In-cab noise reduction on an air-rotary drill rig 

David S. Yantek, David K. Ingram and R. 1. Matetic 

The National Institute for Occupational Safety and Health (NIOSH) has 

investigated engineering noise controls to reduce sound levels in cabs on airrotary drill rigs. A recent investigation revealed that some drillers are exposed to 

A-weighted sound levels exceeding 85 dB even though a cab is used. NIOSH 

studied the in-cab sound levels of one such rig. First, preliminary tests were 

conducted in a controlled environment using accelerometers and microphones 

with spectral analysis to identify the dominant noise sources for in-cab sound 

levels. The results indicate that vibration transmitted from multiple hydraulic 

pumps to the control panel produces a dominant spike in the sound level 

spectrum in the 400 Hz 1/3-octave band. Next, field tests were performed in a 

production environment to evaluate noise controls to reduce in-cab sound levels. 

It was found that utilizing hydraulic noise suppressors reduces the structureborne noise transmitted to the control panel. Further, using hydraulic noise 

suppressors and enhancing soundproofing reduced the in-cab A-weighted sound 

levels by as much as 4 dB. 

I INTRODUCTION 

Exposure to excessive noise over ti me can cause 

permanent hearing loss. In 1996, NIOSH published the 

National Occupational Research Agenda, which identified hearing loss as the most common job-related 

disease in the United States. \ The 1998 NIOSH publication, "Occupational Noise Exposure, Revised Cri teria" recommends limiting exposure to A-weighted, sound levels no greater than 85 dB fo r 8 hours : 

However, federal and most state regulatory guidelines 

are written such that workers should not be exposed to 

A-weighted, time-averaged sound levels above 90 dB 

for 8 hours. In either case, risk assessment research 

confinns that exposure to sound levels in excess of 

90 dB for 8 hours a day increases the risk of noiseinduced hearing loss (N II-IL). NIHL is pai nless, occurs 

over long periods of lime, and cannot be corrected 

medically. 

NIOSH investigations of the sound levels near drill 

rigs during production drilling identified air-rotary rigs 

as some of the loudest drill rigs used today.) These are 

high-production drilling machines that utilize 

compressed air to hammer-drill and to remove cunings 

from the hole. Many air-rotary rigs are outfitted with a 

diesel engi ne, an air compressor, a large cooling fan, 

and numerous hydraulic pumps. Interest in the sound 

levels in the cab on air-rotary rigs resulted from previous NIOSH investigations of drill rig noise, which 

showed the in-cab A-weighted sound levels exceed 

85 dB, and the growing concern about noise-induced 

hearing loss within the mining industry. 

It has long been recognized that hydraulic systems 

or components on equipment are potential sources of 

noise or vibration.4 5 

• One type of engineering control 

used with much success to reduce hydraulic noise and 

vibration is a hydraulic noi se suppressor. Studies by 

Beck and Martin6 and Wilkes and McLean' show that 

hydraulic systems are significant contributors 10 equipment noise and vibration. Noise and vibration generated by hydraulic systems has prompted the development of hydrau lic mufflers and noise suppressors. The 

implementation of these devices has produced favorable noise reductions in many working environments 

that have hydraulic systems. Based on these successful 

resu lts, NIOSH has investigated the application of 

hydraulic noise suppressors on air-rotary drill rigs to 

reduce the in-cab sound levels. This report describes 

the findings on the installation of hydraulic noise 

suppressors on one air-rotary drill rig. 

2 TESTING AND PROCEDURES 

The test rig (see Fig. I) is a track-mounted rig 

having a mass of 19,389 kg, a 347-kilowatt diesel 

engine, and an air compressor which delivers 
0.425 ml/s of air at a pressure of2.4 MPa. In addition, 

the test rig has a 1.6-meter-diameter cooling fa n, a dust 

collector, and a vibration-isolated cab. Several hydraulic pumps driven by the diesel engine are used to 

operate the drill rig. The hydraulic pumps are 

controlled by mechanical valves mounted directly to 

the control panel which forms the front of the cab. The 

cab has two sl iding doors: an inner door near the drill 

steel and an outer door on the opposite side of the cab. 

Fig. /- Track-molilltet/. air-rotmy drill rig. 

Pilot tests involved measuring the in-cab sound 

pressure level along with the sound levels near exterior 

noise sources. The in-cab sound level at the operator's 

ears was dominated by the sound energy in the 400 Hz 

II3-octave band as shown in Fig. 2. 

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100 

" 

90 

" • 

80 

75 

10 

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50 '

~ . 55 

.. 

45 

l 

1/3.Qctave-Band Center Frequency (Hz) 

Fig. 2-Average A-weighted 1/ 3-octave-band sound level spectrum at operator S left and righl ears for 

baseline test with drill rig al high idle. 

The narrowband 

spectrum of the in-cab microphone signal revealed a 

prominent spike at approximately 381 Hz. This 38 1 Hz 

spike was not observed in the sound spectra of the 

ex ternal microphones. 

It was suspected that the 381 Hz spike was due to 

transmission of vibration infO the cab. Vibration could 

be felt on both the hydraulic lines and the control panel. 

These observations led to the hypothesis that the high 

sound level in the 400 Hz 1 13-octave band was due to 

transmission of hydmulic noise into the cab via the 

hydraulic lines with subsequent radiation of noise by 

the control panel. Two batteries of tests, referred to as 

the Preliminary and Field Tests, were conducted to 

investigate the hydraulic noise. 

2.1 Preliminary Tests 

The preliminary testing, which was performed on a 

sealed asphalt test pad with the drill rig at high idle, 

consisted of five tests designed to determine if the 

dominant source of in-cab noi se was hydraulic-related. 

Table I shows the test descriptions for the five preliminary tests. The main objective of these tests was to 

determine with certainty that the tandem gear pump 

was the source of the 381 Hz spike in the sound level 

spectrum. 

The drill rig was placed on the test pad and allowed 

to warm up to stabilize fluid temperatures. Two accelerometers were mounted on the control panel inside the 
cab. One accelerometer was positioned near the middle 

of the control panel while the other was placed near the 

bottom of the control panel below several hydrau lic 

control levers. Two microphones were secured to a hard 

hat so that the microphones would be approximately 

0.1 meters from the side of the operator's ears. The 

signals from the transducers were recorded with a 

portable data acquisition system. The data were 

post-processed to calculate the A-weighted 1/ 3-octave 

band sound level spectra and un-weighted 1/ 3-octave 

band acceleration spectra with filters meeting 

IEC-225-196688 and ANSI S 1. 11-19869.9 The average 

1/ 3-octave band sound level spectrum was then calculated using the spectra from the left-ear and right-ear 

microphones. 

Table l- PrelimilialY tests on the drifJ rig. 

Test Number Test Description 

I Baseline test. 

2 	 Hydraulic line from tandem gear pump 

suspected as source of lonal noise 

disconnected from control panel. 

3 	 Thermal valve by-passed. 

4 	 Two O.3-metcr-long steel hydraulic lines added 

in series with existing lines to tandem 

pump. 

5 	 Two O.3-meter-long steel hydraulic lines added 

in series with existing lines to tandem 

pump plus O.l-metcr-thick layer of 

sound-absorbing foam added to roofof cab. 

For the baseline test, the overall average A-weighted 

sound level was found to be 87 dB with an 85 dB spike 

in the 400 Hz 1/ 3-octave band as shown in Fig. 2. A 

corresponding spike with an acceleration level of I dB 

was observed in the spectrum of the lower accelerometer signa l as shown in Figure 3. 

D ~-+--+--+-+~ -+-­

~ ~-+--+--+~~~-+-- Ci 

... -10 ~-+----'---+--+-++-+-­

: -15 r-+--,--t---'-t+-+f­

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I I I 

I I 

I 

I I I I I I I 

e:"g i i g ~ ~ g § ; g § ~ § § ~ ~ § § i § ~ r 113~ctave-Band Center Frequency = (Hz) 

Fig. 3- 0ne-third-octave-band acceleration level spectrum for lower accelerometer for baseline test with 

drill rig at high idle. 

This indicates that the 

spike in the 400 Hz 1/3 -octave band sound level 

spectrum is probably vibration-related. 

The narrowband spectra of the time signals from the 

baseline tests were then computed using a Hanning 

window with overlap processing (75% overlap) and a 

spectral resolution of 1.5625 Hz. The resu lting average 

narrowband sound level spectrum from 0 to 1000 Hz 

is shown in Fig. 4. A dominant spike with a sound level 

of 82 dB is present at approximately 381 Hz. This 

spike corresponds to the fundamental frequency for the 

hydraulic noise produced by the tandem gear pump, 

which has 10 teeth per gear and a rotational speed of 

2286 RPM with the drill rig at full idle. 

The vibration measurements showed that the lower 

accelerometer had higher accelerations than the upper 

accelerometer, particularly near 381 Hz. Therefore, the 

lower accelerometer signal was used with the in-cab 

microphones for vibration-noise coherence analysis. 

The coherence between the left and right microphone 

signals and the acce lerometer signals was calculated by 

(I) 'Y.'Y- [ G G :u y)' 

where r..y is the coherence, G..), is the cross-spectrum 

between the microphone and accelerometer signals, 
G.u is the auto-spectrum of the accelerometer signal, 

and Gyy is the aUlo-spectrum of the left or right microphone signa l.

10 The coherence function has a value 

bounded by 0 and 1. Coherence values approaching 

unity indicate a strong relationshi p between the 

assumed source (control panel vibration) and the 

receiver (the microphone). 

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O ~1001~ ~~~~~~~~~mm~~~m_ 

Frequency (Hz) 

Fig. 4- Baseline in-cab narrowband average sOlilul level. 

Lo_r Accelerometer Spectrum 

o 200 600 800 1000 1200 1400 1600 1800 

Coherence 

Fig. 5-Lower accelerometer narrowband RMS acceleration spectrum and coherence between lower 

accelerometer and right microphone f or baseline lest. 

The upper plot in Fig. 5

shows the acceleration spectrum for the lower accelerometer, wh ile the lower plot shows the coherence with 

the lower accelerometer signal as the input and the right 
microphone signal as the response. The lower plot 

indicates that the coherence approaches 1 at 381 Hz 

and several other peaks, indicating a strong relationship 

between the in-cab sound level and control panel vibration at these frequencies. 

The coherence function was then used to calculate 

the coherent sound level for both the left and right 

microphones by 

(2) 

where G"v is the coherent output spectrum between 

either microphone and the lower acceierometer. 1o The 

average coherent and measured sound levels are shown 

in Fig. 6. 

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.. ........ 

,. 

iii .. -­ 1 

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Frtqu.ncy (Hz) 

Fig. 6-Measured narrowband sound level and calculated coherent sound level wilh lower accelerometer 

as 'he reference. 

The data show that the coherent sound level is 

nearly equal to the measured sound level at the 381 Hz 

peak. This finding supports the hypothesis that the path 

of the noise from the hydraulic pump to the opcrator's 

ear is mechanical via the connection of the lines to the 

control va lves and the control panel. Since the coherent 

sound levels match the measured soimd levels at the 

most significant peaks throughout the frequency range, 

the data indicate that a strong relationship exists 

between the in-cab noise and structural vibration. 

During the second test, the sound and vibration 

measurements were repeated with the flow from one 

output of the tandem gear pump disconnected from the 

control valves and re-routed to the hydraulic tank. The 

data were analyzed in the same manner as the baseline 

test data. Figure 7 shows the A-weighted, 1/3-octave 

band sound level spectra for both the baseline test and 

the test with the line re-routed (Test 2). With the 

hydraulic line disconnected from the control panel, the 

average sound level in the 400 Hz 1/ 3-octave band 

dropped from 85 dB to 74 dB, and the overall sound 

level dropped from 87 dB to 83 dB . Figure 8 shows the 

1/3-octave band acceleration spectra for the lower 

accelerometer for the first and second tests. The accel

eration level in the 400 Hz 1/ 3-octavc band decreased 

from I to - 2 1 dB with the hydraulic line disconnected. These results indicate that the dominant noise 

source fo r the 400 Hz l / 3-octave band is the tandem 

10-tooth hydraulic pump. 

The thi rd test consisted of re-connecting the pump 

output to the control valves and bypassing a thermal 

valve at the suggestion of the drill rig manufacturer. 

With the thermal valve bypassed, neither the overall 

sound level nor the sound level in the 400 Hz 

1/ 3-octave band decreased. Clearly, bypassing the 

thermal valve was ineffective. 

Two 0.3-meter-long steel hydraulic lines were added 

in series wi th the existing hydrau lic lines 0.48 meters 

from the tandem pump outlet for the fourth test. It was 

thought that adding a stiff section in series with the 

existing flexible lines could reduce the vibration transmitted to the control panel due to an impedance 

mismatch. Sound and vibration measurement and 

analysis was performed as before. Figure 9 shows that 

the 400 Hz 1/ 3-octave band sound level was 83 dB for 

the fourth test compared to 85 dB fo r the baseline test. 

In addition, the overa ll sound level was 86 dB, a I dB 

decrease from baseli ne conditions. The acceleration 

­
level in the 400 Hz J/ 3-octave band dropped from 

I dB fo r the baseline test to - 6 dB with the steel lines 

added as shown in Fig. JO. However, the acceleration 

level in the 400 Hz 1/ 3·octave band with the line 

disconnected was - 2 1 dB, indicati ng that the added 

stitT lines do not achieve the max imum possible reduction in transmitted vibration. Experimentation wou ld 

have to be performed to determine the optimal length 

and location of the steel hydraulic lines. 

100 = ---1r-r . r , , 1­ . I . . . 

75 

~N 70 .. t- I --t­ , 

P , I - .. I50 U llI l 

11 1 ~ I ~ ; S $ e ! 8 ~ g8 g ~ § ~ ~ 8 § i § § § ~ § § i § i [ 

113-Qctave-Band Center Frequency (Hz) ­

Fig. 7- A-weighred 1/ 3-octave band sOl/lid level spectrum IV/pump Oll/pl/t Unes disconnected/rom valve. 

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at -15 

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~ ; a·s e ~ 8 ~ i 8 S ~ 8 ~ ~ 8 § ~ § § i ~ § § ~ § i [ 

1/3.()ctav ..Band Center Frequency (Hz) 

Fig. 8- 0ne-third octave band acceleration level spec/rum at lower accelerome/er w/pump ou/pu/lines 

disconnectedfrom valve. 

Finally, Test 5 consisted of the steel lines added for 

Test 4 with an additional O.I-meter-thick layer of 

sound-absorbing foam on the ceil ing of the cab. Once 

again, sound and vibration measurements and 

I 13-octave band analyses were performed. Nei ther the 

sound level in the 400 Hz 1/ 3-octave band nor the 

overall sound level changed with the additional acoustic foam. This result was not unexpected since the 
microphone location near the operator's ear is probably 

within the direct field or the noise radiated by the 

control panel. 

The 400 Hz 1/ 3-octave band acceleration levels at 

the lower accelerometer and the 400 Hz 1/ 3-octave 

band and overall sound levels for Test I, Test 2, and Test 

4 are summarized in Table 2. The data indicate that the 

best means of reducing the 400 Hz 1/ 3-octave band 

sound level is by addressing the vibration transmitted 

from the tandem gear pump to the control panel. 

)

! ... ~ .. f,,1-+-I1-+-I1-lI 

Fig. 9- 0ne-third octave band sound level wAl4-meter-Iong steel hydraulic lines added 0.48 meters 

}rom talldem pump outlet. 

· 

~ ~ ~ ~ ~ z i 	~ ! g g ~ 8 § ~ ~ 

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

Fig. IO-One-third octave balld acceleration levels wA}.4-meter-long steel hydraulic lines added 

0.48 meters/rom tandem pump outlet. 

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2.2 Field Tests

After several other options were considered, hydraulic noise suppressors, which decrease pump ripple, 

were selected to reduce the vi bration transmitted to the 

cab. A hydraulic noise suppressor is an in-line device 
consisting of a steel body which houses several perforated tubes surrounded by a nitrogen-pressurized 

rubber bladder. To achieve the maximum noise reduction, the charge pressure of the bladder is adjusted 

based on the operating pressure in the attached hydraulic line. 

Table 2-Summary ofTest Results for Test I, Test 2 and Test 4. 

Test 2­

Hydraulic linc 

disconnectcd TesI4-0A-metcr-long 

Test 1­ from control stccllines 

Measurement Baseline panel added 

Aeccl. Level in 400 Hz 113-octave 'Band - 21 - 6 

at Lowcr Accel. 

(dBrc l g) 

A-wtd Sound Level in 400 Hz I I3 -octave 85 74 8J 

Band 

(dB re 20 ,u Pa) 

A-wtd Overall Sound Level 87 83 86 

(dB re 20 ,u Pa) 

While the Preliminary Tests focused on the in-cab 

noise with the drill rig's engine at full idle without drilling, most of the time during production is spent 

hammer drilling with the engi ne at full idle. Therefore, 

field testing was conducted at a production site to 

exami ne the in-cab sound level spectrum under normal 

operating conditions. 

For a typical blasthole drilling cycle, the rig is 

moved to the desired hole location, the mast is raised, 

and the drill steel is positioned over the desired 

location. The machine is leveled using hydraulicallyactuated outriggers. The cab door may be opened to 

enable the operator to see the drill rod position relative 

to the hole location. Next, hammer drilling commences 

and Ihe cab door is closed. The dust collector system is 

used to mi nimize the operator's exposure to dust while 

drilling. When the drill rod is at its maximum depth, a 

second drill rod may be added to the first. Agai n, the rig 

is at high idle while a drill rod is added and the cab 

door may be opened. Prior to adding a drill rod, the rig 

operator may use the drill rod to flush debris from the 

hole. After addi ng the second drill rod, hammer drilling 

continues and the cab door is closed if it was opened. 

This process is repeated until the desired depth is 

reached. After drilling with the last rod is completed, 

the operator flushes the hole, removes the drill rods, 

lowers the mast, and moves the.rig to the next location. 

Two microphones were positioned near the operator's ears as with the Preliminary Tests. In addition, two 

acce lerometers were mounted to the control panel at 

simi lar locations as those used during the Preliminary 

Testing. The microphone and accelerometer signals 

were recorded while drilli ng a blasthole with the rig's 

engine at full idle. The recorded microphone signals 

were subsequently post-processed to calculate the 

A-weighted sound levels with a slow response. In 

addition, the narrowband sound and acceleration level 

spectra were calculated. It is important to note that, in 

addition to Ihe hammer drilli ng, other noise sources 

such as the dust collector, air compressor, and other 

hydrau lic components that are used during drilling 

were not operating duri ng the Preliminary Tests. These 

sources may be significant to the in-cab sound levels 

observed during production drilling. 

Examination of the narrowband in-cab sound level 

spectrum revealed that two spikes, one al 390 Hz and 

the other at 350 Hz, were now the main contributors to 

Ihe sound level in the 400 Hz 1/ 3-octave band (see Fig. 

II). 

" 

Fr.quency (Hz) 

Fig. II- hl-cab narrowband sound level spectrum 

for high idle portion ofblast hole drill~ 

ing cycle. 

Recall, the Preliminary Testing showed that the 

tandem gear pump was a significant contributor to the 
overall sound level, produci ng a spike at 38 1 Hz with 

the drill ri g's engine operating at 1800 RPM. For the 

Field Tests, the engine speed at full idle was observed 

to be approximately 1840 RPM. All pumps on the drill 

rig had an input speed of approximately 2300 RPM. 

The 10-tooth tandem gear pump would, therefore, 

produce its fundamental tone near 390 Hz. 

Identifying the source of the 350 Hz tone was more 

difficult. Severa l piston pumps on the drill rig have nine 

pistons, and each piston pump produces its fu ndamental tone at 350 Hz. By examining the hydraulic 

schematic the dust collector pump was identified as the 

piston pump most likely to create the 350 Hz tone. 

Since the dust collector can be turned on or off via a 

lever in the cab, a simple on/off test was conducted to 

verify that the 350 Hz tone originated at the dust 

collector pump. The 350 Hz peak was observed to 

appear with the dust collector on and to disappear with 

the dust collector off, verifyi ng that the dust collector 

pump was the source of the 350 Hz tone. 

With the main sources of the in-cab hydraulic noise 

identified, three noise suppressors were insta lled: one 

suppressor at each of the tandem gear pump outlet 

ports and another at the dust collector pump outlet port. 

Since the attenuation depends on the noise suppressor 

charge pressure, a series of vibration measurements 

were performed with various charge pressures for each 

suppressor. The real-time frequency spectra of the 

accelerometer signals were measured, and the acceleration levels at the tandem and piston pump fu ndamental 

frequencies were monitored. The mos.t effective charge 

pressures for the slow feed and outrigger ports suppressors of the tandem pump were found to be 

2 10- 410 kPa and 4\0- 1,240 kPa, respectively. The 

best attenuat ion of the dust collector fundamental 

frequency occurred with the dust collector suppressor 

charged to 6,200 kPa. 

Sound and vibration measurements were conducted 

wi thout and with the noise suppressors installed. Three 

holes were drilled without and with the suppressors 

installed and charged to the previously discussed 

charge pressures. A Personal Digital Assistant (PDA) 

was used to record the drill rig operations (hammer 

drilling, setting up, rig al high id le, etc.) and whether 

the cab doors were opened or closed. The drill rig was 

located on a bench such that the rig was parallel to the 

high wall at a distance of approximately 15 meters. It is 

important to note that drilling this close to the highwall 

may increase the sound level in the cab while drilling 

due to the reflections from the highwall. 

After acquiring the data, each recording for a single 

hole was split into multiple files according to the drill 

rig operation and whether the doors were opened or 

closed. The PDA data were used in conjunction with 

listening to the recordings to categorize each recording 

as follows: 

hammer drilling, doors closed 

hammer drilling, door(s) opened 

high idle, doors closed 

high idle, door(s) opened. 

After the recordings were categorized, the average 

1/ 3-octave band sound levels and the un-weighted 

1/ 3-octave band acceleration levels were calculated for 

each resulting data file that was longer than 

10 seconds. Finally, a global average for each of the 

four categories was ca lculated by weighting each 

spectrum according to the percentage of time fo r the 

specific file relative to the total time for the category in 

question. For example, if a single file accounted for 

10 seconds of the tolal time spent hanuner drilling with 

a door open, and the lolal time spent hammer drilling 

with a door open was 1800 seconds, the resulting 

spectrum would account for 0.56% of the average 

spectrum for hammer dri ll ing with a door open. 

Figure 12 shows a comparison between the 

1/ 3-octave band acceleration levels at the lower accelerometer when hammer drilling without and with the 

suppressors. The figure shows that the acceleration 

level in the 400 Hz 1/ 3-octave band decreased slightly 

whereas the acceleration level in the 315 Hz 

J 13-octave band increased by a few dB with the 

suppressors. These changes may be due to di lTerences 

in drilling conditions between the measurements 

without and with the noise suppressors. Significant 

reductions in the acceleration levels occurred in the 

630 Hz to 6300 Hz 1/3-octave bands with the noise 

suppressors installed. The resulting J 13-octave band 

acceleration levels with the rig at high idle with the 

doors closed wilhout and with the hydraulic suppressors is shown in Fig. 13. The figure indicates that the 

acceleration level in the 400 Hz 1/ 3-octave band 

increases slightly and the acceleration level in the 

315 Hz l / 3-octave band increased by a few dB. These 

changes may be due to a small variation in the engine 

RPM for each measurement. The vibration levels in the 

1000 Hz 10 5000 Hz 1/3-oclave bands are significantly reduced with the noise suppressors installed. The 

reductions at the higher frequencies are probably due to 

attenuation of harmonics of the tandem pump fundamental frequency. 

The 1/ 3-octave band acceleration levels near the 

dust collector control lever without and with the 

suppressors for hammer drilling and high idle are 

shown in Figs. 14 and 15, respectively. Both figures 

show that a substantial reduction in acceleration occurs 
in the 315 Hz through 630 Hz 1/ 3-oclave bands. In 

addition, significant attenuation is shown for the higher 

frequency bands. 

.. ! 

CD ·10 

::" -15 H-+-+++-t--+-+::tI 
 ~ ·20 H+ - if1f++ 


§ .25 ,_,.,'_ F 

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:& -30 

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... ~ ~ S ~ ~ ! g ~ ! g ~ ~ 8 § ~ 8 § ~ § § ~ i i ~ i 

-

1/3-Octa~~.nd Canter Ff*luency (Hz) 

Fig. 12- Comparison oj 1/ 3-octave band acceleration levels 011 lower confrol panel w/o and w/lloise 

suppressors during hammer drilling. 

... 

113-Octave-Band Center Frequency (Hz) 

Fig. J3- Comparisoll 0/ I / 3-octave balld acceleration levels 011 lower COI/lro! panel w/o alld IV/noise 

suppressors at high idle. 

Figures 16 and 17 show the average I 13-octave band 

sound levels without and with the suppressors during 

hammer drilli ng with the doors closed and at high idle 

with the doors closed. Figure 16 shows that a 6-dB 

reduction occurs in the 400 Hz 1/ 3-oclave band with a 

I-dB reduction in the overall sound level. Due 10 

hammer drilling, the sound level spectra in Fig. 16 

exhibit a hump from the 630 Hz to 3150 Hz 

1/ 3-octave band which dominates the spectrum. For 

these tests, the dri ll rig was within 15 meters of a 

high wall causing the drilling noise to be reflected back 

to the cab, thereby increasing the significance of the 

drilling noise. If the drilling noise entering the cab 
could be blocked by improving the cab design or ifthe 

noise generated by drilling could be reduced, using the 

suppressors has the potential to reduce the overall 

sound level by more than I dB while drilling. Figure 17 

indicates that usi ng the noise suppressors reduces the 

400 Hz 1/ 3-octave band sound level by 7 dB and the 

overall sound level by 4 dB with the rig at high idle. In 

addition, significant reductions occurred in the 

630 Hz to 6300 Hz I/3 -octave bands. 

• r-H-H-H---+-H-It-I 

o -+ 

1f3-Octave-Band Center Fl1Iquency (Hz) 

.2 .25 I--' -i-t -+- /"

1-30 J-3 • 

... 

... 

... ~ ~ t ~ C Z g ~ ~ 8 ~ ~ g § ~ g § ~ § § § j I I i 

Fig. 14- Comparison oj I / 3-octave band acceleration levels near dust collector control lever wlo and 

w/noise suppressors during hammer drilling. 

10 

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1f3-OctaYe-Band Center Frequency (Hz) 

Fig. 15- Comparison of 1/ 3-oc/ave band acceleratioll levels near dusl collector control lever wlo and 

w/noise suppressors 01 high idle. 

During testing, a gap of approximately I em in 

width that allows drilling noise to enter the cab was 

noticed between the inside door and the cab. The rubber 

seal in this area was not compressed even when the 

inside door was closed and latched. A quilted fiberglass 

blanket with a lead septum was draped over the 

cab/inside door interface to determine if the gap was a 

significant sound transmission path. Figures J8 and 19 

show comparisons of the average 1/ 3-octave band 
sound levels without the hydraulic suppressors, with 

the hydraulic suppressors, and with the hydraulic 

suppressors plus the lead-fiberglass blanket during 

hammer drilling and high idle with the cab doors 

closed. Figure 18 shows that using the lead-fiberglass 

blanket sign ificantly reduced the sound levels in the 

500 Hz through 10 kHz 1/ 3-Qctave bands. In addition, 

adding the blanket reduced the overall sound level 

during hammer drilling by an additional 2 dB. Figure 

19 shows that the sound levels in the 1 kHz through 

10 kHz 1/ 3-octave bands with the drill rig at high idle 

with the doors closed were substantially reduced by 

adding the lead-fiberglass blanket. However, the overall 

sound level was only reduced by I dB since the 315 Hz 

and 400 Hz l / 3-octave bands dominate the spectrum. 

The results of these tests are summarized in Table 3. 

~~tseli&igs~§~~I§~§~~~ilili[ 

1l3-Octave~nd Center Frequency (Hz) 

Fig. J6--Comparisoll of 1/ 3-octave band sound levels wlo am/III/noise suppressors during hammer 

drilling wilh cab doors closed. 

ii 

~ 

5 

" c 

l 

J 

j 

! 

+ 

~ ~ t ~ C ! i &i 8 ~ ~ § § ~ 8 § ~ § § § i 

tl3-Octaw-Ihnd Centw F,..quency (Hz) 

Fig. J7-Comparison oj 1/ 3-octave band sound levels wlo and wlnoise suppressors at high idle with cab 

doors closed. 
1l3-Octave&nd Cenwr F,..quency (Hz) 

Fig. 18- Comparison of 1/ 3-octave band sound levels w/o and w/nohie suppressors (Ind w/noise suppressors and lead-./iberglass blanket during hammer drilling with cab doors closed. 

iD 

! 

~ 

i

, 

.:! 

1 

~ 

•

•
 4: 

CWIIh NolM 

_WllhMoIH 

75 . t 

70 

"~ 

.. 

I 

§ E§ - ~ " ~ ~ t ~ C ! - ~ §~§88S8888~. 

113-Octava&nd Cant... F,..qlHlncy (Hz) = 

Fig. 19- Comparison of 1/ 3-octave band sound levels wlo and IVlnoise suppressors and w/noise suppressors and lead-fiberglass blanket at high idle wir/r cab doors closed. 

3 DRILL RIG OPERATOR'S NOISE 


EXPOSU RE 


The results rrom the Preliminary and Field Tests 

show that multiple hydrau lic pumps on the tested 

air-rotary rig are the dominant contributors to the 

in-cab noise during drilling operations. In addition, a 

gap between the inner door and the cab was shown to 

be a path ror drilli ng noise to enter the cab. Further, the 

test results show that installing hydraul ic nOise 

suppressors and eliminating the gap will reduce the 

overall in-cab sound levels, thereby reducing the drill 

rig operator's noise exposure. The cab on this dri ll rig 

used sliding doors with a rubber seal. It appeared that 

the inside door was bent on trus machine creating a gap 

along the cab-door interrace. Gaps around doors are 

highly detrimental to the ability or a cab to block 

­
Table 3- Comparisoll 0/accelera/ion and sound levels wi/hou/noise suppressors, with noise suppressors, 

and with noise suppressors pIllS leadjiberglass blankets/or high idle and hammer drilling with 

the cab doors closed. 

High Idle-Doors Hammer DrillingMeasurement 

Closed Doors Closed 

Without With Without With 

Aced. Level in 400 Hz 1 J3-octavc Band 
 5 5 7 9 

at Lower Aced. 


(dBrelg) 


Accel. Level in 400 Hz 1/3-octave Band 
 9 9 - 7 

at Aced. Ncar Dust Collector Conlrol 


Lever 


(dBrclg) 


A-wid Sound Level in 400 Hz 1/ 3-octave 
 88 81 86 80 

Band 


(dB rc 20 }I Pa) 


Accel. Level in 630- 6300 Hz 1/3-octave 
 - 12 - 17 - II - 18 

Bands at Lower Accel. 


(dB rclg) 


Accel. Level in 630-6300 Hz 1/ 3-octavc 
 - J - 12 -8 - 14 

Bands at Accel. Near Dust Collector 


Control Lever 


(dBrc l g) 


A-Wid Sound Level in 630- 6300 Hz 1/ 3­ 86 81 97 94 

octave Bands 


(dB rc 20 II Pa) 


Ovcrall, A-wtd Sound Lcvel 
 91 86 98 95 

(dB re 20 }I Pa) 


120r-~----~'-'-'-'-~==~--'-'---'-'---'-, 

I Hammerl~~::::::::j:1 j =-..:I_J,. Ii 115 

- - - -

~f-' ~'I o" l 

- - - - - - - - -

-

"­.. 


~ 110 

~ 

;.:, ;.:, 

~ ~ !? Co " " 0 " 0 0 

0 0 " " " " 
 " " " " " 


Hammering ~-- Hammering 


Doors Closed Doors Closed 


~ 

~ ~ 0 ~ ~ ~ 0 ~ ~ 0 

"

Co " " " " " Co " " " " " Co 

2J Co 

" Co " Co " Co 0 0 0 0 0 0 0 0 0 0 0 

" " " " " " " " " " " 
 " " " Time " " " of Oay " (HH:MM:SS) " " " " 


~ 

Co ~ Co ~ Co ~ ~ ~ " 0 -

" " ~ ~ 

" Co 0 0 " Co 0 0 " Co 0 0 

" " " " " " " " " " " " 


Fig. 20-ln-cab sound levels measured with a slow re:,ponse fo r during the drilling cycle. 
sound. For example, consider a 0.6-me!er-wide 

X 1.2 meter-hi gh door with a sound transmission loss 

of 25 dB at a particular frequency. A I-cm gap along 

the width of the door would reduce the overall transmission loss of the door by more than 4 dB. Therefore, 

elimination of gaps around doors and windows is 

essential to achieve a cab with a high sound transmission loss. 

Figure 20 shows an example of the sound level time 

history measured using a slow response while drilling a 

blasthole with no added engineeri ng noise controls. 

The tramming, setting-up, flush ing, and drill rod 

removal processes are referred to as 'high idle ' in Fig. 

20, since no drilling is taking place, the engine is at full 

idle, and the sound levels and spectra are similar for 

these processes. ' Hammering' refers to when the rig is 

being used to drill the hole. The horizontal bars labeled 

' Hammering Doors Closed' indicate the time the 

operator spent hammer drilling with the cab doors 

closed. As expected, the sound levels are highest while 

drilling with the door open, reaching sound levels as 

high as 115 dB. When hammer dritling with the doors 

closed, the sound level varied from about 93 to 99 dB 

for this particular hole. 

Table 4 shows the average in-cab sound levels and 

the percentage of time spent with the dritl rig operating 

at high idle with the door(s) open, at high idle with the 

doors closed, hammer drilling with the doors open, and 

hammer dri lling with the doors closed. 

Table 4- A-weighted sound level and percenlage of 

lime al each mode ofoperalion. 

A-wtd Sound Level 

Mode of Operation (dB re 20 JlPa) % ofTime 

Hammering, doors closed 98 82% 

Hammering, door open 112 5% 

High idle, door closed 9 1 8% 

High idle, door open 95 5% 

As the table 

shows, the majority of time was spent with the drill rig 

hammering with the doors closed. However, the sound 

level for hammer drilling with the door open is substantially higher than il is with the doors closed. Therefore, 

it remains difficult to know which mode of operation is 

the most significant contri butor to the operator's noise 

exposure because noise exposure is a function of both 

the sound level and the duration of exposure. 

Noise exposure is measured in dose and is a function 

of the threshold sound level, criterion sound level, 

criterion duration, and exchange rate in addition to the 

sound level at the worker's ear and the actual exposure 

time. The dose may be calculated by 

D = IOO.!. I0 (l i- l ,)( IOlOG It12)/Q (3) 

To 

where D is Ihe percentage of the atlowable dose, Tis 

the exposure lime in hours, Tc is the criterion duration, 

L; is the sound level for the jlh event, Lc is the criterion 

level , and Q is Ihe exchange rale in dB. II Sound levels 

below Ihe threshold sound level do not add to Ihe dose. 

Table 5- Eslimated noise dose for MSHA PEL (90 dB LT' 90 dB Lc. 5-dB exchange I'llle). MSHA He? 

(80 dB LT, 90 dB Lc. 5-dB exchange I'llte), and NIOSH (80 dB LT, 85 dB Lc, 3-dB exchange 

I'llle) dosimeter sellings, assuming 6 hours ofdrilling and 2 hours ofquiellime during an 8-hr 

work shift wilhoul noise suppressors. 

% of Hours AVG 

Drilling in 8-hr Sound MSHA MSHA NIOS H 

Mode of Operation Time shift Level. dB PEL Dose HCP Dose Dose 

Hammer drilling, 82% 4.92 98 186% 186% 1240% 

doors closed 

Hammer drilling, 5% 0.30 112 79% 79% 1920% 

door open 

High idle, doors '8% 0.48 91 7% 7% 24% 

closed 

High idle, door open 5% 0.30 95 8% 8% 38% 

Non-dril ling activity 0% 2.00 <80 0% 0% 0% 

Total Dose 280% 280% 3222% 

Typically, worker noise dose is measured using a 

noise dosimeter with settings corresponding to the 

MS HA Permissible Exposure Level (PEL) or MSHA 

hearing conservation program (HCP) settings. Bolh the 

MSHA PEL and Hep seuings use a criterion level of 

90 dB, a criterion time of 8 hours, and an exchange 

rale of 5 dB per doubling (or halving) of lime. The 

MSHA PEL settings use a threshold level of 90 dB 

whereas the MSHA Hep setti ngs lise a threshold of 
80 dB. NIOSH recommends using a threshold of 

80 dB, a criterion level of 85 dB, and a 3-dB exchange 

rate. 

The estimated noise dose due to each operating 

condition and the tota l dose for the drill rig operator 

setti ngs were calculated based on these dosimeter 

readings, assuming that drilling was performed for 

6 hours during an 8-hour work shift and that no 

additional dose was accumulated in the remaining 

2 hours. The results of these calculations are shown in 

Table 5. As the table shows, in this instance the drill rig 

operator was overexposed no matter which metric was 

used. Due to the use of a 3-dB exchange rate and an 

85-dB criterion level, the dose calculated using NIOSH 

settings is significantly higher than the dose calculated 

with either MSHA setti ng. For the MSHA settings, 

hammer drilling with the doors closed accounts for the 

majority of the operator's noise exposure. Hammering 

with the door open is the most significant mode of 

operation in terms of noise exposure with the NIOSH 

settings. 

Table 6-Estimared lIoise dose for MSHA PEL (90 dB LT. 90 dB Le, 5-dB exchange role), MSHA Hep 

(80 dB LT. 90 dB Lc. 5-dB exchange role), alld NIOSH (80 dB LT. 85 dB Le, 3-dB exchange 

rtlle) dosimeter seltillgs, assuming 6 hours ofdrilling and 2 hours ofquiet time during all 8-hr 

work shift wirhouilloise suppressors. 

% of Hours AVG 

Drilling in 8-hr Sound MSHA MSHA NIOSH 

Mode of 0renation Time shift Level. dB PEL Dose HCP Dose Dose 

Hammer drilling, 82% 4.92 97 162% 162% 984% 

doors closed 

Hammer drilling, 5% 0.30 112 79% 79% 1920% 

door open 

High idle, doors 8% 0.48 87 0% 4% 10% 

closed 

High idle, door open 5% 0.30 95 8% 8% 38% 

Non-drilling activity 0% 2.00 <80 0% 0% 0% 

Total Dose 249% 253% 2951 % 

Table 7- Estimated noise dose for MSHA PEL (90 dB LT, 90 dB Le. 5-dB 

exchange rate) . MSHA He? (80 dB LT. 90 dB Lc. 5-dB exchange 

rate). alld NIOSH (80 dB LT. 85 dB Le. 3-dB exchange rare) dosimeter settings. assuming 6 hours ofdrilling and 2 hours ofquiet 

time during an 8-hr work shift with noise sllppressors inslatted and 

a lead-jiberglass blanker covering Ihe gap between/he inside door 

and the cab frame. 

Mode ofO~ra tion 

% of 

Drilling 

Time 

Hours 

in 8-hr 

shift 

AVG 

Sound 

Level, dB 

MSHA 

PEL Dose 

MSHA 

HCr Dose 

NIOSH 

Dose 

Hammer dri lling, 

doors closed 

Hammer dri ll ing, 

door open 

High idle, doOrs 

closed 

High idle, door open 

Non-drilling activity 

82% 

5% 

8% 

5% 

0% 

Total Dose 

4.92 

0.30 

0.48 

0.30 

2.00 

95 

II ' 

86 

9, 

<80 

123% 

79% 

0% 

8% 

0% 

210% 

123% 

79% 

3% 

8% 

0% 

213% 

620% 

1920% 

8% 

38% 

0% 

2585% 

The dose for each drilling operation and the total 

dose were calculated usi ng the results of the lesting 

with the noise suppressors installed and with the noise 

suppressors plus the lead-fiberglass blanket blocking 

the gap between the inside cab door and the cab body. 

Tables 6 and 7 show the calculated dose with the noise 

suppressors installed and with the noise suppressors 
installed wilh the lead-fiberglass blanket covering the 

gap. With the suppressors, the calcu lated noise dose 

dropped by 31 % for the MSHA PEL settings, 27% for 

the MSHA Hep settings, and 271 % for the NIOSH 

settings. The calculated noise dose with the suppressors 

and lead-fiberglass blanket decreased by 70% for the 

MSHA PEL settings, 67% for the MSHA HCP 

settings, and 637% for the NIOSH settings. With the 

treatments installed, hammer drilling with the doors 

closed remained the main contributor to the noise dose 

calculated with the MSHA settings. With the NIOSH 

setti ngs, hammer drilling with a door open remained 

the most significant contributor to the noise dose. A 

further reduction of the noise dose would require either 

a reduction in the dri ll ing noise or an improvement in 

the cab attenuation. 

4 SUMMARY 

A series of tests was conducted to identify the 

dominant noise sources for the in-cab sound levels of 

an air-rotary drill rig. A tandem gear pump and a piston 

pump were identified as significant sources of in-cab 

noise. Three hydraulic noise suppressors, one on each 

outlet port of the tandem pump and one on the output 

of the dust collector piston pump, were installed in-line 

with the existing hydraulic lines. Field measurements 

with the drill rig at high idle showed that the in-cab 

A-weighted sound level near the operator's ear was 

reduced by 4 dB with the noise suppressors installed. 

When the rig was hammer drilling, however, the in-cab 

sound level was reduced by only I dB. In this instance, 

drilling noise within the cab is dominant. Both reflec

tions from the nearby highwall and the presence of a 

gap at the cab/inside door interface increase the significance of dri ll ing noise relative to the in-cab sound 

level. Covering the gap with lead-fiberglass blankets 

wi th the noise suppressors installed reduced the in-cab 

sound level wh ile hammer drilli ng by 3 dB compared 

to baseline conditions. This indicates that eliminating 

the gap is critical to reduce the sound level inside the 

cab. 

The percentage of time spent during each drill 

function and the A-weighted sound levels for each rig 

function were used 10 estimate the operator's noise dose 

based on MSHA PEL, MSHA HCr, and NIOSH 

recommended dosimeter settings for an 8-hour 

workday with 2 hours of quiet time when no dose was 

­

­

accumulated. For the base line machine, the estimated 

noise doses according to the MSHA PEL, MSHA HCP, 

and NIOSH settings were 280%, 280%, and 3222%, 

respectively. With the hydraulic noise suppressors 

install ed these va lues were reduced to 249%, 253%, 

and 2951%. Finally, adding the lead-fiberglass blanket 

to seal the gap with the suppressors reduced the 

estimated noise doses to 210%, 213%, and 2585%. 

These reduclions correspond 10 reductions of 25% for 

MSHA PEL settings, 24% for MSHA HCP settings, 

and 20% for NIOSH settings. The reductions may have 

been higher if the dri ll rig was not positioned as close 

to a highwall, since reflections of drilling noise from a 

high wall significantly increase the contribution ofdrilling noise 10 Ihe in-cab overall A-weighted sound level 

thereby decreasing the importance of hydraulic noise. 

If the hydraulic noise is reduced by insta lling the 

hydraulic noise suppressors, improving the sound 

transmission loss of the cab to block airborne drilling 

noise may reduce the operator's noise exposure below 

Ihe MSHA PEL settings. 

5 	 REFERENCES 

!. 	 National [nstinne for Oecupationa[ Safety and Hcalth, "National Occupational Rcsearch Agenda," DHHS (NIOSH) Publication No. 96-115, (1996). 

2. 	 National Institute for Occupational Safety and 1·lea[th, "Occu

pational Noise Exposure Revised Criteria:' DHHS (N IOSI-!) 

Publication No. 98- 126 (1998). 

3. 	 David K. Ingram and R. J. Matetic. "Are You OpeT'dting an Air 

Rotary Drilling Rig? [s It LOUD?" National Ground Water As

sociation. fl(uer )fellj. 57(7), 18- 22. (2003). 

4. 	 J. E, Millcr, "Silencing the Noisy Hydraulic System," Mach. 

Des. 45(14),138- 143, ( 1973). 

5. 	 A. L Hitchcox, "Nipping Noisc in the Bud:' Hydraul. Pl1ellm. 

50(10).77- 78, (1997). 

6. 	 A. Beck and H. R. Martin, "Hydraulic Mufflers for Noisc and 

Vibration Control," Proc. 38th Annual National Confcrence on 

Fluid Power. Sponsored by Illinois Institutc ofTechnology, 36, 

33-41, (1982). 

7. 	 Wilkes and McLcan, "Hydraulic Noise Gcts Suppresscd," Design Engineering 80, 24- 25. (1996). 

8. 	 International EJectrote<:hnical Commission, lEC-225-1966. Octave. half-octave and third-octave band filte rs intendcd for thc 

analysis of sounds and vibrations (1996). 

9. 	 American National Standards Institute. ANSI Sl.I t- 1986. 

Spccification for Octave-Band and Fraetional-Octavc-Band 

Analog and Digital Filters, (1986). 

10. J. S. Bcndat and A. G. Picrsol. Engineering Applications of Corrclation and Spectral Analysis, John Wiley & Sons, New York, 

NY (1993). 

II. 	American National Standards Institutc. ANSI 512.19- 1996. 

Measuremcnt ofOccupalional Noise Exposure, (1996). 

­

­