Introduction Noise-induced hearing loss is one of the most common health problems among military service personnel. Exposure to noise in military vehicles constitutes a large proportion of total noise exposure. This pilot study aimed to evaluate in-vehicle noise levels depending on the type of vehicle, riding compartment and road surface.
Method Noise levels were measured in armoured personnel carriers and heavy all-terrain trucks, in the cab and rear passenger compartment, while driving on paved or off-road surfaces. The results were compared with national LLV and allowed noise exposure times were calculated per vehicle and surface.
Results The equivalent noise levels in the cab of SISU XA-188 (p=0.001) and peak noise levels in MAN 4620 (p=0.0001) and DAF 4440 (p=0.0047) were higher on paved road, compared with off-road. The equivalent noise levels in the canvas covered rear compartment of MAN 4620 were significantly higher than in the cab on both paved (p=0.004) and off-road (p=0.0003). Peak noise levels in the cab of DAF 4440 exceeded the parameters measured in the canvas covered rear compartment on both paved (p=0.002) and off-road (p=0.0002). In most cases, peak noise levels were below the LLV (p=0.02–0.0001). The maximum noise exposure to passengers in the canvas covered rear compartment of MAN 4620 despite road surface was calculated 0.6 hours per working day.
Conclusion A high risk of noise-induced hearing loss among military personnel occurs during long distance transportation with vehicles showing noise levels higher than allowed LLV.
- military vehicles
- equivalent noise
- peak noise value
Statistics from Altmetric.com
Noise in military vehicles is significant source of exposure among military personnel.
Armoured vehicles have in general higher noise levels than heavy all-terrain trucks.
In heavy all-terrain trucks higher levels appear in rear passenger compartment.
Higher noise levels tend to appear while driving on paved road compared to off-road.
Exposure to equivalent noise poses higher risk than exposure to peak noise in vehicles.
Noise is a continuing concern to the military and one of the most important health hazards to consider when selecting a military vehicle.1 2 Several studies have shown that exposure to noise in military vehicles constitutes a large proportion of total noise exposure.3 4 A study among Finnish conscripts showed that 89% of personnel were exposed to 85 dB(A) of military noise on a weekly basis throughout their service, and 80% of recruits had a cumulative noise dose of 85 dB(A) at the end of military service.5 Exposure to high and chronic noise levels from different types of military vehicles may cause noise-induced hearing loss (NIHL) among crew members.6 NIHL among military personnel can result from both, chronic exposure such as helicopter, plane or vehicle noise,7 and also acute exposure such as impulse noise from gunfire.8 9
A deleterious relationship between noise and the health of military staff has been revealed by a large number of studies.5 10–14 Therefore, it has been shown that hearing loss is significantly associated with military service experience.10 15 16 NIHL is one of the most common health problems among service personnel that has constituted: 67.9% of officers of Finnish Defence Forces,5 65.7% of Belgian officers10 and 65% of local Defence Forces personnel in Estonia.17 The hearing loss increases in correlation with longer service time,10 18 and therefore, longer exposure time. The audiometry measurements among Canadian Forces personnel in the oldest group showed approximately 17%–26% having a moderate-to-severe hearing loss, a greater proportion in the left ear than the right at 4 and 6 kHz. Half of the sample reported some tinnitus, and of these, one-third perceived the tinnitus to be moderate or loud.18
On the one hand, the risk perception has been moderate, with 27% of personnel of a Finnish battalion (n=117) returning home from Kosovo reporting noise as the most serious risk factor for health among physical risk factors in the military.19 On the other hand, the need to reduce noise and vibration in the driver’s cab of passenger vehicles is recognised by military vehicle manufacturers, who are actively trying to improve the quality of vehicles. However, over a long period of use, the technical parameters of a vehicle may change, altering noise levels in the driver’s cab and rear passenger compartment.20
We hypothesised that noise levels of military heavy all-terrain trucks and armoured personnel carriers might exceed noise limit level values (LLV) and maximum allowed noise exposure levels, increasing the risk of NIHL.
As still relatively little is known on NIHL relation to exploitation of variant models of armoured personnel carriers and heavy all-terrain trucks in the military, we were aimed to measure noise levels in different vehicle models, riding compartments and road surfaces, to compare the results with national LLV, and to calculate maximum allowed noise exposure levels and maximum allowed exposure time of staff using military vehicles during training and transportation.
Noise levels were measured in two different types of vehicles more often used in the Estonian Defence Forces for training and transportation: two armoured personnel carriers (SISU XA-180, SISU XA-188); and five heavy all-terrain trucks (DAF 4440, MAN 4610, MAN 4620, MAN 4640, Mercedes-Benz Unimog 1300) (figure 1).
Research subjects comprised two groups: drivers and persons being transported. Drivers sat in the cab and persons being transported in the canvas covered rear compartment. Nine subjects (seven drivers and two passengers being transported in the rear compartments) were measured using dosimetry. The drivers of the vehicles SISU XA-180 and SISU XA-188 were wearing communication headsets Racal Acoustics RA5001 Raptor (SNR 23 dB) (Esterline Technologies Corporation, USA). The subjects of heavy all-terrain trucks did not wear any kind of hearing protection equipment.
Measurements were carried out by the Testing Centre of University of Tartu conforming to the requirements of EN ISO/IEC 17025:2005 as testing and calibration laboratory. Noise exposure of military personnel was measured using the noise dosimeter set Casella CEL-350/K5 (Ideal Industries, Kemston, UK). The microphone of the exposimeter was fastened to a person’s uniform at the shoulder, that is, close to the auditory channel. The average measuring period in the cab on paved road was 20.0±10.3 min and off-road 16.8±9.4 min. In the canvas covered rear passenger compartment the measuring period was 6.0 min. The noise dosimeter measured equivalent noise and peak noise levels at 1 min intervals.
The noise created by vehicles travelling on two different types of surface was measured, an asphalt road as paved road and a natural landscape uneven road as off-road. The measurements were carried out in winter, when outside air temperatures were –2.6 to –0.2°C. Measurements were taken once vehicles had reached normal motor operating temperature.
Vehicles driving speed was measured by Global Positioning System device Montana 600 (Montana, Garmin). Driving speed showed in heavy all-terrain trucks 38–55 km/hour on paved road and 16–40 km/hour on off-road, in armoured personnel carriers 36 km/hour on paved road and 19 km/hour on off-road.
Noise exposure calculations
Based on the measurement results, daily noise exposure levels of personnel were calculated on the basis of a work day with a maximum duration of 8 hours and a 30 min break. Maximum noise exposure levels, and maximum exposure times, were calculated per vehicle and surface.
The equivalent noise level per working day (daily noise exposure LEX,8h) and per week (LEX,40h) was calculated using the formula:
LEX,To=LAeq,Te + 10 log(Te/T0)
where LAeq,Teis equivalent noise level during the time period (Te),
Te is time exposed to noise during the workday (hour) and
T0 is duration of the working day/week (T0=8 hours, week T0=40 hours).
In accordance with international standards, results were compared with the worst-case scenario limit values provided by the normative document ‘Health and Safety Requirements for the Working Environments Affected by Noise, Maximum Noise Limits for the Working Environments and the Noise Measurement Procedure. Government of the Estonian Republic regulation No. 108 of 12 April 2007’. Accordingly, a daily or weekly noise exposure LLV is 85 dB(A) and action value 80 dB(A), peak noise LLV 137 dB(C) and action value 135 dB(C).21
Statistical analysis was performed using Microsoft Excel 16.0. Equivalent and peak noise levels with uncertainty (U(LAeq) and U(LpC)) were measured in two different types of vehicles, by riding compartment (cab and rear passenger compartment) and road surface (paved and off-road). An unpaired t-test was used to compare the results of different types of vehicles, riding compartment and road surface. One-sample t-test was used to calculate differences between the measured noise levels with LLV. Statistical significance was calculated at p values ≤0.05.
Equivalent noise levels in the cab at the driver’s position in different vehicles and road surfaces are presented in figure 2. In general, equivalent noise levels during off-road driving tended to be lower in all types of vehicles compared with driving on paved road (figure 2), whereas statistically significant difference was measured in the cab of SISU XA-188 (p=0.001).
Equivalent noise levels in the cab of armoured personnel carrier SISU XA-180 was higher than the action level value (80 dB(A)), both on paved road and off-road (p=0.0001–0.001). The equivalent noise level at the driver’s position of SISU XA-180 was 89.4±1.8 dB(A) while driving on off-road surfaces.
Equivalent noise level difference by vehicle model and type of surface was statistically not significant in the group of heavy all-terrain trucks, stronger difference was observed in armoured personnel carriers on off-road between the vehicles SISU XA-180 and SISU XA-188 (p=0.0001).
Equivalent and peak noise levels with uncertainty compared with the action level value and LLV by type of vehicle, road surface and riding compartment is shown in table 1. As shown in table 1, the equivalent noise levels were measured from 77.0 (±1.9) dB(A) to 94.7 (±1.8) dB(A) and peak noise levels between 113.8 (±1.3) dB(C) and 136.8 (±2.2) dB(C).
When to compare the results by type of surface, higher peak noise levels were measured in the cab of MAN 4620 (p=0.0001) and DAF 4440 (p=0.005), while driving on paved road, compared with off-road and lower noise levels detected in the cab of MAN 4610 (p=0.0001), while driving off-road, compared with paved road. Peak noise levels in the driver’s cab depending on type of vehicle and road surface are presented in figure 3.
The difference between peak noise levels in the cab of vehicles depending on vehicle model by surface was statistically not significant in the group of armoured personnel carriers. Statistically significant correlation was found in the group of heavy all-terrain trucks (table 2). Although peak noise levels were below the normative values, more often we see differences of peak noise levels when driving on paved road, except DAF 4440, MAN 4620 and MAN 4640, where peak noise levels showed differences also when driving on off-road (p=0.05–0.0001).
Figure 4 compares the equivalent noise levels (±U(LAeq) in the driver’s cab and the canvas covered rear passenger compartment of the DAF 4440 and MAN 4620. The graph shows that in the canvas covered rear passenger compartment, equivalent noise levels in all cases were higher than in the driver’s cab of MAN 4620, during both paved road (p=0.004) and off-road driving (p=0.0003). Also, the LLV 85 dB(A) was significantly exceeded in the canvas covered rear passenger compartment of the MAN 4620 during paved road 94.2±2.3 dB(A) (p=0.01) and off-road 94.7±1.8 dB(A) (p=0.003) driving. However, we did not see significant compartment and LLV differences of equivalent noise levels in DAF 4440.
Figure 5 compares the peak noise levels (±U(LpC) in the driver’s cab and the canvas covered rear passenger compartment of the DAF 4440 and MAN 4620. When to compare the peak noise levels in the cab of vehicles by riding surface, significantly higher peak noise levels were measured in MAN 4620 (p=0.0001) and DAF 4440 (p=0.005), while driving on paved road, compared with off-road. Contrary, the higher off-road peak noise levels in the cab of MAN 4610 were measured (p=0.0001), compared with paved road. The peak noise levels in the cab of DAF 4440 exceeded the parameters measured in the canvas covered rear compartment: on paved road (p=0.002) and off-road (p=0.0002). Contrary, the off-road peak noise parameters in the canvas covered rear compartment of MAN 4620 exceeded the cab parameters (p=0.007).
To assess whether noise requirements in military vehicles are being exceeded, it is necessary to calculate maximum allowed noise exposure time per working day in the canvas covered rear passenger compartment of the vehicles in which personnel were transported. In most cases, the vehicles were safe and had allowable exposure times in terms of daily noise exposure time (8 hours) during off-road driving than paved road driving. Among the studied vehicles all the driver’s cabins of heavy all-terrain trucks could be used for a whole working day (8 hours). In the canvas covered rear passenger compartment of the DAF 4440 and the MAN 4620, the maximum allowed exposure time for pave road driving was between 0.6 and 5.0 hours, for off-road driving between 0.6 and 4.5 hours, respectively. In the driver’s cab of armoured personnel carrier SISU XA-180, the maximum allowed noise exposure time on off-road was 1.7 hours, in the driver’s cab of armoured personnel carriers SISU XA-180 and SISU XA-188 the maximum allowed noise exposure time on paved road was 1.4 and 3.1 hours, respectively, which might be exceeded during real training process.
Heavy all-terrain trucks
Measured noise levels in military vehicles by type of vehicle, riding compartment and road surface were compared with a daily noise exposure LLV 85 dB(A) and peak noise LLV 137 dB(C) (table 1). In most cases, the equivalent noise levels in the cab of vehicles exceeded LLV and tended to be higher while driving on paved road, compared with off-road. The highest equivalent noise level was detected in the canvas covered rear passenger compartment of the MAN 4620: 94.7±1.8 dB(A) when driving off-road and 94.2±2.3 dB(A) on paved road. With such noise levels, the calculated maximum allowed safe off-road noise exposure time was just 0.4 hour. As equivalent noise levels exceeded LLV in the driver’s cab and canvas covered rear passenger compartment of the MAN 4620 both during paved road and off-road driving, it is necessary that passengers in the rear compartment limit noise exposure time or use effective personal protection equipment. According to the national action value, levels between 80 and 85 dB(A) must be taken under serious consideration.21 Thus, the DAF 4440, MAN 4610, MAN 4620 and MAN 4640 need to be considered as potential risk factors to hearing health. The maximum allowed noise exposure time should be limited in both the driver’s cab and canvas covered rear passengers compartment of the MAN 4620 and in the canvas covered rear compartment of the DAF 4440.
In general, the peak noise levels in the cab of vehicles stayed below the LLV parameters. The highest peak values in the driver’s cab were measured in the four-wheel drive truck DAF 4440: 136.8±2.2 dB(C) on paved road and 129.5±1.3 dB(C) off-road. Significantly higher peak noise levels were measured in the cab of MAN 4620 and DAF 4440, while driving on paved road, compared with off-road. Contrary, the higher off-road peak noise levels in the cab of MAN 4610 were detected, when compared with paved road.
Our results of noise level in the driver’s cab compared with the canvas covered rear passenger compartment of heavy all-terrain truck were quite similar to the results by Aziz et al,22 where the equivalent noise level in the rear passenger compartment was higher than in the driver’s cab. Contrary, the peak noise levels in the cab of DAF 4440 exceeded the parameters measured in the canvas covered rear compartment on both paved and off-road. Only the peak noise levels measured the canvas covered rear compartment exceeded the cab parameters of MAN 4610, while driving off-road.
Both the older model SISU XA-180 and the newer model SISU XA-188 had higher noise levels in the driver’s cab over an 8-hour working day than the 85 dB(A) limit. The SISU XA-188 also had a significant difference in equivalent noise levels in the driver’s cab between paved road and off-road driving. Thus, it is highly recommended that personnel use personal protection equipment, especially during paved road driving. Our results are in line with those of the armoured vehicles MTVE and M113A223 and similar to the vehicles LAW III, Bison and M113A2 ADATS, which had overall noise levels that exceeded the Canadian Labour Code 8 hour noise exposure limit of 87 dB(A).3
Possible measures to reduce noise exposure
Daily noise exposure level in the case of an 8-hour working day must not exceed 85 dB(A) and peak noise pressure (eg, impulsive sound) must not exceed 137 dB(C). Analysis of the collected data showed that equivalent noise values higher than 85 dB(A) occurred in the majority of armoured vehicles and total daily noise dose is important in relation to the health of the service personnel. Once a person’s noise exposure level exceeds 80 dB(A) or peak noise pressure exceeds 135 dB(C), measures should be taken to reduce the impacts of noise. One of the simplest measures is to use personal protective equipment (PPE) such as earplugs and earmuffs. However, the use of PPE by personnel in military vehicles is often infrequent.4 24 25 In addition, hearing conservation programmes, which include noise level measurements, noise control, safety instructions and educating military personnel, the use of hearing protection devices, repeated audiometric testing,13 might be useful to national defence forces to reduce the risk of hearing damage from military vehicles.
The strength of the study was selection of different models of military vehicles and the equivalent and peak noise measurements in different driving compartments and road surfaces. Whereas, the main limitations of our study were that we conducted a study with small sample size representing opportunities of the country. The vehicles for the study were chosen occasionally and the measurements were designed close to experimental conditions, in purpose selected riding compartments and road surfaces. In most cases, it was possible to measure noise in the driver’s cab. The noise measurements were carried out only in two canvas covered rear passenger compartments (MAN 4620 and DAF 4440). To avoid infrequency in the future studies, it is necessary to design more correct and systematic measurement methodology, using more comparable measuring time for each vehicle, driving compartment and road surface. To make convincing conclusions and to compare the results with previous studies, it could be reasonable to carry out more in-vehicle noise measurements in different models of military vehicles with what passengers are being transported.
Regular noise measurements in military vehicles are reasonable. Acoustic barriers, absorption materials, constrained-layer damping systems and new component designs could be implemented on military vehicles. Enforcing obligatory use of individual protective equipment is needed when military vehicles are used to train and transport personnel.
Our pilot study shows that different models of military vehicles have different noise levels, which depend on the driving surface. Noise levels were higher in the canvas covered rear passenger compartments compared with the driver’s cab, especially when vehicles were travelling along the paved road. While transporting military personnel in canvas covered rear passenger compartment area, DAF 4440 should be preferred to MAN 4620. Also the newer models of military vehicles (eg, SISU XA-188) should be preferred to older versions (eg, SISU XA-180), as newer vehicles are more in line with health and safety requirements for the working environments affected by noise, and have less noise inside the driver’s cabin and rear passenger compartment.
As noise LLV were exceed in several cases, personal protection equipment should be essential accessory in-vehicle and obligatory to wear to decrease noise exposure of military personnel. New and effective measures to protect the hearing impairment of persons working in the military and further studies of NIHL among defence forces personnel with hearing conservation programme should be a high priority.
The authors acknowledge the Estonian Defence Forces for their kind support in providing vehicles for this study. Any findings or recommendations expressed in this article are those of the authors and do not necessarily represent the view of the Estonian Defence Forces.
Contributors HO and EM were responsible for the project coordination. SK, RJ and HO carried out measurements and data collection. AL, MR and EM performed the data analysis. AL drafted the manuscript. All authors read and approved the final version of the manuscript.
Funding The Estonian Ministry of Defence project 'Assessment and management of health risks among military personnel' financed this research.
Competing interests None declared.
Patient consent Not required.
Ethics approval Research Ethics Committee of the University of Tartu.
Provenance and peer review Not commissioned; externally peer reviewed.
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