Practical application of a partial cab to 

reduce the A-weighted sound level at the 

operator’s station on surface drill rigs

D.S. Yantek, P. Jurovcik, Jr., and D.K. Ingram

Research engineer, mechanical engineer and health communications coordinator, respectively, 

National Institute for Occupational Safety and Health, Pittsburgh, Pennsylvania

Abstract

The National Institute for Occupational Safety and Health (NIOSH) identified hearing loss as the most 

common job-related disease in the United States in its 1996 National Occupational Research Agenda. 

Previous studies by NIOSH have shown that operators of surface drill rigs without cabs are commonly 

exposed to A-weighted sound levels exceeding 90 dB. During hammer-drilling, A-weighted sound levels 

exceeding 100 dB were recorded at the operator station. NIOSH collaborated with a local drilling company to design and install a partial cab between the operator and the drill steel in an effort to reduce the 

sound level at the operator station. Sound level measurements in the field show the partial cab reduces 

the A-weighted sound level by 5 to 9 dB at the operator’s left ear and 2 to 5 dB at the operator’s right 

ear while hammer-drilling. This paper briefly discusses the preliminary design of a cab and the testing 

of materials for construction of the prototype. An explanation of the ideas behind the prototype design 

and fabrication are covered. Finally, the field test method and results are discussed in detail.1

Introduction

Noise, which is any unwanted sound, is present throughout 

the mining industry. Continued exposure to high noise levels 

can cause damage to the inner ear. The eventual result is a 

permanent shift in hearing thresholds, known as noise-induced 

hearing loss (NIHL). NIHL is the most common occupational 

disease in the United States today, with 30 million workers 

exposed to excessive sound levels or toxicants that are potentially hazardous to their hearing (NIOSH, 1996). For noise 

dosimetry, NIOSH recommends using a criterion level of 85 

dB(A) and a 3-dB exchange rate, whereas the Mine Safety 

and Health Administration (MSHA) uses a criterion level of 

90 dB(A) with a 5-dB exchange rate (NIOSH, 1998, Federal 

Register, 1999)

Surface drill rigs are used for a variety of purposes, including the installation of water wells, environmental monitoring 

wells, gas wells and pilings. Additionally, they are commonly 

used in the mining industry to drill blast holes at surface mines 

and access holes for underground mines. When used on mine 

property, drill rigs are subject to MSHA regulations. Prior 

NIOSH research concluded that air-rotary rigs produced the 

highest sound levels at the operator station when compared 

to cable tool rigs, auger rigs and probe impact rigs. Typically, 

this type of rig uses a pneumatic/hydraulic hammering device 

that is either located at the bottom of the drill string (downhole hammer) or at the top of the drill string (top hammer). 

The hammering action is needed to break up hard materials 

to expedite the drilling process. After the rock is fractured, air 

rotary rigs use high-pressure air to force the drill cuttings from 

below the drill bit and out of the hole.

Field investigations identified the drill steel as the major noise 

source on the drill rig during-hammer drilling. One particular 

study involved three different air-rotary drill rigs with three 

different drill rig operators (Ingram and Matetic, 1993). Results 

of this study indicated all three drill rigs produce A-weighted 

sound levels above 90 dB during the development of domestic 

water wells. In two of the three cases, the drill-rig operators 

exceeded their recommended daily allowable noise exposure 

limits within four hours of drilling activity. For and 8-hour 

shift, the projected doses for these operators were 276%, 268% 

and 130%. These dose values equate to time-weighted averages (TWA) of 97.3, 97.1 and 91.9 dB, respectively, based on 

the MSHA criteria. In general terms, the TWA is the constant 

A-weighted sound level that would result in the same noise 

dose as the time-varying sound level (ANSI, 1996). This study 

also found A-weighted sound levels at the operator station in 

excess of 100 dB during hammer-drilling and, particularly, 

while hammering well casing.

Many of the air-rotary rigs used for blast hole drilling are 

 1 The findings and conclusions in this report are those of the 

authors and do not necessarily represent the views of the 

National Institute for Occupational Safety and Health.
equipped with cabs to protect the operator from dust and noise. 

However, rigs used for mine support services such as access 

holes, dewatering holes and degas holes do not usually have 

operator cabs. Additionally, a large percentage of the air-rotary 

rigs used in non-mine-related work do not have cabs. These 

rigs are sometimes owned and operated by family businesses 

in the water-well and gas-drilling industries, which are regulated by OSHA.

Rig designs vary with manufacturer and model. However, 

the air-rotary rigs NIOSH observed in this research were 

similar. The rig controls used by the operator were located 

in close proximity to the drill steel, and no barrier existed 

between the drill steel and the operator’s platform (Fig. 1). 

Additionally, some operators spent a large percentage of their 

day at the operator’s platform. As can be seen from Fig. 1, the 

operators of drill rigs of this style are in very close proximity 

to the major noise source of the rig.

In general, the best approach for reducing noise is to address the dominant noise source. However, developing noise 

controls to address noise at its source may require a substantial 

investment of time, money and resources. The current effort 

focused on the development of a field-worthy noise control with 

limited time, money and resources. The goal of this research 

was to design an effective barrier to reduce the operator’s exposure to noise radiated by the drill steel. To be fully accepted, 

the barrier would have to be easy to use, uncumbersome and 

economical.

Many of the rigs without cabs do not have the automated 

drill-steel handling equipment that is typically installed on rigs 

used for blast-hole drilling. The operators of the non-cab rigs 

who were observed by NIOSH have to add and remove drill 

steels by hand during the process of completing a hole. While 

adding a drill steel, a jib crane on top of the mast is used to 

move the drill steel directly through the area in front of the 

control panel. Therefore, any barrier located near the operator’s 

platform would have to allow the operator to reach the drill 

string when adding and removing drill steels. In addition, the 

presence of a full cab could impact the size and maneuverability of the rig and thus limit the location of drilled holes. 

The overall cost of the barrier also needed to be considered. 

According to a manufacturer of air-rotary drill rigs, a full 

operator’s cab could cost approximately $20,000, depending 

on the rig, cab options and cost of materials at the time the rig 

was manufactured. This high cost could be one reason why 

only 5% of the rigs sold by the manufacturer included operator 

cabs as an option. NIOSH concluded that a partial cab design 

could be retrofitted to these rigs to help protect the operator 

from the sound emitted from the drill steel.

Concept and testing of the initial partial cab

After researching the sound levels around air rotary rigs, the 

idea of a partial cab was discussed by researchers at NIOSH. 

The partial cab was envisioned as a small, lightweight, inexpensive barrier that could be added to almost any drill rig 

on the market. The detailed design of the partial cab would 

vary between rigs, but the overall effect of the cab would be 

to protect the drill rig operator from direct exposure to sound 

emitted from the drill steel. A concept cab was tested to see 

if a simple partial enclosure could reduce the sound levels at 

the operator location.

The first concept of the partial cab was a five-sided box (Fig. 

2). The box would screen the operator down to the shoulders 

and would be equipped with windows to allow the operator 

to view the control panel and drilling deck. The cab could be 

quickly and easily retracted using a winch to allow the drill 

operator to move around the drill rig.

This basic concept was tested in the lab using a representative layout of an air-rotary rig (Fig. 3). For the initial tests, 

the concept partial cab consisted of a box constructed with 6-

mm- (1/4-in.-) thick plywood for the left, right, top and back 

and acrylic sheet for the front. The box was approximately 

610 mm (24 in.) long, 1,000 mm (39 in.) wide and 610 mm 

(24 in.) high. Barrier-absorber material, with a thickness of 

31.8 mm (1.25 in.), was added to the interior of the box on the 

four plywood surfaces for the secondary testing. The materiFigure 1 — Example of an operator station on an air-rotary 

drill rig.

Figure 2 — The initial partial cab concept shown on an 

air-rotary rig.
als selected were used because they were 

available at the time of the initial testing. 

Loudspeakers were placed in the locations 

where the cooling fan and drill steel would 

be positioned on the rig. A Bruel & Kjaer 

4188 microphone2 was then placed on a 

tripod where the operator station would 

be on a drill rig. During prior research, 

the sound pressures near the drill steel 

and cooling fan were simultaneously 

recorded while hammer-drilling with a 

drill rig in the field. Because the sound 

pressures at these locations were recorded 

simultaneously, the relative contributions of the individual noise sources are 

maintained. These recordings were used 

for the laboratory testing. The testing was 

conducted in a large open building at the 

Pittsburgh Research Laboratory (PRL). 

The test set up was positioned at least 7 

m (23 ft) from large objects to reduce the 

potential for sound reflecting off of the 

objects and affecting the sound levels 

recorded at the operator position. The set 

up described above was also used to test 

later cab designs.

Using a laptop computer, the recorded 

signals were fed from the laptop’s soundcard to an amplifier that drove the two 

loudspeakers The signal recorded near the 

drill steel was played through the speaker 

positioned where the drill steel would be 

and the signal recorded near the fan was 

played through the other speaker. An 

LMS Pimento portable data-acquisition 

system was used to record the resulting 

sound pressure at the microphone using 

the Time Data Acquisition Module with a 

sample rate of 25,000 samples per second 

and 16-bit resolution. While playing the 

recordings without the concept partial cab 

covering the microphone, the volume for 

the loudspeakers was adjusted to produce 

an overall unweighted sound pressure level 

of 100 dB at the microphone. This level was 

considered the baseline level to measure 

the attenuation of the partial cab concept. 

Prior to performing measurements with 

the loudspeaker-generated noise, the 

background noise was measured. The 

background noise was more than 10 dB 

below the loudspeaker-generated noise at 

all frequencies of interest.

Figure 4 shows the A-weighted 1/3-octave-band sound 

levels near the drill steel from the field data (plotted on the left 

y-axis) and at the A-weighted 1/3-octave band sound levels 

at the microphone resulting from playing the field recordings 

(plotted on the right y-axis). Although the A-weighted sound 

levels for the lab testing are lower than what was observed in 

the field, the frequency content of the sound generated by the 

loudspeakers is similar to that of the field data. Because the 

goal of the testing was to determine changes in sound levels 

due to using the concept partial cab, the sound generated by 

the speakers is sufficient for the laboratory testing.

After the baseline sound level was established, the concept 

cab was positioned to cover the microphone and the attenuation 

tests were conducted. The sound pressure at the microphone 

was measured without the partial cab, with the unlined partial 

cab and with the lined partial cab using the recordings made 

with the rig at high idle and while hammer-drilling. For each 

measurement, the data were post-processed to calculate the 

overall A-weighted sound level and the A-weighted sound 

levels in 1/3-octave bands. The plywood cab reduced the Aweighted sound level at the microphone by 8.0 dB. The attenuFigure 3 — Laboratory test setup of speaker and microphone locations. Rig 

outline shown to display locations of speakers relative to equipment on the rig. 

Rig was not used during testing.

Figure 4 — Comparison of A-weighted 1/3-octave-band sound levels from field 

data on a drill rig with lab sound levels generated from playback of field recordings in the lab.

2 Reference to specific brand names does not imply endorsement 

by NIOSH.
ation increased to 11.6 dB when the barrier-absorber material 

was added. The reductions in the A-weighted 1/3-octave-band 

sound levels resulting from the tests are presented in Fig. 5. 

The figure shows the cab was not effective at reducing the low 

frequency sound levels at frequencies below 400 Hz 1/3-octive-band. This can be expected because the thickness of the 

partial cab is not sufficient to block low-frequency sounds, 

and low-frequency components diffract around partial barriers 

more easily than high-frequency components (Beranek, 1998). 

However, Fig. 5 shows the partial cab was effective at reducing the sound levels in the 400 through 8,000 Hz 1/3-octave 

bands. Because drilling noise is the most significant contributor to the A-weighted sound level at the operator’s location 

and because drilling noise is dominated 

by high frequencies (refer to Fig. 4), the 

testing proved that a partial cab could be 

an effective noise control for non-cab drill 

rigs. Therefore, a more detailed design for 

a partial cab was pursued.

Research and design for prototype

Determination of required panels. Subsequent to the findings discussed above, a 

new research effort was initiated at PRL 

to design and fabricate a functional partial 

cab for installation on a working air-rotary 

rig. The first step in this process was to 

determine some of the fundamental design 

characteristics of a functioning partial cab, 

including the number and heights of the 

sides necessary to substantially reduce the 

A-weighted sound level at the operator’s 

ear. Because the final version of the partial 

cab would not consist of a full enclosure, 

choosing a material with the highest transmission loss was not necessary because 

the maximum attenuation of a partial enclosure is limited 

due to flanking paths (Bell and Bell, 1994). For convenience, 

18-mm- (3/4-in.-) thick plywood sheets were used to form 

the sides of the structure.

Using the previously described test set up (see Fig. 3), a 

series of tests were conducted at PRL. The test configurations 

included adding 18-mm- (3/4-in.-) thick plywood panels, 

which represented the sides of a partial cab, to a wooden 

framework placed around the microphone. The microphone 

was placed to the left of the speaker playing the sound recorded near the drill steel. This location was similar to the 

operator station of several rigs studied earlier by NIOSH. 

Plywood panels were then added to the faces of the wooden 

framework. The panels were categorized as front, back, left, 

right and top, depending on the relationship of the operator 

facing forward at the operator station. For reference, the 

panel located between the microphone and rear speaker was 

the right-side panel (see Fig. 6).

After completing the tests with various heights and sides 

consisting of plywood (see Fig. 7 (a), (b) and (c)), two additional 

series of tests were conducted using some of the configurations from the first series of tests in conjunction with a layer 

of 25-mm- (1.0-in.-) thick urethane acoustic foam attached to 

the inside surface of the plywood. For one series of these tests, 

the entire surface of the plywood was covered with foam (see 

Fig. 7 (d)), while the other tests were performed with foam only 

around the perimeter of each panel because added windows 

would not be covered with foam (see Fig. 7 (e)).

For this series of tests, the LMS Pimento Octave Acquisition 

Module was used to measure the A-weighted 1/3-octave-band 

sound levels in the 20 through 10 kHz 1/3-octave-bands. Using the Octave Acquisition Module eliminated the need to 

post-process the data. In addition to the microphone signal, 

the voltages used to drive each loudspeaker were measured to 

ensure that the noise generated was consistent for each test. 

At the beginning of each test sequence, the microphone was 

placed at the operator station without the plywood partial cab 

and the speaker output was adjusted until the sound pressure 

level at the microphone was 100 dB.

For each sequence of tests, the initial configuration had 

panels on the front and right (see Fig. 7 (a)). A top panel was 

Figure 6 — Plywood partial cab test setup.

Figure 5 — Reduction in A-weighted 1/3-octave-band sound levels with partial 

cab concept without and with a barrier-absorber lining.
added for the second configuration. For 

the third configuration, a panel was added 

to the back of the frame. Finally, the last 

test in a sequence had a panel added to the 

left side (see Fig. 7 (c)). The first, second 

and third test sequences used full-height, 

waist-height and shoulder-height plywood 

panels, respectively. Because the frame 

rested on the floor, the full-height configuration with panels on all sides formed 

a complete enclosure. A fourth sequence 

of tests was conducted with configurations similar to the third sequence using 

shoulder-height panels, except that a 0.15-

m- (0.5-ft-) wide L-shaped panel was used 

in place of a full top panel. Additional test 

sequences were conducted to evaluate the 

improvement in the plywood partial cab 

performance due to using acoustic foam 

to line the inside surfaces of the structure. 

Four sequences of tests were conducted 

with configurations similar to sequences 

one through four and a few additional 

variations in the configuration were tested 

as well. These tests were completed with 

acoustical foam covering the entire interior 

surface of the panel (“full foam”) and with 

a narrow strip of foam installed around 

the perimeter of the panels (“perimeter 

foam”). This configuration was tested 

because windows cannot be treated with 

acoustic foam. In all, 36 configurations 

were tested.

Figure 8 shows the reduction in Aweighted sound level for the ten best configurations. The full-height panels covering 

all sides of the framework using full interior 

foam or perimeter interior foam provided 

the highest attenuation, 22 dB. Full-height 

panels covering the top, front and right 

sides with full foam or perimeter foam 

were the next best configurations (18-dB 

reduction). The best unlined configuration 

used full-height front and right side panels 

with a top panel (16-dB reduction). Waistheight with top, front and right panels with 

full or perimeter foam is the best partialheight configuration (12-dB reduction). 

The unlined, five-sided enclosure reduced 

the sound level by 12 dB. The unlined 

configurations approaching a full enclosure 

had poor results due to the reverberant 

conditions created within the enclosure. 

Adding either full foam or perimeter foam 

to this configuration increased the sound 

level reduction to 22 dB.

The “three-sided” configurations (front, 

top, right) with full-height panels seem to be 

the best compromise between noise reduction and worker mobility. The rear panel 

and left (or panel opposite the drill steel) 

would not be installed on the prototype 

cab. This layout would give the operator 

protection from direct exposure to sound 

emitted from the drill steel, while allowing 

Figure 7 — Panel test configuration examples: (a) full-height, front and right 

panels, no foam; (b) shoulder-height, front and right panels, no foam; (c) waistheight, front, right, top, back and left panels, no foam; (d) full-height, front, left 

and right panels, full foam; (e) full-height, front, right and top panels, perimeter 

foam.

Figure 8 — Reduction in A-weighted sound level for the ten most effective cab 

configurations.
the operator free ingress and egress to the enclosure without 

moving the cab panels. The “front” panel would include the 

operator control panel. Additional height or length could be 

added as needed. Because flanking around the cab would be 

the limiting factor for attenuation, the transmission loss of the 

barrier was not as important as its location relative to the drill 

steel and operator. Even if a barrier with a high transmission 

loss were used, flanking would limit the maximum attenuation 

achievable from the partial cab (Bell and Bell, 1994).

Aluminum mockup testing. Based on the findings of the 

plywood panel tests, the overall layout of the cab was decided 

— a three-sided enclosure would be the basis for the design. 

The next step was to determine what materials to use for the 

prototype design. Clearly, plywood is not an acceptable material for field-worthy partial cab. The panels for a partical cab 

installed on a working drill rig would need to be lightweight, 

strong and durable. Also, windows would be necessary for 

observation of the drill steel and drill mast. Originally, it had 

been planned to test a variety of materials for the panels. However, the researchers discussed the possibilities and decided 

to use panels constructed of powder-coated aluminum sheets 

installed on a steel framework. The powder-coated sheets would 

provide a lightweight, rustproof, maintenance-free covering for 

the cab. Steel sheet was also considered. But, after taking into 

account the potential for rusting and the additional weight, the 

team decided to rule out the use of steel sheets. The researchers 

contemplated using either acrylic or polycarbonate sheet for the 

windows. However, the decision was to use laminated glass for 

the windows due to its superior scratch resistance compared to 

the other materials. To support the cab, the researchers chose 

to use a painted steel framework.

The objective of these tests was to determine how several 

design choices affected the attenuation provided by a partial 

cab constructed of aluminum panels attached to a square tube 

frame. An aluminum mockup cab was constructed and placed 

on the same wooden framework used for the plywood panel 

testing. Once again, the test setup and procedures using the 

recorded drilling noise played through loudspeakers were followed. An OROS OR38 Multi-analyzer was used for these tests 

to measure the resulting A-weighted 1/3-octave band sound 

levels. One goal of this testing was to determine if attaching a 

layer of aluminum sheet to both the inside and outside of the 

steel frame would improve attenuation. A three-sided, waistheight cab with a single layer of aluminum sheet attached to 

the outside of the steel frame with no windows or acoustical 

foam was used for the initial tests (Fig. 9 (a)). Further tests 

on the aluminum mockup included the addition of acoustical 

foam, a second layer of aluminum sheet attached to the inside 

of the steel frame and windows. A rubber skirt made from a 

roll of rubber floor mat was added to block 

reflections from the ground from reaching 

the microphone (Fig. 9 (b)). A full-length 

panel was not desirable due to the added 

weight of the framework and panels. Also, 

the drill rigs are commonly used on uneven 

ground. Under these conditions, a fulllength door could hit obstructions and not 

be usable. For one configuration, acoustic 

foam was added to the inside surface of 

the single layer of aluminum. In another 

configuration, acoustic foam was encapsulated by the layers of aluminum attached 

to the steel frame.

In all, 11 aluminum mock-up configurations were tested and the resulting reductions of the A-weighted sound level were 

determined (see Fig. 10). The single-layer 

and double-layer aluminum with encapsulated foam configurations each provided 

the lowest attenuation, 8.3 dB. The doublelayer aluminum configuration resulted in 

an attenuation of 8.9 dB. Examination of 

the results for the single-layered configurations highlights the importance of adding 

sound-absorbing foam and blocking ground 

reflections from reaching the operator loFigure 9 — Test arrangement showing: (a) three-sided 

double-layer aluminum cab with no windows or foam and 

(b) three-sided cab with windows, acoustical foam and a 

rubber skirt.

Figure 10 — A-weighted sound level reduction of aluminum mock-up designs.
cation. The maximum attenuation, 15 dB, 

was achieved with the mockup consisting 

of a single layer of aluminum lined with 

acoustic foam and with the added rubber 

skirt. Adding the rubber skirt to the singlelayer, foam-lined configuration improved 

the sound level reduction by almost 5 dB. 

Clearly, the partial cab would have to protect the operator from sound that emanates 

from the hole or is reflected by the ground. 

Full-length doors were not practical due 

to the additional weight of the door. Also, 

because the rig often works in areas with 

uneven ground, a flexible material was 

necessary to allow the door to slide without 

catching on objects at ground level. Several 

additional configurations were performed 

to determine if two panes of acrylic sheet 

would result in a higher sound level reduction than a single pane. A few tests were 

conducted with a 3.2-mm-thick pane of 

acrylic sheet attached to the outside of the 

frame. More tests were conducted with a 

1.6-mm-thick pane of acrylic sheet on the 

inside of the frame with the 3.2-mm-thick 

pane of acrylic sheet on the outside of the 

frame. As Fig. 10 shows, the three-sided, 

single-layer aluminum, waist-height configuration with the rubber skirt, using two 

panes of acrylic sheet, improved the attenuation of the cab by less than 1 dB.

Figure 11 shows the reductions in the 

A-weighted 1/3-octave band sound levels for the mockup cab 

tests. The results for the mockup were similar to the results 

observed with the concept design (refer to Fig. 5). The aluminum 

mockup designs achieved reductions of 10 dB or more in the 

1,000 Hz 1/3-octave band and beyond. At 125 Hz, the sound 

level reductions were less than 1 dB. The sound levels were 

amplified with the mock-ups at 100 Hz and below. However, 

because the sound level at the operator’s station is dominated 

by high frequencies, a partial cab similar to the aluminum 

mockups should provide a significant reduction in the sound 

level at the operator’s station.

Based on the mockup test results, it was concluded that the 

partial cab prototype would be constructed of a single layer of 

aluminum attached to a steel frame. Large single-pane windows 

made from laminated safety glass would be installed on the side 

of the partial cab facing the drill steel and on the top surface 

of the partial cab. To add absorption, acoustical foam would 

be installed wherever possible around the windows and inside 

the compartment. A dense vinyl barrier would be hung from 

the bottom of the partial cab where possible to block noise 

emanating from ground level. A dense vinyl barrier with a 

surface mass density of 4.84 kg/m2 was used instead of the rubber skirt for field-testing, because the vinyl barrier had a much 

higher surface mass density. Because the sound transmission 

loss, which is a measure of a materials ability to block sound, 

increases with surface mass density, the vinyl barrier would 

be better than the rubber skirt at blocking noise from entering 

the bottom of the partial cab (Harris, 1991).

Design and fabrication of the prototype partial 

cab

Using the information gathered from the testing above, the 

design of a working prototype partial cab could begin. The 

Figure 11 — Reduction in A-weighted 1/3-octave band sound levels for selected 

mock-up configurations.

researchers collaborated with a local drilling company that 

was willing to have the cab installed on one of their production 

drill rigs. The selected rig was a truck mounted air-rotary rig. 

The rig had an onboard air compressor and would use a second 

or third skid-mounted air compressor to help flush cuttings 

when large or deep holes were drilled. The operator station 

for the selected rig was located to the right of the drill mast 

(Fig. 12). The rig’s operators indicated the partial cab should 

not add any width to the rig or reduce ground clearance. The 

rig operator commonly needs to access the drill steel and drill 

deck while standing at the control panel. The operator grabs the 

drill steel when adding pieces of drill steel to the drill string. 

The operator also needs to slide the thick steel plate (Fig. 12) 

that is used to tighten or loosen the drill steels. Therefore, the 

researchers decided the operator must be able to quickly and 

easily retract the partial cab. Also, depending on the layout of 

the drill rig, the operator would need to retract the cab so it 

was not in the way when adding and removing drill bits, or 

adding drill steels to the drill rig.

After examining the rig in detail it was determined that 

the basis for the cab would include a sliding canopy covering 

the left side and top of the operator station. The control panel 

would make up the majority of the front panel. The cab would 

be constructed of a one-piece door that would slide between 

the control panel and the drill platform and over the top of the 

control panel. The biggest obstacle to overcome was the limited 

space to the left of the control panel (Fig. 13). The distance 

between the drill mast and control panel was approximately 

63.5 mm (2.5 in.). This meant the one-piece door would have 

to be approximately 38 mm (1.5 in.) in overall thickness to 

reliably clear any obstacles.

Thorough measurements were taken of the drill rig’s control 

panel and operator platform. The measurements were then 
used to complete a three-dimensional model of the control 

panel using a computer-aided design (CAD) program. Using 

the model, the design for the partial cab was completed (Fig. 

14). The design of the cab consisted of a one-piece cantilevered door constructed from 25.4-mm (1-in.) steel tubing and 

covered with 1-mm-thick aluminum sheet. The door would be 

supported on a 25.4 mm (1 in.) steel tubular framework added 

to the drill rig’s control panel. The door would slide forward 

and backward using linear bearings. Laminated glass windows 

measuring 0.53 x 0.57 m (21 x 22 in.) and 0.51 x 0.91 m (20 x 

36 in.) were added to the door on the top and sides, respectively. 

Where necessary, areas of the support framework would be 

covered with aluminum sheet to block direct paths from the 

drill steel to the operator. All available surface area inside the 

partial cab would be covered with acoustical foam to reduce 

reflected noise. The existing supports for the operator platform 

would be modified to make room for the door between the 

control panel and drill mast.

Arrangements were made with the drilling company to have 

the drill rig available at PRL for one week for installation of 

the partial cab. Prior to the rig’s arrival, the majority of the 

components for the cab were constructed. After the rig was 

delivered to PRL, the components were fitted to the machine 

and final fabrication was completed. Components were then 

painted and installed. The one-piece door was then taken to a 

local glass shop for installation of the laminated glass. When 

the door returned, the final steps, including adding aluminum 

to the support framework and control panel, were finished. An 

acoustic barrier made from a dense filled vinyl-based polymer 

was added to the bottom of the door and control panel (Fig. 

15). The researchers considered adding a barrier beneath the 

operator’s platform that is constructed of an open grate. However, this could cause this area to become packed with dirt and 

debris creating a slipping hazard. Therefore, a barrier was not 

added beneath the platform.

The cost of materials for the partial cab was less than 10% 

of the estimated cost of purchasing a complete operators cab 

from the manufacturer. The total material cost and material list 

is included in Table 1. The cab was installed at no cost to the 

drilling company collaborating with NIOSH for this project. 

The estimated design and fabrication time for the project was 

approximately 200 personnel hours. The door and support 

framework was constructed of steel tubing. All welding was 

completed using a MIG wire feed welder. All construction 

Figure 13 — Available clearance between control panel 

and drill platform. The clearance is approximately 63.5 

mm (2.5 in.).

Figure 14 — Three-dimensional CAD design of the partial 

cab.

Figure 12 — Drill rig operator station prior to installation 

of partial cab.
materials were readily available from either on-line or local suppliers. The most 

expensive components for the cab were the 

precision guide blocks and guide rails that 

support the cab and allow it to move. These 

rails and bearing blocks need to be strong 

enough to support the entire weight of the 

cantilevered door, which was estimated at 

75 kg (165 lb). The amount of time required 

to design and fabricate the NIOSH partial 

cab was adversely affected by the limited 

availability of the rig for measurement and 

fabrication. A complete CAD model would 

not be required when constructed by the rig 

owner/operator. The cab could be designed 

and built during rig downtime and as time 

allowed during drilling operations. Using 

this method of design and fabrication the 

rig operator could install an effective partial cab without appreciable labor cost. The owner/operator could also save additional capital by choosing to install a partial cab without the 

retractable functionality requested by the company partnering 

with NIOSH for this research.

Field testing of the prototype partial cab

After the partial cab had been successfully installed on the drill 

rig, researchers evaluated the attenuation provided by the cab 

under actual drilling conditions. While the testing completed 

at PRL was conducted in an area approximating a free-field 

environment, the conditions surrounding the drill rig during 

operation can vary greatly. The rig that the cab was installed 

on was primarily used to drill holes into underground mines 

for utility access or methane degassing operations. The rig 

would travel into both remote locations and mine operation 

yards. In remote locations, the surrounding area resembled 

a free-field condition unless the rig was located near a high 

wall. When located on mine property sites, the rig could be in 

a more complex acoustic environment having multiple reflections due to the presence of buildings and other large pieces of 

machinery. However, even in these locations, the researchers 

believed the partial cab would still reduce the sound level at 

the operator’s station. The researchers expected the drilling 

noise reaching the operator via the direct path from the drill 

steel to the operator station to overshadow the noise reflected 

from the surroundings.

The researchers followed the drill rig to a variety of locations and performed both sound pressure time history and noise 

dosimetry measurements. The sound pressure measurements 

were later post-processed to determine the A-weighted sound 

levels in 1/3-octave bands. Much of the field testing was similar 

to an earlier investigation involving sound pressure measurements, dosimeter recordings and time-activity study of four 

different air rotary drill rigs with mounted cabs (Ingram and 

Jurovcik, 2005a). Two types of sound measurements were 

taken at each field site during this early investigation. One 

focused on the exterior sound pressure generated by the rig 

during hammer-drilling, the loudest sound levels during hole 

development. The other measurements focused on the operator’s 

noise exposure during hole development. This prior research 

showed a factory installed full cab could substantially reduce 

the A-weighted sound levels at the operator’s ear when properly 

used. Researchers observed a 17-dB reduction in the A-weighted 

sound level with the original equipment manufacturer (OEM) 

cab. Furthermore, for drill rigs designed with the operator 

control panel adjacent to the drill steel, simply opening the 

OEM cab’s door facing the drill steel would increase the Aweighted sound level by 15 dB, indicating the barrier blocking 

the direct sound waves between the drill steel and the operator 

is very important for this style of rig (Ingram and Jurovcik, 

2005b). During the field research associated with the partial 

cab, the sound pressure levels at the operator’s platform were 

measured to show the attenuation in sound level provided by 

the partial cab at the operator station. Noise dosimetry was 

conducted to review the impact of the partial cab on the drill 

operator’s noise dose.

While evaluating the partial cab in the field, sound pressure 

measurements were performed by placing Bruel & Kjaer 4188 

microphones near the operator’s left and right ears while the rig 

was hammer-drilling (Fig. 16). An LMS Pimento portable data 

acquisition system was used to record the time waveforms of 

Figure 15 — Completed partial cab installed on the drill 

rig.

25-mm-thick square steel tubing m 24 $5.00 $120.00

1-mm-thick aluminum sheet each 2 $100.00 $200.00

25-mm-thick open cell acoustic foam m2 1 $30.00 $30.00

6-mm-thick laminated safety glass m2 0.8 $220.00 $176.00

3-mm-long rivets box 1 $15.00 $15.00

15-mm-wide, 820-mm-long precision rails each 2 $220.00 $440.00

57-mm-long precision guide blocks each 2 $100.00 $200.00

Bellows cover for rails each 2 $45.00 $90.00

4.84 kg/m2 dense vinyl barrier m2 1.7 $20.00 $34.00

 Total $1,305.00

Item Unit Quantity Unit cost Total cost

Table 1 — Material list with pricing for the partial cab installed by NIOSH.
the microphone signals at a sample rate of 25,000 samples per 

second with 16-bit resolution. The sound pressure generated 

by the drilling process varies due to several factors including 

drilling depth, the hardness of the material that is being drilled 

and whether an auxiliary compressor is being used to expedite 

drilling. Due to these variables, the overall A-weighted sound 

levels vary from measurement to measurement leading to differences in the attenuation of the partial cab. To account for 

this variability, several sets of sound pressure recordings were 

made with the partial cab pulled out to protect the operator and 

pushed in to the storage/transport (or unprotected) position. 

Three 15-second sound pressure recordings were made with 

the cab pulled out to block the direct path from the drill steel 

to the operator. Then, the partial cab was pushed back into its 

retracted position and three additional 15-second recordings 

were collected. The recordings for each set of measurements 

with the partial cab out and in were completed within a few 

minutes of each other to ensure the drilling conditions had not 

significantly changed. A total of three visits were made to field 

sites resulting in nine sets of recordings.

The recorded data were post-processed to calculate the 

overall A-weighted sound level and the A-weighted sound 

levels in 1/3-octave bands. For each set of measurements, the 

average A-weighted sound level and 1/3-octave-band sound 

levels were calculated for the measurements with the cab pulled 

out (protecting the operator) and with cab pushed in (not protecting the operator). The attenuation was computed by simply 

taking the difference in the average sound levels for each set 

of measurements. The partial cab attenuated the A-weighted 

sound level by 5 to 9 dB at the operator’s left ear and by 2 to 

5 dB at the operator’s right ear when used while hammer-drilling. Figure 17 shows a comparison of the A-weighted sound 

levels in 1/3-octave bands with the cab pushed in (operator not 

protected) and with the cab pulled out (operator protected) for 

one set of measurements. The data indicate that the partial cab 

reduces the sound levels by more than 10 dB at 1,000 Hz and 

above. This is similar to the results from the mock-up testing 

conducted at PRL (refer to Fig. 11).

The reduction in the overall A-weighted sound level using 

the installed partial cab was less than the reduction seen in the 

lab for the mockup for a variety of reasons. The recordings 

used for the mockup testing were for a different drill rig and 

likely generated a slightly different spectrum than the rig the 

partial cab was tested on. For the lab tests, the noise emanates 

from only two locations, which cannot replicate the full sound 

field around a drill rig, which consists of many distributed noise 

sources. Additionally, the drilling noise does not come from 

a point location such as a speaker, it radiates from the entire 

drill steel, which is 12 m (40 ft) long, as well as the drill mast. 

Reflections from surrounding equipment at a drill site result 

in additional paths for sound to reach the operator. Finally, the 

partial cab installed on the rig did not have the same dimensions 

as the mockup cab. Because the operator 

did not want the cab to block his access 

to the drill steel, the depth of the door 

between the operator and drill steel had to 

be reduced. The mockup cab had a depth 

of 189 mm (7.4 in.), while the door for the 

cab installed on the drill rig was limited to 

a depth of 112 mm (4.4 in.).

The attenuation achieved by the partial 

cab will be dependant on the drilling conditions as well as the environment surrounding 

the rig. For this rig, the A-weighted sound 

levels at the operator station increased by 3 

to 5 dB when the auxiliary air compressors 

were turned on. The increased amount of 

air helps flush drill cuttings from the hole 

and causes the pneumatic hammer on the 

bottom of the drill string to impact harder, 

which in turn increases the noise emitted 

by the drill string.

Further data collection and analysis 

was performed to assess the potential 

reduction in operator noise dose due to 

use of the partial cab. Two Larson Davis 

Figure 16 — Sound pressure measurements at operator’s 

ears.

Figure 17 — A-weighted 1/3-octave-band sound levels at operator’s left ear 

when not protected and protected by the partial cab.
Spark dosimeters were used to measure 

the operator’s noise exposure. During the 

initial visit to record the sound pressures 

using the LMS Pimento, dosimeters were 

placed on the left and right shoulder of the 

operator. Time-motion data for the operator 

was collected using a personal digital assistant (PDA) while observing the operator 

for several hours.

The collected data indicates the partial 

cab has the potential to significantly impact 

the noise dose for the operator. Figure 18

shows the A-weighted equivalent continuous sound level (LEQ) in 5-second increments for 20 minutes of the dosimetry data. 

The data presented here were collected 

while the operator stood on the operator’s 

platform while the rig was hammer-drilling. 

The figure indicates the A-weighted LEQ 

at the operator station while the operator 

is protected by the partial cab was approximately 90 to 93 dB at the operator’s 

left ear and 91 to 94 dB at the operator’s 

right ear.

The levels increase dramatically twice 

during the observations. The first increase, 

which was approximately 7 dB for the 

operator’s left ear, occurred when the operator leaned around the partial cab toward 

the drill steel, exposing the left dosimeter 

microphone and the left side of his head 

to the noise radiated by the drill steel. The 

second increase, which was approximately 

4 dB, occurred because the operator was 

asked to push the partial cab in while the 

data were collected for the unprotected 

position. Additionally, the data show the 

LEQ at the operator’s left ear is typically 

lower than the LEQ at the right ear when 

the partial cab is pulled out to protect the 

operator from drilling noise.

When the partial cab is pushed in to the 

storage, or unprotected, position the sound 

level is higher at the operator’s left ear than 

at the right ear. For the protected position, 

the sound level at the right ear may be 

higher than the sound level at the left ear 

for several reasons. First, sound radiated 

from the drill string or other noise-generating components on the rig, such as the 

engine and cooling fan, could be flanking 

the partial cab and reaching the right ear. 

Secondly, sound waves could be reflecting 

from objects surrounding the drill rig and 

reaching the operator’s right ear. 

After examining the time-motion and dosimetry data, the 

team decided to add an additional side to the partial cab to 

reduce the levels at the operator’s right ear. A removable partition constructed of overlapping clear vinyl noise barrier strips 

was added. Once again, the sound pressures at the operator’s 

left and right ear were recorded, using Bruel & Kjaer 4188 

microphones and the LMS Pimento portable data-acquisition 

system, and the data were post-processed to calculate the Aweighted overall sound level and 1/3-octave-band sound levels. 

Two recordings were made without the clear barrier followed 

by two recordings with the clear barrier. After approximately 

20 minutes had passed, two additional recordings were made 

with and without the clear barrier. Analysis of the hammerdrilling data showed the sound level reduction due to adding 

the clear barrier was less than 1 dB for the left ear. However, 

the clear barrier provided a 2 dB sound level reduction at the 

right ear (see Fig. 19).

Conclusions

Laboratory tests were conducted using recorded drilling noise 

played through loudspeakers to determine the required number 

Figure 18 — Five-second, A-weighted LEQ from dosimetry data for operator’s 

left and right ear.

Figure 19 — A-weighted 1/3-octave-band sound level at the operator’s right ear 

without and with a clear barrier added to the right side of the partial cab.
and size of panels for a partial cab for an air-rotary drill rig. The 

data showed that a three-sided mockup constructed of full-height 

aluminum sheets, steel tubing and acrylic sheet could reduce 

the A-weighted sound levels by approximately 13 dB. A partial 

cab fabricated using aluminum sheets, safety glass, steel tubing, 

acoustic foam and vinyl barrier was attached to an air-rotary 

drill rig. Field testing showed that using the partial cab reduced 

the A-weighted sound levels at the operator’s left ear and right 

ear by 5 to 9 dB and 2 to 5 dB, respectively. The sound levels in 

the 1,000 Hz 1/3-octave band and above were reduced by more 

than 10 dB with the partial cab protecting the operator. Adding 

a clear barrier to the right side of the partial cab increased the 

sound level reduction at the operator’s right ear by 2 dB.

To be effective at reducing noise and to be accepted by the 

operator, the partial cab must be small, lightweight, durable 

and easy to use. The NIOSH cantilever design of the partial 

cab can be applied to many of the drill rigs with the operator 

station located adjacent to the drill mast. For other styles of 

drill rigs, an effective partial cab can be installed by following 

the basic design principles discussed above. The partial cab 

installed by NIOSH was well received by the drilling company 

and the cab is still in place on the rig after more than a year. 

An effective partial cab can be installed for a small cost when 

compared to the cost of installing an OEM cab. Materials for the 

partial cab installed by NIOSH were estimated at $1,300. The 

estimated cost of adding an OEM cab to a drill rig is $20,000 

according to a rig manufacturer. The cab can be fabricated and 

installed by the rig owner/operator, or by a local welding and 

fabrication shop. Additionally, the partial cab does not limit 

the maneuverability of the drill rig for use in tight locations 

and adds limited weight to over the road drill rigs.

Commercially available full cabs can provide high reductions in sound levels. However, their initial cost, added size, 

added weight and limited operator access make OEM cabs 

unattractive to many drill rig operators. The partial cab is an 

effective compromise for drill rig operators and owners who 

want lower sound levels at the operator station while maintaining the flexibility and maneuverability of their drill rig.

Acknowledgments

The authors would like to thank the following individuals for 

their effort and assistance with the laboratory and field testing 

required for this research project: David Dwyer, Timothy Lutz, 

Timothy Matty and Patrick McElhinney of NIOSH, Pittsburgh, 

Pennsylvania, and Rick Enoff of Gene D. Yost & Son, Inc., 

Mount Morris, Pennsylvania.

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