Experimental methods to reduce noise generated by haul trucks and LHDs

Abstract

The National Institute for Occupational Safety and Health (NIOSH) maintains the Pittsburgh Mining Research Division (PMRD) where a wide variety of mining-related health and safety research is conducted. Part of this research is devoted to reducing the incidence of noise-induced hearing loss (NIHL) among the nation’s mining workforce. The need for this research is particularly important, as NIHL is the second most common occupational-related disease among miners. Many types of equipment operators are overexposed to noise, and NIOSH has worked to develop noise controls that reduce the sound level at the equipment operator’s location and, thus, operator noise exposure. Examples of these include a urethane-coated flight bar chain for continuous mining machines and a drill bit isolator for roof bolting machines. This article discusses the development of a retrofitted noise control package for haul trucks and load-haul-dumps (LHDs) used in underground metal/nonmetal mines. Experimental methods under discussion include dosimetry and time motion studies, to determine when an operator accumulates the most noise dose. Noise source identification techniques are used to determine the primary noise contributors to the sound level at the operator’s position. Proof-of-concept testing using rudimentary noise controls is undertaken to confirm that treating the suspected noise sources will actually reduce the sound level at the operator’s location. Next, a description is given of the development of noise controls—an iterative process where noise controls are fabricated, evaluated in an acoustic laboratory, refined, and tested again. Those noise controls that show promise are then field tested under actual mine operating conditions.

1. INTRODUCTION

The NIOSH Pittsburgh Mining Research Division (PMRD) conducts research on a wide variety of issues addressing the health and safety concerns of the nation’s mining workforce. The Hearing Loss Prevention Team of the PMRD Workplace Health Branch is charged with conducting health and safety research to reduce noise-induced hearing loss (NIHL) among the nation’s mining workforce.

The Mine Safety and Health Administration (MSHA) sets its permissible exposure level (PEL) for noise exposure at a time-weighted average (TWA) of 90 dB(A) for an 8–hour shift. An 8–hour noise exposure of 90 dB(A) equates to 100% noise dose, as noise dose is expressed as a percentage of the allowable noise exposure. As part of a surveillance program, NIOSH conducted surveys of underground metal mine equipment operators to document their noise exposures. The survey results indicated that greater than 90% of the noise exposures for equipment operators exceeded the MSHA PEL¹. Haul truck and load-haul-dump (LHD) operators had noise exposures that exceeded 200% of the PEL on average.

The above results prompted NIOSH to initiate a noise control project in which the objective was to reduce the noise emissions of haul trucks and LHDs used underground. Reducing noise emissions would, in turn, reduce the sound level at the operator location, the operator’s noise exposure, and the operator’s risk of NIHL. Enclosing the operator location with a cab is a straightforward method to reduce the airborne noise that reaches the operator, and cabs have proven to be effective at reducing noise exposures. However, in certain situations, cabs are not a viable solution, given limited space in a mine. Over-boring of the mine might be necessary to create clearance for a cab, but over-boring may weaken the surrounding geology, which could create additional safety hazards.

Underground mining operations use haul trucks and LHDs to move ore from the ore face to dumping locations where the ore is transported to a processing site. Figures 1 and 2 show a representative haul truck and an LHD above ground. Shown in Fig. 3 are an LHD and haul truck operating in tandem; the LHD is loading a haul truck with ore for transport to a dumping location. A key difference between haul trucks and LHDs is that haul trucks employ dump beds to carry ore while LHDs use buckets, similar to front end loaders. This research project investigated smaller haul trucks (less than 15 tons) and LHDs (less than 5 cubic yards).

Fig. 1—

Mining haul truck above ground.

Fig. 2—

Load-haul-dump (LHD) above ground.

Fig. 3—

LHD dumping ore into a haul truck.

This article discusses a general approach NIOSH uses to develop engineering noise controls and then presents a case study on how this methodology was applied to haul trucks and by extension, LHDs.

2. METHODOLGY

2.1. Dosimetry and Time-Motion Studies

Effective miner noise exposure assessment can begin in the field, e.g., via a collaborating mine, with a noise dosimeter, watch, and log sheet. For this purpose, NIOSH uses Larson Davis Inc. Spark 705+ dosimeters (Fig. 4). Each 705+ essentially functions as four dosimeters, as NIOSH configures the instrument to simultaneously collect data per four sets of specifications. Table 1 lists the typical settings for each the four dosimeters for a Spark 705+. NIOSH employs the MSHA PEL data most often for post-processing of data and to quantify pertinent information, e.g., noise dose or exposure. For quality control purposes, the dosimeters are calibrated prior to and after each measurement interval. After the measurement interval, NIOSH offloads the data into Microsoft Excel and analyzes the data using Visual-Basic algorithms. In Excel, the incremental noise dose for each sample interval (1 second) is calculated and these incremental noise doses are summed to provide an overall noise dose for the sampling interval. This allows for a graphical representation of the noise dose accumulation over time.

Fig. 4—

Spark 705+ dosimeter used by NIOSH to log noise exposure data.

Table 1—

Dosimeter settings and MSHA PEL settings typically used for data analysis.

	Dosimeter 1 MSHA PEL1	Dosimeter 2 Wide range2	Dosimeter 3 NIOSH REL3	Dosimeter 4 OSHA action level4
Weighting	A	A	A	A
Threshold level (dB)	90	40	80	80
Exchange rate (dB)	5	3	3	5
Criterion level (dB)	90	85	85	85
Response	Slow	Slow	Slow	Slow
Upper limit (dB)	140	140	140	140
Sample interval (s)	1	1	1	1

¹Settings per the MSHA permissible exposure level.

²Settings to collect data per a low threshold level.

³Settings per the NIOSH recommended exposure level.

⁴Settings per the OSHA action level.

These data may or may not be collected for a full mining shift. However, a representative sampling interval of the machine under test is also observed. Calculating the rate of dose accumulation for given tasks lends itself to the development of a “modeled shift” where total noise dose may be estimated using the dose accumulation rate and exposure time for each task. In addition, NIOSH uses dosimeter data to determine the 8-hour time-weighted average (TWA₈) sound level for each test. Dosimetry and time-motions are conducted to quantify an equipment operator’s baseline noise exposure (prior to the implementation of engineering noise controls) and periodically during the development of the controls, whenever the controls are field evaluated. The objective, of course, is to reduce the equipment operator’s TWA₈ to less than 90 dB(A), bringing the operator’s noise exposure into compliance, or to reduce the operator’s noise exposure by 3 dB.

A machine’s duty cycle is determined and delineated into the requisite tasks. Figure 5 shows a portion of a log sheet developed for haul truck and LHD time-motion studies. During the study, the NIOSH investigator observes the equipment under test during its usual mining operation and logs the times associated with each of the duty cycle tasks. Using this time data and Visual-Basic algorithms allows the investigator to analyze subsets of the dosimeter data. As an example, analysis can be conducted on “Load” data only so that NIOSH can determine, as examples, mean and maximum sound levels during loading. With these tools, a researcher can quantify an individual’s noise dose and, equally importantly, when that individual accumulated the noise dose. Further, the times at which an operator is exposed to the highest noise levels can also be found.

Fig. 5—

Time-motion study log sheet example for a haul truck or LHD.

2.2. Noise Source Identification and Proof-of-Concept Testing

Noise source identification (NSID) is crucial to the development of noise controls. The ranking of noise sources is also an essential task to be performed. Consider a piece of machinery with three individual incoherent noise sources, L_p,1, L_p,2, and L_p,3, that make up the total sound level radiated by the machinery, L_p,TOT. The sound level of the machine would be determined by:

$$L_{\mathrm{p},\mathrm{TOT}}=10\mathrm{LOG}({10}^{0.1L_{\mathrm{p},1}}+{10}^{0.1L_{\mathrm{p},2}}+{10}^{0.1L_{\mathrm{p},3}})$$ (1)

Because sound levels are logarithmic, differences in the sound levels of the individual sources that are as small as 2 dB are significant. The most efficient means of reducing the total sound level is to decrease the contribution of the dominant sources. Table 2 shows several cases with assumed sound levels for the three individual sources that add up to the same total sound level for a machine.

able 2—

Examples of noise source reductions and the effect on the overall sound level.

Case		Lp,1 (first source) Sound level (dB)	Lp,2 (second source) Sound level (dB)	Lp,3 (third source) Sound level (dB)	Lp,TOT dB
1	Baseline	85	85	85	90
	Reduce Lp,1 10 dB	75	85	85	88
	Reduce all 5 dB	80	80	80	85
2	Baseline	85	87	82	90
	Reduce Lp,2 10 dB	85	77	82	87
	Reduce Lp,1 and Lp,2 7 dB	78	80	82	85
3	Baseline	89	80	80	90
	Reduce Lp,2 and Lp,3 10 dB	89	70	70	89
	Reduce Lp,1 10 dB	79	80	80	84

For case 1, each individual source contributes equally to the total sound level. In this case, it is likely that each individual noise source would have to be treated to reduce the total sound level. For example, reducing the sound level of the first source by 10 dB would reduce the total sound level to 88 dB, whereas reducing the sound levels of each individual source by 5 dB would reduce the total sound level to 85 dB.

In case 2, the second source contributes slightly more than half of the total sound energy, but the first source is also significant. The sound levels of both the first and second sources would have to be decreased to efficiently reduce the total sound level. Reducing the sound level of the second source by 10 dB would only reduce the total sound level to 87 dB. However, if the sound levels of both the first and second sources were reduced by 7 dB, the total sound level would be reduced to 85 dB.

In case 3, the first source is clearly dominant. Reducing the sound levels of both the second and third sources by 10 dB would reduce the sound level to 89 dB, whereas reducing the dominant source by 10 dB would reduce the total sound level to 84 dB. The most efficient means of reducing noise is to rank order the noise sources by the cumulative noise exposure and control the noise starting with the source causing the greatest cumulative noise exposure.

NIOSH employs several methods to locate the primary noise sources of equipment under investigation. In the field, this is most easily accomplished with source path contribution (SPC) analysis. In the laboratory, beamforming (BF) or SPC methods are used to localize noise sources. In prior research, NIOSH had conducted beamforming testing on continuous mining machines, vibrating screens, and roof bolting machines. NIOSH maintains a Brüel & Kjær Pulse system and an 84-microphone array (for sound data collection) and software (for data analysis) in a hemi-anechoic chamber (HAC) at its research facility near Pittsburgh, PA.

For the beamforming technique, a delay is assumed for each microphone based on its location. Individual delays from each microphone are chosen such that the plane waves arriving from a chosen direction will add up coherently. A large main lobe is concentrated in the direction of interest, and side lobes appear in directions of noninterest. This measurement system is used to pinpoint acoustic “hotspots” on the equipment under test.

In summary, the following steps are followed to use the beamforming technology.

1. A picture is taken of the equipment under test with a USB camera in the center of the microphone array.

2. The equipment is energized and operated under normal conditions.

3. Several seconds of noise data are sampled.

4. The equipment is deenergized.

5. The BF software computes the source sound levels.

6. The BF software generates the color noise contour map and overlays it on the picture of the equipment under test.

This color contour map shows the sound pressure levels in a plane at some distance in front of the array. Figure 6 shows representative beamforming data collected while conducting roof bolting machine research. Here, the drill steel clearly stands out as a primary noise source for roof bolting machines.

Fig. 6—

Example beamforming data collected during roof bolting machine research.

The second method utilized by NIOSH, SPC analysis, can be used for airborne or structure borne noise sources. However, this discussion will be limited to airborne noise sources.

Two sets of noise measurements are needed to identify the most prominent sources and paths. A transfer function from a prospective noise source or sources to a receiver, typically an equipment operator location, is required. Developing this transfer function requires using a known noise source — for NIOSH a volume velocity source (VVS). The VVS is configured as a white-noise source, generating a broadband noise signal. For transfer function testing, the Device Under Test (DUT) is stationary and unpowered, with the VVS serving as the sole noise source during these measurements.

The VVS is placed at a prospective noise source location and energized. Sound level measurements are sampled at the VVS location, at each of the other prospective source locations, several additional redundant locations, and also at the receiver location. Thus, there is a known input (the broadband VVS source at the noise source location) and the associated output (the sound level at the receiver location).

This testing was repeated with the VVS located at each of the prospective noise source locations. This data set serves to generate a matrix of transfer functions between the prospective noise source locations and the receiver location. A second set of noise measurements involves the operating data of the DUT. For these tests, the DUT serves as the sole noise source during the data collection. The DUT remains stationary and is operated under normal conditions. Again, noise data are collected at the prospective noise source locations, the redundant locations, and the receiver location.

Analysis of this data yields an estimated contribution of the sound level emanating from each source to the sound level at the receiver location. These sources can then be ranked, revealing the noise sources that should be treated first to reduce receiver location sound levels. Once the sources causing the greatest cumulative noise exposure are determined, proof-of-concept testing is conducted to verify that reducing noise emissions at a given noise source will reduce the sound level at a receiver location. Simple noise controls are tested to ensure that the research is following a viable path in pursuing controls to address a particular noise source. These noise controls do not have to be durable, as they only have to survive for the duration of the testing. They also do not have to be practical, as they are tested only to ensure that the research direction shows promise in reducing noise emissions. Should the proof-of-concept testing show a worthwhile reduction in the sound level at the receiver location, then more durable and practical controls will be investigated.

2.3. Noise Control Development

Key to the development of noise controls is determining a baseline noise emission for the equipment under investigation. Unlike sound pressure, sound power is independent of the environment. It is the actual noise energy emitted by a device. NIOSH maintains a NVLAP-accredited Acoustic Test Chamber (ATC) for precision-grade sound power level measurements per ANSI/ASA S12.51/ISO 3741^2,3. Engineering-grade measurements are conducted per ISO 3743–2⁴. Calculating the sound power emission of equipment before and after the application of noise controls will quantify the reduction in the noise energy of the equipment. This is useful, as it reveals the reduction in the sound pressure levels to which the equipment operator and miners in nearby areas are exposed. NIOSH uses the comparison method via a calibrated reference sound source (RSS) to calculate the sound power. Later, after the development and implementation of the noise controls, sound power tests would be repeated and the results compared to the baseline data collected earlier.

The ATC is equipped with 15 optimally spaced microphones to sample sound levels. To ensure that the equipment noise levels are not adversely affected by background noise, the background noise is sampled by the Pulse system. Next, the reference sound source is operated at its calibrated rotation speed, and its sound level is sampled. Finally, the untreated DUT is brought into the chamber and a series of tests is conducted. From these three measurements, the sound power level is calculated. Three tests are then conducted per condition (e.g., low idle, high idle, etc.), and the results are averaged to calculate a mean sound power level. The DUT is removed from the ATC and the noise control installed. Then, the tests are repeated, including the background and RSS noise measurements and another series of DUT noise measurements. These results are averaged and compared to the results of the untreated DUT. An effective noise control should reduce the noise emission (sound power) of the DUT.

Simultaneously with the beamforming testing, NIOSH collects operator location sound pressure level data. This also allows for spectral analysis of the sound levels, providing key information pertaining to the technical approach for developing requisite noise controls.

Should laboratory testing show that the noise control does not perform as hoped, NIOSH will revise the control and repeat the laboratory evaluation with BF, sound pressure level, and sound power testing. Total life cycle cost, sound engineering judgment, and practicality of the noise control all play a role in the decision to continue a particular noise control approach, combine it with additional noise controls, or abandon the approach entirely.

2.4. Noise Control Evaluation

Those controls that show promise are field-evaluated in a manner similar to early NIOSH field testing, when the cumulative dose and exposure time associated with each task for an operator were determined via dosimetry and time-motion studies. Sound pressure levels at the operator’s position and frequency spectral data are collected, and NIOSH compares the cumulative noise dose when operating an untreated machine with the cumulative noise dose when operating a machine with noise controls installed. The operator’s 8-hour time-weighted average is the standard used for comparison. A single control or a combination of controls is considered a success if either of the following criteria are met:

• The control brings an operator previously out of compliance with the MSHA PEL into compliance.

• The control results in a 3-dB or greater reduction in the 8-hour time-weighted average sound level.

3. CASE STUDY — AN UNDERGROUND HAUL TRUCK

3.1. Dosimetry and Time-Motion Studies

PMRD researchers conducted dosimetry and time-motion studies on multiple haul truck operators at an underground mine. In this case study, per the NIOSH Hearing Loss Prevention Team protocol, the duty cycle for a haul truck was observed, and the required tasks were determined to be the following:

• Load, while the haul truck was being loaded

• Haul, while the haul truck was traveling to the dump site while loaded with ore

• Dump, while the haul truck was dumping

• Return, while the haul truck was traveling to the loading site empty, logged as a second haul cycle

• Haul + return: haul and return data combined

To reduce operator noise exposure, it is important to know what the operator was doing when accumulating the bulk of the noise dose. Time and money can be wasted developing noise controls to address intermittent and/or short-duration loud noise sources that do not make a significant contribution to the operator’s noise dose. Ore striking the dump bed of the haul truck during loading or the dump chute during dumping is an example of intermittent and/or short-duration noise. Rather, it is best to first determine the tasks that contribute the most to the operator’s noise exposure and then address the noise sources that are active during those tasks.

A summation of task observation times for three underground haul trucks is shown in Table 3. Here, the haul truck activities were documented for partial shifts, roughly 4 to 5 hours per machine. The bulk of the time was spent hauling loaded and returning unloaded with a lesser amount of time spent during loading. Only 3% to 5% of the time was spent dumping ore during the observations.

Table 3—

Haul truck time-motion study observation times (hh:mm).

Task	Haul truck A	Haul truck B	Haul truck C
Load	0:36	0:36	0:27
Haul	1:37	1:47	1:33
Dump	0:12	0:10	0:10
Return	1:53	2:23	1:34
Total	4:19	4:55	3:44

Figure 7 shows the duty cycle tasks and the associated cumulative noise dose for each of the three haul truck operators. As shown, minimal noise dose was accumulated during the load and dump tasks. Therefore, engineering noise controls developed to address noise that was generated specific to these tasks would do little to reduce an operator’s noise dose. Most of the operator’s noise dose was accumulated during hauling and returning, considered to be the “operating” tasks.

Fig. 7—

Haul truck operator noise dose by duty cycle task.

Figure 8 shows the time-weighted average sound level for each task. As mentioned previously, noise controls designed to reduce sound levels during loading or dumping would not significantly reduce noise exposures of haul truck operators, because most of their noise exposure is due to hauling ore and returning to the loading site. The most efficient means of reducing noise exposure of these operators was to reduce the noise generated while the haul truck is being driven between the loading and dumping locations. Addressing primary noise sources during the hauling and returning duty cycle tasks would be essential to reducing noise exposures.

Fig. 8—

Haul truck operator TWA sound level by duty cycle task.

Based on the data of Fig. 7, the reduction in a haul truck operator’s noise exposure for a given reduction in the sound levels during hauling and returning was calculated. Figure 9 shows the estimated noise dose for a haul truck operator for hauling/returning sound level reductions of 0 (baseline), 2, 3, 4, 5, and 6 dB. This assumes that all other noise the operator is exposed to remains unchanged, such as when dumping or loading or at any undocumented time. In this example, the operator’s baseline noise dose was roughly 133%. Given a 2-dB reduction in the hauling/returning noise level, the operator’s noise dose would be roughly 111% and then would continue to drop as the operator location sound level decreases. The source or sources of the “operating” noise must be found and suitable noise controls to address this noise source or sources be implemented.

Fig. 9—

Graph demonstrating how reducing operating noise will reduce operator noise dose.

3.2. Noise Source Identification and Proof-of-Concept Testing

Prior haul truck testing in a hemi-anechoic chamber at the PMRD facility in Pittsburgh indicated that airborne noise sources are probably the most prominent contributors to noise at the operator’s location. Additionally, field tests at the University of Arizona’s San Xavier Mining Laboratory also demonstrated that the likely sources of haul truck noise were airborne in nature. Therefore, in this case study, NIOSH visited a collaborating mine and investigated a haul truck underground in a shop area using SPC techniques for NSID⁵. Seven microphones were positioned near potential noise source locations. These included the air intake, the engine right side, the engine top, the engine exhaust, the engine pump location, at the fan, and underneath the engine (bottom). Two redundant microphones were used to improve the quality of the data, one near the fan and a second near the rear of the engine (Figs. 10 and 11). Two additional microphones were positioned at the operator location to represent right and left ear positions. In total, 11 microphone positions were used.

Fig. 10—

Test microphones at noise source locations for SPC testing.

Fig. 11—

Test microphones at the top of the engine and near a hydraulic pump at the rear of the engine.

NIOSH completed two sets of noise measurements to identify the sources and paths that contribute the most cumulative worker exposure. The initial set of measurements involved collecting data to develop a transfer function for the SPC analysis. The potential sources are noted above and the receivers are at the operator’s ear locations. NIOSH developed a transfer function involving VVS as the known noise source (Fig. 11). The VVS was configured as a white-noise source, generating a broadband noise signal.

For these tests, the haul truck was stationary and unpowered; the VVS served as the sole noise source during these measurements. The VVS was placed at a noise source location, such as that shown in Fig. 12 at the fan, and turned on. Sound level measurements were collected at each of the seven source locations, the two redundant microphones, and the two operator location microphones. Thus, there is a known input (the broadband VVS source at the noise source location) and the associated output (the sound level at the operator’s location). This testing was repeated with the VVS located at each of the seven noise source locations, and an associated set of sound level measurements was taken. These data served to generate a matrix of transfer functions between the seven expected noise source locations and the two receiver positions at the operator location as well as the transfer functions between the VVS position and the other noise source locations.

Fig. 12—

Using the volume velocity source to simulate a noise source at the fan location.

The second set of noise measurements involved the operating data of the haul truck. For these tests, the haul truck served as the sole noise source during the data collection. The haul truck was stationary and operated under both low and high idle conditions. Again, noise data were collected at the seven noise source locations, the two redundant locations, and the right and left ear operator locations.

To complete the SPC analysis, the transfer function and the operating data from the second set of measurements facilitated an estimate of the sound level at the operator’s location, given the operating noise level at the seven noise source locations.

Figure 13 shows the resulting contributions for each source location per the SPC analysis. The estimated contribution from each of the source locations is shown for both the operator’s left and right ear. The three primary source locations with respect to the proximity of the operator’s position are the pump near the rear of the engine, the cooling fan, and the bottom of the engine. The contribution from the bottom of the engine was likely a combination of engine and fan noise. The engine pump location, which is inside the engine compartment (Fig. 11), is the primary source location that is closest to the operator location. The cooling fan is also near the operator location (Fig. 9). Contributions from the engine exhaust and air intake locations are insignificant compared to engine and fan noise. In this case, engine and fan noise sources must be treated first to reduce the operator location sound level and, thus, the operator noise exposure.

Fig. 13—

Calculated noise source location contribution to the operator location sound level.

At a later date, NIOSH returned to the mine and conducted proof-of-concept testing of the same model haul truck as in the SPC testing. Here, the objective was to test simple noise control concepts to see if they reduced the sound level at the operator location. Two concepts were tested. One was to better enclose the engine compartment of the haul truck with a barrier material to determine if engine noise should be addressed. Sound level meter measurements at the operator location with the haul truck at high idle indicated a 2-dB reduction in operator location sound level (Fig. 14). The second concept was to simply disengage the cooling fan. Of course, it is not practical to completely eliminate fan noise, but for the sake of this experiment, this approach served the necessary purposes. Several short-duration sound level meter measurements were conducted. Here, a 6-dB reduction in the sound level was achieved (Fig. 15). From this, reducing fan noise served as the focus for the development of haul truck engineering noise controls.

Fig. 14—

Haul truck proof-of-concept testing with better engine compartment barriers resulted in a 2-dB reduction in the operator location sound level.

Fig. 15—

Haul truck proof-of-concept testing by disengaging the fan resulted in a 6-dB reduction in the operator location sound level.

3.3. Noise Control Development

It was most important to develop a noise control package focused on the cooling fan, which would reduce noise emissions while maintaining the required airflow for cooling. This prompted NIOSH to develop a fan test apparatus (FTA) for laboratory testing (Figs. 16 and 17)⁶. The intention was to conduct testing based on haul trucks and then extend the solutions to LHDs. The fan test apparatus was designed to duplicate a haul truck engine compartment as closely as possible, duplicating the noise-generating components and airflow paths through a haul truck engine compartment. Using this approach, sound level and airflow measurements could be made and used to determine an optimal configuration of noise controls.

Fig. 16—

Fan test apparatus for duplicating a haul truck engine compartment.

Fig. 17—

Fan test apparatus and equipment used for airflow testing in the PMRD hemi-anechoic chamber.

NIOSH conducted testing in a hemi-anechoic chamber per ISO 3744⁷ and in a reverberation chamber per ISO 3743–2 to determine the sound power levels of various cooling package configurations^8,9. In series with these tests, NIOSH collected airflow data for each of the cooling package configurations. A baseline sound power level and airflow were determined using the stock cooling fan and rotation speed. Then, other cooling package configurations were tested, seeking to determine a configuration that emitted the least amount of noise while maintaining at least the stock airflow and thus maintain cooling capacity^8,9.

NIOSH used the FTA to test a variety of axial fans to access their ability to generate the required airflow rate while generating less noise⁸. A summary of fan configuration variables tested is listed in Table 4. PMRD tested three types of Multi-Wing America axial fan designs, an airfoil, a high pressure airfoil, and a sickle fan (Figs. 18 and 19). Two fan diameters have been tested: 76-cm airfoil and high-pressure airfoil fans and 76-cm and 81-cm sickle fans.

Table 4—

Fan configurations laboratory tested with the FTA.

Fan type	Airfoil, high pressure airfoil, sickle
Fan diameter (cm)	76, 81
Insertion %	0, 25, 33, 50, 67, 75, 100
Pitch angle (°)	35, 40, 45
Rotation speeds (rpm)	2000, 2200, 2450, 2600

Fig. 18—

Stock airfoil fan.

Fig. 19—

Sickle type fan used as part of the noise cooling package.

Table 5 compares the stock cooling configuration to the cooling package. In the first column is the stock or baseline configuration for the FTA. This was the configuration that most accurately met the actual haul truck cooling package configuration. The most successful results have been increasing the fan diameter, from 76 to 81 cm (30″ to 32″) and changing from a Multi-Wing America airfoil fan to a Multi-Wing America sickle fan. Note that the fan insertion percentage, blade angle, rotation speeds, etc. for the stock condition and the larger diameter fan are the same.

Table 5—

Tested cooling package configurations and their parameters.

	Stock	Cooling package
Fan type	Axial	Axial
Fan geometry	Airfoil	Sickle
Fan diameter (cm)	76	81
Insertion (%)	67	67
Blade angle (°)	35	35
Fan-to-block-distance (cm)	10.2	10.2
Fan-to-core-distance (cm)	7.6	7.6
Rotation speed (rpm)	2450	2100

Shown in Fig. 20 are data collected using the stock 76-cm diameter airflow fan and an 81-cm diameter sickle fan. For this example, the required airflow rate is 27,900 m³/h and the sound power emission was 116.1 dB(A). At the required flow rate, the 81-cm fan sickle fan emits 113.5 dB(A). In the laboratory, this was accomplished by reducing the rotation speed when using the 81-cm sickle fan. Thus, by changing the fan geometry (axial to sickle), increasing the fan diameter (76 cm to 81 cm), and reducing the rotation speed, a configuration was determined that met the airflow rate requirement while generating less noise.

Fig. 20—

Graph showing that the 81-cm sickle fan met the airflow requirements of 27,900 m³/h and emitted less noise.

To reduce the engine noise reaching the operator, NIOSH sought to improve the noise barrier the mine used on its haul trucks. NIOSH selected Duracote Durasonic 5356 as a replacement for the barrier material that the collaborating mine was typically using. The Durasonic 5356 is a mass-loaded vinyl barrier with a vinyl facing and an acoustical urethane foam underlay. The Durasonic 5356 is lightweight and flexible, making it easier for the mine to install. It can also be easily shaped to better enclose the engine compartment.

3.4. Noise Control Evaluation

Based on the above noise control package research, NIOSH conducted time-motion and dosimetry studies on a haul truck and an LHD at the same collaborating mine to determine operator noise exposures. These studies were performed with the machines outfitted as they normally are (stock) and with noise controls supplied by NIOSH⁹.

Figure 21 shows the 8-hour noise dose (Dose (8)), the noise dose accumulation per hour, and noise exposure (TWA (8)) for a haul truck operator. The data are shown for three conditions, defined as follows:

1. Stock 30″ airfoil fan, stock fan pulley (1:1) — This is the stock condition for the haul truck, stock cooling fan, and pulley. Also, the noise barrier material that the mine normally used to enclose the engine compartment was installed on the haul truck.

2. Stock 30″ airfoil fan, stock fan pulley (1:1), Durasonic 5356 — Here, the stock cooling fan and pulley are used, but the usual barrier material has been replaced with a barrier material part number Duracote 5356, manufactured by Durasonic.

3. 32″ sickle fan, fan pulley (0.9:1), Durasonic 5356 — Here, a larger fan of a different design was installed as well as a different fan pulley, used to reduce the fan rotation speed to 90% of stock. The Duracote 5356 barrier material was also installed.

Fig. 21—

Haul truck operator 8-hour noise dose and exposure, stock and with noise controls installed.

Analysis of the data shows that replacing the mineinstalled barrier material with the Duracote 5356 resulted in a 2-dB reduction in the operator exposure (condition 2 above). Then, while still using the Duracote 5356, NIOSH replaced the stock fan with a larger sickle fan and reduced the rotation speed (condition 3 above). This resulted in a 9-dB reduction in operator noise exposure from the stock condition. To illustrate the significance of this, Fig. 22 shows that the operator can work much longer with the noise controls installed (5.1 hours as compared to 1.6) before reaching the MSHA PEL.

Fig. 22—

Haul truck operator time to reach MSHA PEL, stock and with noise controls installed.

Key advantages to this approach of improving the engine compartment noise barrier and changing the fan type, size, and rotation speed are that:

1. The operator noise exposure is reduced.

2. The mine can perform the noise control installation in-house. The larger sickle fan, the fan pulley, and the Duracote barrier material were all installed at a collaborating mine by the mine’s own mechanics.

3. The tested parts are stock parts and can be purchased directly from an original equipment manufacturer (OEM).

4. Parts installation can be completed during the normal rebuild cycle or whenever the machine is in a shop for other maintenance.

It was important to ensure that the addition of these noise controls did not create a condition where the haul truck overheated. During this field testing, adequate airflow was maintained, the haul truck did not overheat, and the haul truck operator was able to work at his normal pace.

Figure 23 shows the 8-hour noise dose (Dose (8)), the noise dose accumulation per hour, and noise exposure (TWA (8)) for an LHD operator. The data are shown for three conditions, as follows:

1. Stock 24″ airfoil fan, stock fan pulley (1.1:1) — This is the stock condition for the LHD, stock cooling fan, and pulley. Also, the noise barrier material that the mine normally used to enclose the engine compartment was installed on the LHD.

2. 26″ sickle fan, fan pulley (0.96:1), Dura-sonic 5356 — Here, a larger fan of a different design was installed as well as a different fan hub to reduce the fan rotation speed to roughly 87% of stock. The mine-supplied barrier material was replaced with the Duracote 5356.

3. 26″ sickle fan, fan pulley (0.87:1), Dura-cote 5356 — Here, another fan pulley was installed to further reduce the rotation speed by an additional 8% from stock.

Fig. 23—

LHD operator 8-hour noise dose and exposure, stock and with noise controls installed.

The data show that replacing the stock cooling fan and pulley with the larger sickle fan and fan pulley to reduce the rotation speed resulted in a 2-dB reduction in the operator exposure (condition 2 above). During the testing, the LHD did not overheat. Figure 24 shows that the LHD operator can work roughly 25% longer (3.5 hours as compared to 2.8 hours) before reaching the MSHA PEL. Again, this approach of improving the engine compartment noise barrier and changing the fan type, size, and rotation speed resulted in a reduction of operator noise exposure and allowed for the operator to work longer before reaching the MSHA PEL. The advantages listed earlier for the haul trucks apply here as well.

Fig. 24—

LHD operator time to reach the MSHA PEL, stock and with noise controls installed.

The reduction in LHD operator exposure was significantly less than that of the haul truck operator (2 dB as compared to 9 dB). In an attempt to document greater reductions in LHD operator exposure, NIOSH conducted additional testing with the fan rotation speed reduced even further, with a 0.87:1 fan pulley (condition 3 above). An additional 3-dB reduction in operator exposure was achieved (Fig. 22) but at the expense of cooling ability. During this testing, the LHD overheated, indicating that the rotation speed had been reduced too much.

4. SUMMARY

The Hearing Loss Prevention Team of NIOSH PMRD conducts noise control research with a variety of testing methods and techniques that lend themselves to the development of noise controls to reduce mining equipment noise emissions with the goal of reducing hearing loss in mining. A summary of some of these techniques has been presented in this paper as well as a case study looking at the development of noise controls for underground haul trucks and LHDs. This case study demonstrates how the noise control development methods used by PMRD are applied in practice.