics of the 

environment. Sound power gives a direct comparison of noise 

generated by any drill tested under the same conditions. 

The sound power radiated by the SECO S215 rock drill 

and the modified SECO S215 rock drill (CSIR prototype) were 

determined for normal operating conditions. For each test 

hole drilled, sound power and the penetration rate were 

determined. Since the test results showed an overall environmental K factor of -.31, -.31 was added to the surface sound 

pressure level. Calculations of surface sound pressure level 

were determined from the equation below: 

Lpf = surface sound pressure level 

L’p = measured sound pressure level 

K1 = correction factor for background noise 

K2 = test environment correction factor based on room 

absorption 

The surface sound pressure level was then used to 

calculate the sound power level. 

Also, it should be noted that the water amount used for 

flushing the material out of the hole had no effect on noise 

levels or penetration rates. The goal was to compare both 

drills under the same conditions for sound power and 

performance. 

Results for drilling in concrete demonstrated that the CSIR 

prototype sound power was reduced by 10 dB(A) as 

compared to the SECO S215. This is shown in Figure 5. 

However, the penetration rate or performance of the CSIR 

prototype decreased for all operating pressures. This can be 

seen in Figure 6. 

Figure 5—Sound power comparisons while drilling in concrete 

Figure 6—Performance comparisons while drilling in concrete 

Using the manufacturer’s recommended operating 

pressures of 496 kPa (72 psi), for the SECO S215, the tests 

results showed that the CSIR prototype sound power level 

was 10dB (A) less then the SECO S215. The penetration rate 
also dropped for the CSIR prototype by almost .203 metres 

per minute (8 inches per minute). However, it should be 

noted that the SECO S215 was designed for a hard medium 

and not a soft medium like concrete. 

Results for drilling in granite indicate that the CSIR 

prototype’s sound power level was reduced by 10 dB(A) as 

compared to the SECO S215 (Figure 7). However, the 

penetration rate of the CSIR prototype did not drop as much 

as when drilling in concrete and, in fact, it outperformed the 

SECO S215 at 552 kPa(80 psi). This can be seen in Figure 8. 

Figure 7—Sound power comparisons while drilling in granite 

Figure 8—Performance comparisons while drilling in granite 

In summary, it was found that by using the 

manufacturer’s recommended operating pressure of 496 kPa 

(72 psi), the CSIR prototype’s sound power was 10 dB(A) 

less then that of the SECO S215. Also, the penetration rates 

of both drills were within 6% of each other. This indicates 

that the noise control was effective without sacrificing 

performance. By increasing the thrust pressure to 552 kPa 

(80 psi) the CSIR prototype actually outperformed the SECO 

S215 and the sound power level was still reduced by 10 

dB(A). 

While conducting the tests on the CSIR prototype, it was 

noted that the type of oil used with the automatic oiler was 

critical to the proper operation of the drill. The type of oil 

required is dependent on the ambient temperature. If the 

wrong oil was used, the drill would stall. The CSIR prototype 

was cumbersome and required two people to set it up. First, a 

support hole for the drill had to be drilled into the working 

face. The two people then had to manoeuvre the drill in order 

to insert the support rod of the drill into the newly drilled 

support hole. The feed pressure to the CSIR prototype was 

critical to its proper operation. Approximately 69 to 103 kPa 

(10 to 15 psi) was required. When the feed pressure 

exceeded 103 kPa (15 psi) the drill stalled. When the feed 

pressure was too low, it would not drill. 

Conclusions 

Test results demonstrated that the flow rate of the water used 

for flushing the material out of the hole had no effect on 

sound power level or the penetration rate of either drill. The 

SECO S215 performed best, having a penetration rate of 

0.439 metres/minute (17.3 inches/minute) while drilling in 

concrete at the recommended operating pressure of 496 kPa 

(72 psi). However, when drilling in granite, the SECO S215 

penetration rate dropped to 0.315 metres/minute (12.4 

inches / minute) for both operating pressures of 496 (72 psi) 

and 552 kPa.(80 psi). Granite is a hard medium and is more 

representative of the medium for which the SECO S215 rock 

drills were designed. The sound power level of the SECO 

S215 had little change for any of the tests ranging from 120 

to 123 dB(A). 

The CSIR prototype performed best while drilling in 

granite. When drilling in granite with an operating pressure 

of 552 kPa (80 psi), it out performed the SECO S215 with a 

penetration rate of 0.353 metres/minute (13.9 inches/ 

minute) as compared to 0.315 metres/minute (12.4 

inches/minute) for the SECO S215. The sound power level for 

the CSIR prototype was consistently 10 dB(A) below that of 

the SECO S215 with the same operating conditions. It 

appears that the CSIR prototype is potentially a viable 

engineering noise control for rock drills. However, there are 

some issues that need to be addressed: the set-up 

requirements and the feed pressure. The CSIR prototype is 

too difficult for one person to operate and would take too 

long to set up underground. The feed pressure needs to have 

a presetting range for the operator based on the material 

conditions. 

References 

1.	 SUMMERS, C.R. and MURPHY, J.N. Noise Abatement of Pneumatic Rock Drill. 

Bureau of Mines Information Circular 7998, 1974. pp. 1–45. 

2.	 VISNAPUU, A. and JENSEN, J.W. Noise Reduction of a Pneumatic Rock Drill. 

Bureau of Mines Information Circular 8082, 1975. pp. 1–23. 

3.	 HARPER, G.S. and SCANLON, T. Develop a quiet non-atmosphere polluting
 

blast hole drilling system. Final Report, Safety in Mines Research
 

Advisory Committee. GEN 207 November 1997. pp. 1–70.
 

4.	 Acoustics—Determination of sound power levels of noise sources using
 

sound pressure—Engineering method in an essentially free field over a
 

reflecting plane. ISO 3744, Second edition 1994-05-001. pp. 1–31.
 

5.	 Drill rigs—Safety. EN 791:1995. pp. 1–47. 

6.	 ALJOE, W.W., STEIN, R., and BARTHOLOMAE, R.C. Test Apparatus for
 

Measuring Sound Power Levels of Drills. Bureau of Mines Information
 

Circular 9166, 1987. pp. 1–35.
 

7.	 A Guide for Manufacturers to the Evaluation of Uncertainties. The Noise 

Emission in the Environment by Equipment for Use Outdoors Directive, 

2000/14/EC, URN 00/605, First edition July 2000. pp. 1–11.