Mining haul truck cab noise:


an evaluation of three 


acoustical environments
 

S.B. BEALKO 

S.B. Bealko, member SME, is mining engineer with the 

Pittsburgh Research Laboratory, National Institute for 

Occupational Safety and Health, Pittsburgh, PA.

Abstract

 Mining haul trucks comprise the majority of the equipment used in underground limestone mining operations and 

are known to emit high levels of noise. A previous study 

conducted by the National Institute for Occupational Safety 

and Health (NIOSH) indicates that 70-90 % of all miners 

have a noise-induced hearing loss (NIHL) great enough 

to be classified as a hearing disability by retirement age. 

These results demonstrate the public health need to protect 

the hearing of workers in the mining industry, including 

haul truck drivers. 

 Cab enclosures present an opportunity to isolate the haul 

truck operator from both truck and other noise in the mining environment. A total of 25 haul truck cabs were studied. 

They were divided into three style (treatment) categories 

determined by soundproofing features and technology 

for noise reduction: old-, new- and retrofi tted-style. This 

study examines the contribution of cab acoustics, operator 

performance and maintenance to noise reduction for each 

cab style. Dosimeters were used to measure eight-hour 

time weighted average sound pressure levels (TWA8

 SPLs) 

inside and outside the cabs. The main objective was to 

determine the noise levels inside of the three types of cabs 

(with different acoustical treatments) and determine if the 

noise levels were significantly different. Adherence to the 

Mine Safety and Health Administration (MSHA) permissible exposure limit (PEL) of 90 dB TWA8

 (with a 90 dB 

threshold) was used as the main indicator of overall noise 

reduction achieved. Dosimetery results indicated that all 

but two samples measured outside of the cab exceeded the 

MSHA PEL. However, only 2% of the samples measured 

inside of the cabs exceeded the PEL, but samples could still 

be reduced much further. Descriptive and comparative statistics indicate that noise levels inside the new-style cabs are 

significantly lower than the other two cab styles. Also, data 

suggest that there is no difference in noise exposures when 

comparing the old-style to retrofi tted cab styles. Operator 

infl uence (opening doors and windows) was a signifi cant 

factor for increasing noise exposure. 

 This paper demonstrates that properly designed cabs can 

achieve major noise reductions, but noise levels could still 

be reduced much further below the MSHA PEL. New-style 

cabs, equipped with modern noise-reduction treatments, 

exhibit much lower noise exposures than the other two cabs 

styles, and the effectiveness of the current noise-reduction 

treatments for retrofitted cabs is questionable. Haul truck 

driver observations indicate that improved noise exposure 

reduction training is needed. Finally, specific targets for 

future noise reduction research are suggested that will further contribute to the prevention of hearing loss for haul 

truck operators. 

Introduction

This study investigates haul truck 

cab noise in underground limestone 

mines that employ nearly 2,000 workers at 117 mines across the United 

States. In this industry, hazardous noise 

is present from drilling, blasting, rock 

crushing operations and the predominance of large and noisy equipment. 

Continued exposure of miners to high 

noise levels can cause damage to the inner ear.The result of 

this damage is a permanent shift in the hearing threshold, 

known as a noise-induced hearing loss (NIHL). A NIHL 

makes it difficult to hear and understand everyday speech 

and is irreversible. 

Of special interest is the haul truck (Fig. 1) because 

it comprises of the largest class of equipment used in the 

underground limestone industry. With these trucks and 

most diesel-powered equipment, the engine is generally a 

major source of noise. Engine noise may emanate from the 

exhaust, the intake and the cooling fan. Other signifi cant 

noise sources include the transmission, drive train and hydraulic system. 

Noise from these sources reaches

the ear via several paths, both directly, by airborne paths and indirectly, 

by refl ections from various surfaces. 

In addition, sound in the form of vibrations may travel along or through 

structures (Daniel et al., 1981). An 

approach to eliminate or control 

noise at its source, engineering controls, is through the use of muffl ers, 

gaskets and control of reflected noise. Another way to 

lower noise levels is to identify, isolate and treat the many 

paths along which noise travels with barriers, absorbers 

and dampers. 

Control of haul truck cab noise is important because 

haul truck operators spend a majority of their time inside 

the cab. Most mine policies require haul truck operators to 

remain inside their cab throughout the entire shift except 

for restroom use, attendance at safety meetings, during 

maintenance and sometimes during lunch breaks. Therefore, it is typical for operators to spend almost the entire 

shift (eight to 10 hours) inside the haul truck cab. 

According to Daniel et al., “Cab enclosures generally 
 

are the most efficient way to prevent the radiation of sound 

through the cab walls.” The effectiveness of noise reduction 

is greater if the cab is lined with an acoustically absorptive 

material. Most newer haul truck cabs are manufactured 

with features that are designed specifically for noise reduction (new-style cabs). These features are typically not 

found as original components in cabs of older trucks (oldstyle cabs). Sound-proofing materials may be added to 

the older cabs to upgrade their noise reduction potential 

(retrofi tted cabs). 

FIGURE 1 

Haul truck and cab. 

This study examines noise exposure inside haul truck 

cabs experienced during a typical workday with normal 

operator practices, the effect of noise-reduction features 

inside the cab, the consequence of disabling noise controls 

(unnecessary open doors/ windows) and the signifi cance of 

haul truck and cab maintenance factors. The objectives of 

this study were to: 

•	 Determine if current haul truck cabs provide 

enough protection to prevent a noise overexposure 

(as defined by the MSHA PEL) during normal 

operations. 

• 	 Determine if there is a significant difference in the 

noise exposure as measured inside the old-style, 

new-style and retrofi tted cabs. 

• 	 Analyze critical factors that contribute to the cab 

noise protection potential. 

•	 Observe and consider haul truck operator activities 

(opening of doors or windows) relative to established operating procedures and to determine this 

effect on the noise exposure inside the cab. 

• 	Suggest specific research areas to further improve 

noise reduction in haul truck mining cabs. 

Federal regulation of noise exposure in mining 

Efforts to combat NIHL in miners began in 1969 with 

the enactment of the Federal Coal Mine Safety and Health 

Act (Public Law 91-173). This law set forth requirements 

for protecting coal miners from among other hazards, exposure to excessive noise. Later, the Federal Mine Safety 

and Health Act of 1977 (Public Law 95-144) broadened 

the scope of the law to include noise protection for all miners of all mineral types (the Acts are detailed in 30 CFR, 

Subchapter O, Part 70, Subpart F, 1997). MSHA enforced 

a PEL that was an eight-hour, time-weighted average 

(TWA8) of 90 dBA(slow) (with a 90 dB threshold), but a 

hearing conservation program was not mandated unless a 

citation was issued for overexposure (Joy and Middendorf, 

2007). 

On Sept. 13, 2000, there was further progress in controlling mining-related noise when MSHA established the 

new Health Standards for Occupational Noise Exposure 

(Federal Register, 1999).This standard adopted a provision 

similar to Occupational Safety and Health Administration’s (OSHA’s) Hearing Conservation Amendment (29 

CFR 1910.95), where a miner is required to be enrolled in a 

hearing conservation program (HCP) if the full-shift noise 

exposure is at or above the action level (AL) of TWA8

 85 

dBA (slow)(80 dBA threshold). 

With the PEL remaining the same, other requirements 

of the new regulations included the primacy of engineering 

and administrative controls for noise exposure reduction, 

the implementation of a noise exposure monitoring system and the relegation of the use of hearing protection to 

the hearing conservation program. The implementation of 

these regulations has served to reinforce the importance of 

noise reduction throughout the mining industry. 

Methodology 

Categorization of haul truck cabs. Presently, haul truck 

cabs are manufactured with built-in design features for 

noise reduction (new-style cabs) whereas cabs on older 

vehicles lack many such features (old-style cabs).There are 

also cases where the cab components have worn out before 

the haul truck is taken out of service. Often, to extend the 

life of the truck, the original components of these cabs are 

upgraded with materials (e.g. foam-materials on cab walls 

or new gaskets around doors and windows) to reduce noise 

exposures (retrofitted cabs). In order to compare the noise 

reduction characteristics for each cab style, criteria were 

established to divide cabs into the three cab styles or treatment groups. A description of each cab style, along with 

illustrations, follows. 

FIGURE 2 

Examples of new-style cab, a) full-upholstery and window seals b) newer gaskets, door handles and window 

cranks. 

New-style cabs: Noise control features for new-style 

cabs include sound absorption, vibration damping and 

sound barriers. Sound absorption materials are soft and 

porous materials (e.g., fl exible polyurethane foam) where 

the amount of sound absorption is directly related to the 

amount of treated surface area. Vibration damping materials reduce the amount of vibration energy transmitted 

between surfaces and are constructed of rigid materials. 

Sound barrier materials combine mass and fl exibility to 
reduce the sound energy passing between the noise source 

and the controlled area. Sound barriers, combined with 

sound absorption materials, can be very effective in controlling noise (Mohanty et al., 2000). 

Besides new sound-proofing materials and technology, 

research has been conducted to reduce structure-borne 

noise by determining the best placement for sound-absorbing materials in cabs. Researchers created and tested cab 

designs using computer-aided-engineering methods. Some 

examples of the new-style cab characteristics are shown in 

Fig. 2. This figure demonstrates full-upholstery cabs that 

absorb noise and seals, gaskets and latches that minimize 

noise leakage and all-around cab vibration. 

Old-style cabs: It is not uncommon to encounter haul 

trucks that are 20-years-old or older still in use at underground limestone mines. Needless to say, these cab enclosures lack some of the new technologies that reduce noise. 

A typical cab of this type has a hard steel interior that acts 

as a noise refl ecting surface and there is little use of noise 

reducing materials. Because these cabs experience wear 

over time, the original components including cab sealants 

and gaskets may lose their effectiveness and allow noise 

leakage. In addition, cab integrity may deteriorate and increase cab noise due to the vibration of doors, windows or 

latches. Preserving the integrity of these components is 

crucial for noise reduction and thus requires a proactive 

maintenance program. Some examples of old-style cab 

characteristics are shown in Fig. 3. 

FIGURE 3 

Examples of old-style cab, a) and b) steel interior with 

older sealants and gaskets and c) older door handle. This fi gure illustrates 

steel interior cabs with noise reflective surfaces, and olderstyle latches, gaskets and seals. 

Retrofi tted-style cabs: This style cab has been upgraded 

with new technology or materials such as floor mats, insulation, special glass and other methods to reduce noise. The 

upgrade may be due to original component deterioration 

or wear from extensive usage. Figure 4 demonstrates an 

old-style cab that was upgraded with a material approved 

by MSHA. 

FIGURE 4 

Examples of retrofitted cab with foam sound-absorbing 

material with sealants and gaskets around a) doors, b) 

windows and c) walls. 

This foam material shown was engineered for 

use in high noise environments and meets the MSHA fl ammability standards (adopted UL 94 HF-1). It is faced with 

an aluminized polyester material layer that refl ects radiant 

heat. The facing and foam were fused during processing 

to create a bond that resists delamination. This material is 

ideal for sound absorption in enclosed equipment, such as 

compressors, motors, generators, and pumps (TUFCOTE, 

2006). This material is just one example of the many commercially available soundproofing products.There are also 

products that reduce noise echo, stop vibrations and lower 

noise transmission through glass. 

Associated co-variables 

This study was designed to evaluate cab noise exposure 

levels given the current maintenance condition, in the typical environmental noise surroundings, and with the haul 

truck operators performing as usual during a typical workday. Therefore, besides noise-reduction features in cabs, 

there are other variables that effect sound levels inside the 

cab. As part of the methodology, two of these variables referred to as co-variables, were monitored closely throughout the survey.These two co-variables are maintenance and 

operator performance. Other potential co-variables were 

considered including maintenance down time, extreme 

weather and road conditions, and shortened work shifts. 

Maintenance: Although the effect of noise reduction 

for specific cab components is difficult to identify and measure, it is evident that improper maintenance of haul trucks 

and cab enclosures can lessen (or degrade) cab attenuation. Also, deteriorated door and window seals should be 

replaced and holes in the cab frame should be patched 

because air gaps and holes can also allow noise leakage. 

Figure 5 shows examples of cab degradation including inadequate sealing around doors, holes in the cab frame and 

the deterioration of soundproofing materials on the roof. 

Another essential maintenance issue is a functional air 

conditioning system to sustain a comfortable and healthy 

work environment. If the system is in disrepair, haul truck 

operators will, out of necessity, open doors and windows to 

seek relief from the heat. When this occurs, the haul truck 

operators circumvent some of the protective cab features 

and allow outside noise, dust or diesel exhaust to enter the 

cab. 

Operator infl uence: Haul truck operators are encouraged to follow safe operating practices. Most procedures 

require equipment operators to keep their windows and 

doors closed as much as possible. However, observations 

of haul truck drivers during operations reveal that some 

operators do not always adhere to these practices. One 
reason, which has already been mentioned, is to cope with 

malfunctioning air conditioning systems. Another reason is 

the preference for fresh wind and outside air, regardless of 

a guarantee for a noisier environment. Smoking and chewing tobacco use can also cause operators to frequently open 

their windows and doors. 

Operators are also encouraged to stay inside the cab 

and away from noisy environments as much as possible 

and to report any maintenance needs. Adherence to good 

practices and procedures for equipment operation can help 

to reduce noise exposures. 

FIGURE 5 

Examples of poor maintenance (a) inadequate sealing 

around roof, (b) hole in cab frame and (c) deteriorated 

roof materials. 

Mine characteristics 

Haul truck cab noise was studied at fi ve underground 

limestone mines. The typical mining sequence for each 

mine included drilling the face, blasting the rock, and extraction using front-end loaders and haul trucks. The blasted material was transported to the crushing and screening 

facilities where it was processed into various sized aggregates. One mine had the crushing/screening plant located 

underground while the other mines had the facilities located outside the mine approximately 91 - 182 m (100 - 200 

yards) from the mine portal. 

Mine production ranged from 1.4 to 1.8 Mt/a (1.5 to 

2 million stpy) of raw product and employment ranged 

from nine to 30 underground employees. Mining heights 

averaged 6.7 m (22 ft) and the mining widths were approximately 12 m (40 ft). Most of the underground equipment 

was diesel-powered with some smaller equipment powered 

by electricity. 

Study procedure 

Several tasks were completed prior to the start of the 

shift. Twenty-five haul trucks were examined and the cabs 

were categorized as follows: five old-style, 17 new-style and 

three retrofi tted cabs. A pre-shift maintenance inspection 

of the cab was conducted, including noting the operational condition of the air conditioning unit and any obvious 

acoustic material maintenance needs. 

Haul truck operators were then interviewed about their 

habits, activities and common practices. The company’s operating procedures for haul trucks were discussed including 

the requirement to keep the windows and doors closed, reporting of any maintenance problems and proper radio volume. Additional information was collected including truck 

engine data (e.g., horsepower, year, make/model), weather 

or road conditions and planned maintenance activities. 

Upon completion of the pre-shift tasks, noise dosimeters were attached at two locations inside and outside 

of the cabs. In all, there were 44 samples collected over 

the 13-shift study period. For each sample, two noise dosimeters measured noise (dBA TWA8

) using the MSHA 

PEL exposure criteria during the full shift. The dosimeter 

placed outside the cab to get some idea of how loud noise 

levels could be without the protective aid of the haul truck 

cab. In addition, operator activities were monitored and 

noted throughout the shift as closely as possible. However, researchers could only observe the haul trucks when 

they were outside of the mine. At the end of the shift, the 

noise dosimeters were recovered and the data recorded. 

Finally, a second interview of the haul truck operator was 

conducted to determine driver activities and other potential co-variables, discuss maintenance issues and to receive 

feedback or concerns about the noise study. 

FIGURE 6 

Dosimeter inside cab. 

FIGURE 7 

Dosimeter outside cab. 
Instrumentation and data collection 

Worker noise exposures were monitored using Quest 

Q-400 noise dosimeters. This instrument is preferred over 

a sound level meter when the noise levels must be measured over a lengthy period and vary due to intermittent 

nature. Most dosimeters available today provide outputs 

in dose or TWA8

 using various exchange rates (e.g. 5 dB), 

response rates (fast or slow), eight hour criterion levels, and 

sound measurement ranges. 

Noise dosimeters are typically used to measure personal noise exposures of employees, but can also be used 

to measure noise exposures as area samples where the dosimeter stays in a stationary location. The dosimeters used 

in this study measured and stored the sound levels during 

a time period and computed the readout as a percent dose. 

Equation 1 below was used to convert the MSHA PEL 

dose percent to TWA8 SPL. 

TWA8 SPL (dBA) = 16.6 x log10 (Dose %/100) + 90 (1) 

Prior to the shift start, the noise dosimeters were calibrated and set to monitor an MSHA PEL of 100% or a 

TWA8 of 90 dB Threshold, 5 dB Exchange Rate, a Slow 

Response, and a 140 dB Upper Limit. For uniformity, the 

dosimeter microphone inside the cab was placed as close to 

the operator’s right ear as possible. As shown in Fig. 6, the 

microphone was placed 0.3 - 0.45 m (1 - 1.5 ft) to the right 

of the operator next to the engine-side window. Actions 

such as securing the microphone mount with tie wire/electrical tape, placing cloth or clothing around the dosimeter 

and utilizing microphone covers were taken to ensure the 

dosimeters or microphones did not vibrate or touch the 

window and produce structure-borne noise. 

Measures were again also taken to protect the outside 

dosimeters and microphones from damage and vibration 

against the cab frame. Outside dosimeters were attached 

to the frame above the operator’s door as shown in Fig. 

7. This location was chosen to most closely represent the 

noise levels that the operator would experience if he/she 

did not have the protective measure of the cab and also out 

of convenience for fastening the dosimeters (handles and/ 

or bars around or above the cab door in which to secure the 

dosimeter). There were a few occasions during the study 

where it was raining heavily. On these days, dosimeters 

were attached to the frame on the opposite side of the cab 

(engine-side) and were more directly exposed to engine 

noise than if they were located on the operator’s side. 

Results 

A histogram of the TWA8 SPLs for all of the measurements inside the cab is shown in Fig. 8.

FIGURE 8 

Histogram of the TWA8 SPL sample data. 

The shape of the 

histogram (skewed left) suggests that the data may not 

be normally distributed, but perhaps a larger sample size 

would lead to a more normal or log-normal sample distribution. It could also be a bi-modal distribution where the 

high number of quieter cabs (new-style) represents the 

first peak and the larger group of noisier cabs (older and 

retrofitted) represents the second peak. Again, the normality characteristics of the histogram could be clarifi ed with 

a larger sample size. 

The descriptive statistics for all of the sample measurements are as follows: sample mean, 75.1; median, 81.3; 

standard deviation, 15.4; and 95% confidence intervals of 

the mean, 70.5 (lower limit) and 79.8 (upper limit). There 

was one extreme outlier (97.7 dBA) in the sample measurements that was not used in the data analysis. When 

compared to the highest measurement from the study data 

(90.1 dBA), the sound energy from the outlier was as much 

as four times greater. This reading was so high that it was 

improbable that the haul truck driver could have operated 

for a full shift without reporting discomfort from excessive 

cab noise levels. Therefore, this sample measurement is 

extremely unlikely. Researchers found no plausible explanation for this reading except that the dosimeter may have 

malfunctioned or it was not properly secured to the cab 

frame causing excessive vibration noise. 

Figure 9 graphically represents the data as the TWA8 

SPLs for each sample collected inside the cab for each cab 

type. 

FIGURE 9 

Sound level measurements for each type of cab. The black data points represent samples where the 

haul truck drivers had unnecessary intervals of open doors 

and windows. Figure 9 also shows the limits for the PEL 

(TWA8

 90 dBA) displayed in red. Only one out of the 44 

sample measurements was above the MSHA PEL. However, 14 out of 44 (32%) did exceed levels of TWA8

 85 

dBA, which may be considered hazardous. Furthermore, 

Fig. 9 shows that the new-style cab samples were fairly 

spread out, but only one sample came near to exceeding 

the MSHA PEL. In contrast, the measurements for the oldstyle cabs and retrofitted cabs were spread out only over 

the upper range (right side) of the graph that contained the 
higher noise level samples. Table 1 displays the descriptive 

statistics of the TWA8

 SPLs for each of the three cab styles 

including the mean, median, standard deviation, and 95% 

confidence interval of the mean. 

 

 

Table 1 

Descriptive statistics for three cab styles. 

Retrofitted New style Old style 

n=7 n = 26 n = 11

 TWA8 TWA8SPL TWA8SPL 

SPL (dB) (dB) (dB) 

Mean 85.1 67.7 86.3 

Median 86.1 82.1 86.0 

Standard deviation 3.86 16.2 3.0 

95% CI on mean 

Lower 81.5 61.2 84.3 

Upper 88.7 74.2 88.3 

Figure 10 depicts a box plot of the data showing the 

median, and upper and lower quartiles for each cab type. 

FIGURE 10 

Box plot of TWA8 SPL for the 3 cab styles. 

It appears that the median values of the MSHA PEL noise 

exposure for the old-style and retrofitted cabs are very 

similar, but both differ from the new-style cab median. The 

median value of the new cab style is much lower than the 

other two cab styles. 

Because of questions regarding the underlying distribution of the sample data, the parametric ANOVA and 

non-parametric Kruskal-Wallis tests were performed to 

determine if there were significant differences in the TWA8 

SPLs inside the cab between the three cab styles. ANOVA 

tests were performed with Tukey post hoc comparisons. 

Although signifi cance is usually set at alpha equal to 0.05, 

these tests were done to see if the p-values approached 

significance (p-value <0.05). Table 2 displays the results 

from all of these tests. 

The null hypothesis for an ANOVA test is typically that 

there is no difference or effect among groups and a p-value 

close to zero signals that the null hypothesis is rejected. 

The ANOVA test of the sample data achieved a p-value 

of 0.0001, which achieves signifi cance (p-value<0.05), suggesting that the null hypothesis should be rejected and that 

there is a difference in at least one of the three cab styles. 

As seen on Table 2, the Kruskal-Wallis test on the data 

achieved a p-value of 0.0001, which was the same as the 

ANOVA test. Results from the Tukey post hoc multiple 

comparison test on this data show that a p-value of 0.001 

was achieved between the old-style and new-style cabs and 

a p-value of 0.007 was achieved between the retrofi tted 

and new-style cabs. These two comparison tests indicate a 

significant difference between the groups for each test. The 

comparison for old-style and retrofitted cabs did not show 

significance (p-value = 0.987) that there was a difference 

between the groups. 

The outside dosimeter measurements (shown as a range 

of measurements in dB) are provided below. All but two of 

the data measurements were found to be above the MSHA 

PEL. Furthermore, the outside noise levels for the haul 

trucks with retrofitted cabs, overall, tended to be slightly 

lower than the trucks with new-style and old-style cabs. 

The measurements that were taken on the engine-side of 

the cabs were considerably higher than the ones taken on 

the operator’s side of the cab. 

•	 89.3 – 93.8 dB for retrofitted cabs (driver-side

measurements). 

•	 91.3 – 95.8 db for old-style cabs (driver-side 

measurements). 

• 	 91.3 – 95.1 dB for new-style cabs (driver-side 

measurements). 

• 	 97.5 – 99.5 dB for old-style cabs (engine-side 

measurements). 

• 	 98.8 – 99.8 dB for new-style cabs (engine-side 

measurements). 

The category of “open doors and windows” was determined if the operator admitted that he/she had doors 

or windows open (an inch or more) for unnecessary reasons or that this practice was observed by the researcher 

more than three times throughout the shift.The occurrence 

of unnecessary open doors or windows was observed in 

13 out of the 44 samples (30%). These observations were 

seen in three out of the four highest noise levels measured 

during the study, including the highest measurement of 90.1 

dBA. Table 3 shows the mean noise exposure for samples 

with open doors or windows versus the mean for samples 

where open doors or windows were not observed for each 

cab style. The increase in noise was 8.5 dBA for the newstyle cabs, 4.0 dBA for the old-style cabs, and 1.2 dBA for 

the retrofitted cabs. This data shows a signifi cant increase 

in noise for the new-style and old-style cabs. A broken air 

conditioning unit was the reason for the higher noise measurement in only one of the samples. Operators provided 

the following additional reasons why they opened their 

doors or windows: fogged interior windows, tobacco use, 

the preference for outside air and the need to hear the 

horn/signal from the front-end loader. 

Good road conditions were noted throughout the study 

and normal weather conditions were observed. Some cab 

deterioration was visible in the old-style and retrofi tted 

cabs, but little in the new-style cabs. Aside from wear and 

tear due to normal use, the seals, gaskets and latches were 

in fair to good condition. 

Discussion and conclusions 

The noise levels measured outside of the cab clearly 

indicate that most noise levels that would be experienced 

by the operator would be above the MSHA PEL had they 
not had the noise-reducing protection of a cab. The results 

from this study show that 43 out of 44 (98%) measurements inside cabs were below the MSHA PEL, regardless 

of open doors and windows, the cab maintenance condition 

or the cab style. However, noise reduction measures should 

be made to reduce noise even further to prevent NIHL. 

 

Table 2 

Comparison statistics for three cab styles.

Parametric test F statistic P-value

ANOVA 10.7 0.0001

Tukey post hoct test 

(multiple comparisons) 

New-style vs. retrofitted 0.007

Old-Style vs. retrofitted 0.987

New-style vs. old-style 0.001

Non-Parametric Test Chi-square 

statistic P-value

Kruskall-Wallis 20.9 0.0001

 

 

Table 3 

Mean SPL for open vs. closed doors and windows 

for each cab style. 

Doors and windows

 Open (dBA) Closed (dBA)

New-style cabs 73.6 (n=8) 65.1 (n=18) 

Old-style cabs 88.8 (n=4) 84.8 (n=7) 

Retrofi tted cabs* 86.1 (n=1) 84.9 (n=6) 

Multiple comparison tests and descriptive data both 

show that there is a significant difference between the oldstyle and new-style cabs and the retrofi tted and new-style 

cabs.Therefore, efforts to control noise inside the new-style 

cabs have been effective and mine management should 

continue their efforts to purchase haul trucks that have 

cabs equipped with these state-of-the-art noise-reduction 

features. Fortunately, as mines begin to replace old haul 

trucks with new haul trucks that have the new-style cabs, 

noise overexposures should become infrequent as long as 

operators keep the doors and windows closed. 

Finally, multiple comparison tests and descriptive statistics suggest there is no difference between the old-style and 

retrofitted cab styles, highlighting the difficulty of designing and implementing retrofit noise controls. The multiple 

comparison tests show a lack of signifi cant benefit from the 

retrofitted cabs, but with such a limited sample size, one 

cannot entirely rule out retrofits as a useful noise control 

strategy. The multiple comparison tests (Tukey post hoc 

tests) were sensitive to normality assumptions and without 

additional data measurements to test normality, making 

strong conclusions regarding the differences is somewhat 

questionable. Furthermore, some potential shortcomings 

were identified that, if corrected, could lead to more effective retrofits. Improved interventions to reduce door 

and window vibration could serve to further reduce these 

noise levels. Also, noise treatment of the cab fl oor could 

improve noise reduction signifi cantly because of the relative proximity of the cab floor to the engine noise sources. 

Finally, treating the outside of the cab (engine side of the 

firewall and under the hood) with sound absorbing materials should also help to reduce the amount of noise inside 

the cab. 

Open doors and windows will increase the noise levels 

inside the cab and measures should be taken to encourage operators not to disable these protective cab features. 

Improved education and training of operators is needed 

regarding noise source awareness and noise overexposures. Furthermore, enforcement of noise policies should 

be strengthened.Technical interventions, such as alarms or 

lighted warnings that alert the operator when a window 

is open (similar to seat belt dash warnings), may heighten awareness that operators are at higher risk of a noise 

overexposure and encourage them to take appropriate 

action. 

Three factors add a degree of uncertainty in this study. 

The first is the fairly small sample size for old-style and 

retrofitted cabs and the second is that the research was 

conducted at only five mine sites. Further studies of additional haul truck cabs (at a variety of mine sites) would 

enhance the certainty of the study results. In addition, the 

observations of operator activities could only be made 

while a haul truck was visible while on the surface and the 

activities that occurred underground could not be monitored. The collection of data on underground activities 

depended on the reliability of the information provided by 

the haul truck operator and could be affected by self-report 

bias and errors. More direct observation techniques, such as 

sensors around the windows or video cameras, could help 

alleviate this problem for future studies. Finally, no plausible explanation could be provided as to why the outside 

noise levels were lower for the trucks with retrofi tted cabs 

except for that the majority of these vehicles had engines 

with lower horsepower than the other engines (old- and 

new-style), thus possibily producing less noise. 

Disclaimer 

The findings and conclusions in this report have not 

been formally disseminated by the National Institute for 

Occupational Safety and Health and should not be construed to represent any agency determination or policy. 

References 

Bauer, Eric R. and Daniel R. Babich (2004), “Administrative Controls for 

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