Noise and vibration measurements in a Viking military vehicle

Gurmail S. Paddan , Marietta L.L. McIlraith

a Institute of Naval Medicine, Crescent Road, Alverstoke, Hampshire, PO12 2DL, UK

b Formerly at Institute of Naval Medicine, Crescent Road, Alverstoke, Hampshire, PO12 2DL, UK

Keywords:Noise Whole-body vibration Vibration dose value Viking military vehicle

ABSTRACT Noise and whole-body vibration measurements were made in a Viking military vehicle to determine the variation that should be expected during repeat measures, the effect of speed (up to 60 km/h in 5 km/h increments),and during travel over different types of terrain(comprising concrete road,gravel track and rough cross-country). Measurements were made at various crew positions (including the driver and commander)in both the front and the rear cabs in the vehicles.Three translational axes of vibration were measured in each seat. Two speeds were investigated over road (35 km/h and 55-60 km/h) and gravel(20 km/h and 35 km/h) surfaces. The effect of varying speed of the vehicle on the measured noise and vibration magnitudes was also investigated. The highest sound pressure level (LAeq) of 104 dB(A) was measured at the commander’s standing position during travel over concrete road at 55 km/h. Higher noise levels occurred for a standing commander compared with when sitting on the seat. A maximum single axis frequency-weighted vibration magnitude of 1.0 m/s2 r.m.s.was measured on the driver’s seat during travel over track at 35 km/h. Higher vibration magnitudes occurred during travel over track compared with travel over road. Both noise and vibration exposure of crew within the Viking vehicle increased with increasing speed of the vehicle.

1. Introduction

Military vehicles are known to expose occupants to high levels of noise and vibration.For example,Paddan and Griffin(2002a)[1]showed that, based on measurements in 100 different vehicles,vibrations measured in military vehicles were among the highest compared with other types of transport.Griffin(1990)[2]presents data that show ‘caution’ should be exercised if exposure to vibration occurs in a military vehicle travelling at speed (4.5 m/s;16.2 km/h) for a period exceeding 30 s. Regarding noise exposure,Wijngaarden and James (2004) [3] discuss the need for hearing protection for crew travelling in military vehicles where “heavy tanks”generally showed highest noise levels.Military vehicles will necessarily travel over different types of terrain including rough cross-country and at high speeds. Vibration magnitudes generally increase with increasing speed of the vehicle (Griffin, 1990) [2].Lower sound pressure levels have been reported with vehicles travelling over paved road compared with off-road terrain (Luha et al., 2019) [4].

Many studies have been reported on noise and vibration occurring in military vehicles. Measurements made in three armoured vehicles (Bison; Light Armoured Vehicle III; M113A2 antitank air defence system) used by Canadian Forces have been reported by Nakashima et al. (2007) [5]. The noise and vibration levels increased with increasing speed of the vehicles. Communication between crewmembers was affected by the high levels of noise measured within the vehicles. Ruffa et al. (2013) [6] present an interim study where noise and vibration measurements made in a tracked combat vehicle used by the Argentinian Army, an Argentine Medium Tank. The data showed that some form of hearing protection should be worn by crewmembers during travel.A relatively short measurement period of approximately 4 min showed a vibration magnitude of 0.77 m/s2r.m.s.during travel over cross-country terrain. A study conducted by Aziz et al. (2014) [7]investigated noise and vibration in a 3-tonne truck used by the Malaysian Army.Measurements were made with the vehicle idling and travelling at 60 km/h.The noise and vibration levels increased from 66.5 dB(A)to 77 dB(A)and from 1.37 m/s2r.m.s.to 2.26 m/s2r.m.s. with change from idle to 60 km/h, respectively. Although these three studies were conducted over the past two decades,measurements of noise and vibration exposure in military vehicles have been the subject of investigation for more than 50 years (for example, Bates and Sparks,1964) [8].

When determining the effect of a particular variable on the noise and vibration occurring in a vehicle, a measure, or an indication, of the variation that should be expected during repeat measurements would be required. For example, if the effect of speed were to be investigated on the noise and vibration occurring in a vehicle,then the variation that should be expected over repeat measurement runs would be required. That is, the intra-variation.This would involve maintaining and keeping constant as many of the variables as possible.That is,ensuring the same driver is used in the repeat measures (to ensure manner of operation is maintained); maintaining constant the speed of the vehicle and the terrain traversed; ensuring that the weather conditions were maintained. In such a study, the only variable that would not be maintained would be time:the measurements would be conducted over successive periods. This would provide a quantitative indication of the variation (intra-variation) over repeat measures. Then,the effect of speed, or any other parameter, can be investigated whilst attempting to keep all other variables the same. In this context,intra-variation would be the variation over repeat measure(within repeat measures), and inter-variation would be the variation between different parameters (or measures). The intravariation would be used to determine statistically the effect of speed, that is, inter-variation.

This study is concerned with the measurement of noise and whole-body vibration in a Viking amphibious armoured vehicle.The aims of this study were to determine: (i) the variation that should be expected over repeat measures, (ii) the effect of vehicle speed, and (iii) the effect of different types of terrain.

2. Equipment and procedure

2.1. Vehicles tested

Noise and vibration measurements were made in two Viking vehicles based at the Royal Marines Armoured Support Company,Bovington, UK. These vehicles, which were fully armoured and amphibious, were manufactured by BAE Land Systems H¨agglunds.The vehicles are referred to as‘Carrier Full Tracked Articulated’and are used by the British Royal Marines.The Viking is an ATV(P) (All Terrain Vehicle (Protected)) and is signified as BvS10 by the manufacturer; this vehicle comprises many variants. The vehicles consist of two cabs (front and rear) with an articulated steering system. The forward cab accommodated the driver and three combat troops, and the rear cab (in the Troop Carrying Variant)could carry eight fully equipped troops.

Two Viking vehicles(with identification numbers RE 97 AB and RF 54 AB)were used in the trial;one of these(RE 97 AB)is shown in Fig.1. (The vehicle with number RE 97 AB was used for studies on the effect of speed and terrain,and the vehicle with number RF 54 AB was used to study the variation in repeat measures.) The rear cab used in the measurements was attached to Viking RE 97 AB;the chassis number of the rear cab was BXI0000254R serial number 0942-002. The total weight of a standard Viking vehicle was specified as 13000 kg(gross weight)(6800 kg for the front cab and 6200 kg for the rear cab).The front cab was powered by a Cummins in-line six-cylinder turbocharged diesel engine producing a maximum road speed reported to be 65 km/h(40 mph)and a water(amphibious) speed of 5 km/h (3 mph). Viking RE 97 AB had an engine with capacity of 6.7 L whereas Viking RF 54 AB had a smaller engine with capacity of 5.9 L. The total length of the vehicle was 7.71 m,width was 2.20 m,and the height was 2.25 m.Steering was achieved by a change of direction between the front and the rear chassis.Each cab had 12 road wheels,2 support wheels and two 12-tooth twin driving wheels.The track length separation was 90 mm.

2.2. Noise measurements

Noise measurements were made in both the front and the rear cabs for the Viking vehicles;these comprised four locations in each of the front cab (driver, commander seat, commander standing,crew right) and the rear cab (seat 1, seat 2, seat 5 and seat 6). The approximate locations of all seats in the two cabs and those used in the measurements are shown in Fig.2. Seat locations were similar in both Viking vehicles.

Audio recordings were made at the left shoulders(about 15 cm from the left ear) of four crew members in each cab using a ?”condenser microphone(G.R.A.S.,type 40AR).Each microphone was fitted with a spherical foam wind shield approximately 30 mm in diameter.The microphone preamplifiers(G.R.A.S.,type 26AC)were connected to the input channels of a microphone power supply(Brüel and Kj?r,type 5935).The output from the power supply was connected to an Edirol R-4 recorder where 24-bit time histories of sound pressure were acquired simultaneously at a sampling rate of 48,000 samples per second.Sound pressure levels were acquired at the various crew locations in the Viking vehicle during normal driving.

Calibration of the complete recording system was carried out by recording sinusoidal calibration tones of 94 dB and 114 dB at a frequency of 1000 Hz using an acoustic calibrator(Brüel and Kj?r,type 4321). The calibration procedure was repeated following the measurement of noise in the vehicle which showed the equipment to be stable over the measurement period.

2.3. Vibration measurements

Vibration measurements were made in both the front and the rear cabs for the Viking vehicles;these comprised five locations in the front cab (driver, driver floor, commander seat, commander standing floor,crew right)and three locations in the rear cab(floor,seat 1,seat 4).The approximate locations of all seats in the two cabs are shown in Fig.2.Also shown in Fig.2 are the directions in which the seat occupants faced (that is, the fore-and-aft direction). The seats in the front cab in Viking RE 97 AB used in the measurements are shown in Fig. 3. All seats in the front cab comprised a conventional foam seat squab(and backrest)covered with hard plastic as shown in Fig.3.The seats in the rear cab,shown in Fig.4 were of a basic construction with a fold-down mechanism for the squab;seat 1 has been folded down whereas seat 4 has been folded up.(The rear seats in the front cab(crew left and crew right)could also be folded down.)Vibration measurements were made on top of the seat squab.Personnel were required to be sat in the seats during the measurements.Weights of the personnel sitting on the seats were required as these may affect vibrations on the seats;Table 1 shows weights of people in the front cabs of the Viking vehicles. The weights of the personnel sitting in the rear cab were:seat 1-74 kg,seat 4-58 kg.

Accelerations were measured at the various seats and floor locations in the Viking vehicles during normal driving. Measurements were made of three translational axes of acceleration at each location(fore-and-aft(x-axis),lateral(y-axis)and vertical(z-axis)).The measurements made were in accordance with International Standard ISO 2631-1 (1997) [9]. Vibrations were measured using triaxial piezoelectric ICP accelerometers (AP2081-100). The accelerometers were mounted in mutually orthogonal axes encased in a semi-rigid mounting disc for measuring vibration on the seat surface.The flexible plastic disc was placed under the subject’s ischial tuberosities for seated people and under the feet for floor measurements for a standing person (commander).

Fig.1. The Viking RE 97 AB used in the measurements.

Fig. 2. Approximate locations of the crew in the Viking vehicle. (Arrows denote the fore-and-aft direction of each seat. Not to scale.).

Fig. 3. Seats in the front cab in Viking RE 97 AB used in the measurements.

The signals from the accelerometers were fed into Fylde FE-376-1PF amplifiers and low-pass filtered at 200 Hz using anti-aliasing filters with an attenuation rate of 48 dB/octave with a Butterworth filter response.The signals were digitised at a sample rate of 900 samples per second using DATAQ module DI-710. Analyses of the data were carried out using a data analysis package HVLab(v3.81).The duration of each measurement run ranged from 5 min to 15 min depending on the purpose of the run.

All accelerometers were calibrated prior to the vibration measurements using a Rion calibration exciter type VE-10(Rion,Japan)that produced a sine wave signal at a magnitude of 10 m/s2r.m.s.at a frequency of 160 Hz. The calibration procedure was repeated following the measurement of vibration in the vehicles which showed the equipment to be stable over the measurement periods.

Fig. 4. Seats in the rear cab attached to Viking RE 97 AB.

Table 1 Weights of the seat occupants in the front cabs of the Viking vehicles.

2.4. Procedure

The commander would not always be seated on the seat(seat to the right of the driver)in the front cab,but could be standing on the floor in the centre of the vehicle so as to look out of the hatch(behind the driver and the commander’s seat).The commander was standing during these measurements, but a passenger sat in the front(commander’s)seat.

Noise and vibration measurements were made with the vehicles being driven in their normal manner over different types of terrain:All Weather Driving Circuit (concrete), gravel track and crosscountry. All measurements were made with the vehicles travelling over Wool Heath at Bovington Training Area, Bovington,Dorset. The speeds of the vehicles were measured using a Global Positioning System (GPS) controlled Velocitek Speed Puck and these correspond to the average speed over the measurement period.Tables 2-4 show the journey details of the different sets of measurement,the requested nominal speeds and the actual speeds of the vehicles. Table 2 shows data for the vehicle travelling at different speeds and over different terrain. Data for the vehicle travelling in speed increments of 5 km/h are shown in Table 3.Table 4 shows data for the repeatability study conducted over cross-country terrain and concrete road.There was some deviation from the intended (nominal) speed to the actual speed of the vehicle.Also shown in Tables 2-4 is the chronological order of the conduct of the trial.The vibration measurement durations for most of the runs investigating the effect of terrain were 15 min(Table 2);the durations for the speed increment tests were 5 min per speed increment(Table 3).The vibration measurement duration for each run for the repeatability study was 10 min (Table 4). The measurement durations for the noise study were 15 min for the effect of terrain; 3-5 min for the effect of speed; and 10 min for the repeatability study. Two repeat measures were made when investigating the effect of speed and terrain (Table 2). During crosscountry travel, the speed of the vehicle was considered to be‘normal’ and very dependent on the local terrain.

2.5. Analysis of recordings

2.5.1. Noise measurements

The equivalent continuous sound pressure level (Leq) is the steady sound pressure level,over a specified period of time,which would produce the same energy equivalent to the actual fluctuating sound. It is described in the following equation where tm is the specified period of time,P1is the instantaneous sound pressure and P0is the reference sound pressure corresponding to 20 μPa.

Table 2 Operational details for the effect of speed and terrain (vehicle RE 97 AB).

Table 3 Operational details for the vehicle RE 97 AB driving on concrete road in 5 km/h speed increments.

Table 4 Operational details for the vehicle RF 54 AB driving during the repeatability study in the front cab.

Leqvalues were calculated using both the C-weighting and the A-weighting filters. The C-weighting filter is more suitable for assessing peak sound pressure levels whereas the A-weighting filter is more suited for assessing continuous noise exposures. The human ear does not respond equally at all frequencies therefore the A-weighting is applied to the audible frequency range to represent the reduction in sensitivity to the low frequencies.

2.5.2. Vibration measurements

Root-mean-square,r.m.s.,vibration magnitudes were calculated for measurements made in the different axes on the seat surface using the following equation:

where a(t)is the frequency-weighted acceleration time history (in m/s2)at the input to the body and T is the measurement period(in seconds).

The vibration dose value, VDV, gives a measure of the total exposure to vibration and the risk of injury. The VDV reflects the total, rather than the average, exposure to vibration over the measurement period. VDVs were calculated for each axis and vibration measurement using the following equation:

where a(t)is the frequency-weighted acceleration time history (in m/s2)at the input to the body and T is the measurement period(in seconds).

Table 5 shows the frequency weightings specified in ISO 2631-1(1997)[9]for assessing the health effects of seated persons exposed to vibration. These frequency weightings apply for vibration measurements at the seat surface and the backrest for seated persons.Also shown in Table 5 are the multiplying factors that are to be used when assessing the health aspects of exposure to vibration: a multiplying factor of 1.4 is used for vibrations in the horizontal axes. (Note that different multiplying factors apply depending onwhether the data are to be assessed with respect to the health of exposed persons or the effects of vibration on comfort and perception.)

Table 5 Frequency weightings for health aspects as specified in International Standard 2631-1 (1997) for seated persons.

According to ISO 2631-1 (1997) [9], vibrations should be measured on the seat and the backrest to assess the risks posed by exposure to vibration (see Table 5). Furthermore, measurements made at the backrest are not included in the assessment of vibration. The standard states that:

“Measurements in the x-axis on the backrest … … are encouraged. However, considering the shortage of evidence showing the effect of this motion on health it is not included in the assessment of the vibration severity … ”.

The measurement of vibration on the backrest is “encouraged”by ISO 2631-1 (1997) [9] although the data are not used in the assessment of vibration exposure. Therefore, no data were measured at the backrest.When assessing the effect of vibration on health in accordance with ISO 2631-1 (1997) [9], the “… highest frequency-weighted acceleration … in any axis on the seat pan” is used in the assessment.

The vibration dose values, VDVs, can be used to determine the effect of the vibration on the health of persons exposed.The VDV is a cumulative measure of the exposure to vibration, and therefore,an indication of the measurement duration must be considered together with the VDV.

3. Results and discussion

3.1. Variation in noise and vibration during repeat measures

In determining the noise and vibration exposure of personnel operating in these vehicles it is important to observe the variation that may occur during ‘normal’ transit. Both noise and vibration measurements were made in the front cab of Viking RF 54 AB to determine the variation that should be expected during repeat measures.These data would be required to determine the effect of other confounding variables on the measured values. The vehicle was driven over cross-country terrain and over concrete road.The aim during the measurements was to ensure that the speed of the vehicle was maintained constant over the run and between the successive runs; the corresponding speeds are shown in Table 4.The average speeds over cross-country and concrete road were 18 km/h (standard deviation 1 km/h) and 40 km/h (standard deviation 0.6 km/h),respectively.The driver was able to maintain the speed of the vehicle to a greater degree when travelling over concrete road compared with travel over cross-country terrain. Six repeat measurements were made over each surface.

Fig. 5. A- and C-weighted equivalent continuous sound pressure levels (LAeq, LCeq)measured in front cab of the Viking obtained during repeat measures comprising travel over cross-country terrain at 18 km/h and over concrete road at 40 km/h(＋-LAeq;-median LAeq; × - LCeq; - median LCeq).

Fig. 5 shows the sound pressure levels for both A-weighting(LAeq, dB(A)) and C-weighting (LCeq, dB(C)) measured at the four locations in the front cab of the Viking vehicle (RF 54 AB) during repeat measures over cross-country terrain and concrete road.Also shown in Fig. 5 are median values corresponding to the six repeat measurements runs. Median and interquartile values for the LCeqand LAeqsound pressure levels are shown in Table 6.It is seen that for all measurements, the sound pressure levels measured using the C-weighting are higher than the values calculated using the Aweighting demonstrating the attenuating behaviour of the Aweighting filter.These data show mostly higher noise during travel over concrete road compared with cross-country terrain. The variation in LAeqfor the six repeat runs appears visually similar for the data corresponding to travel over cross-country compared with concrete road. The highest variation is observed at the commander’s location (when standing) during travel over crosscountry (range of 97.5 dB(A) to 99.2 dB(A)) and in the crew seat during travel over concrete road(range of 93.9 dB(A)to 96.2 dB(A)).The highest value measured for these data being 101.3 dB(A)(commander standing, concrete road, run 3) and the lowest value being 92.2 dB(A) (commander sitting, cross-country, run 6). With the exception of the outliers observed at these locations,the overall variation appears to be low visually and it is likely that these would represent the minimum variation that should be expected.

Fig. 6 shows the frequency-weighted vibration magnitudes measured at five locations in the front cab of the Viking vehicle(see Fig. 2). (No reliable data were collected for the x-axis on the floor with the commander standing.) These data show higher vibration magnitudes during travel over cross-country terrain compared with concrete road.Also,the variation in the vibration magnitudes for the six runs is higher for the data corresponding to travel over cross-country compared with concrete road. An example of the maximum variation in vibration magnitudes is seen for the vertical axis measured in the driver’s seat during travel over cross-country.The range of vibration magnitude for the six runs was 0.21 m/s2r.m.s. to 0.73 m/s2r.m.s.; the highest value being nearly 3.5 times the lowest vibration magnitude.Other axes and locations appear to show lower visual variation.Median and interquartile values for the vibration magnitudes are shown in Table 7.As many of the extrinsic factors as possible were kept constant as possible,including terrain,speed and driver. Therefore, the variation presented in Fig. 6 and Table 7 probably represents the minimum variation that should be expected, as other factors that would affect the variation such as manner of driving, terrain, vehicle differences, etc.

Table 6 Median and interquartile equivalent continuous sound pressure levels(LAeq,LCeq)measured in front cab of the Viking during transit over cross-country terrain and concrete road.

3.2. Effect of vehicle speed

The effect of speed on sound pressure levels and vibration magnitudes was investigated in the front and rear cabs in Viking(RE 97 AB). The vehicle was driven over concrete road in 5 km/h increments ranging from stationary (engine running but vehicle stationary) up to the maximum speed of the vehicle, as shown in Table 3.Figs.7 and 8 show sound pressure levels,LAeq,measured in the front and rear cabs while travelling at different speeds over concrete road. The maximum speed tested for each cab varied depending on weather conditions:the road surface was wet during tests on the front cab thus restricting the maximum speed to 40 km/h. Sound pressure levels measured in the rear cab show a large increase from about 65 dB(A) with the vehicle stationary to about 90 dB(A) with the vehicle travelling at 15 km/h. Gradual increase in sound pressure levels (in the rear cab) occurred with further increase in speed:90 dB(A)at 15 km/h to 97 dB(A)at 60 km/h.

The graphs show an increase in cabin noise with increasing speed. An advantage of gathering noise data at increasing speed increments would be to determine if any resonances resulting in higher noise can be observed in the data. For example, in the rear cab of the Viking vehicle(Fig.8),slightly elevated noise can be seen at 35 km/h; this was not observed in the front cab.

Figs. 9 and 10 show frequency weighted vibration magnitudes measured in the front and rear cabs, respectively, during travel at different speeds.The data shown in Figs.9 and 10 were calculated using a unity multiplication factor for all axes,since these data are used to show the effect of speed on vibration rather than the health effects of changing speed. Multiplying factors of 1.4 would be applied to the vibration magnitudes in the horizontal axes (x-axis and y-axis)to determine the health effects of vibration magnitudes.Applying the health-effect multiplication factors would not alter the highest vibration magnitudes which occurred in the vertical axis (z-axis).

The data for the front cab in Viking in Fig. 9 show that the vibration magnitudes in the different locations tended to reach a maximum at a particular speed and a further increase in speed did not significantly affect the vibration. For example, vibrations measured on the driver’s seat showed greatest magnitude (about 0.5 m/s2r.m.s.)in the vertical axis and this was the highest at 10,20 and 40 km/h. Crew right seat shows only a small, but gradual, increase in the vertical axis vibration with increase in speed from about 15 to 40 km/h.

The main advantage of vibration magnitudes that have been measured at increasing speed increments would be to determine if any resonances could be observed in the vibration data. Maybe such resonance behaviour would not be evident if incremental speeds were not used.One such resonance of note is seen in Fig.10 for measurements on the floor of the rear cab of the Viking:a local resonance is seen in vibration in the vertical axis which appears to be excited with the vehicle travelling at 15 km/h. This could be indicative of a specific vibration problem with this vehicle.Further investigation could focus to determine the cause of this high vibration and, if necessary, appropriate remedial action taken.Although this resonance occurred on the floor of the vehicle, this was not transmitted to the seat closest to the floor location(seat 1,see Fig. 2).

3.3. Effect of different types of terrain

The A-weighted broadband equivalent continuous sound pressure levels LAeqare presented graphically in Figs.11 and 12 (front and rear cabs) for the Viking vehicle (RE 97 AB) travelling over different types of terrain and varying speeds.The values in Figs.11 and 12 correspond to average sound pressure levels for two different measurement runs. (No reliable data were collected for the commander’s seat while travelling at 20 km/h over track.)

The highest noise value obtained for the front cab of the Viking vehicle was 104 dB(A) and was measured at the commander’s location when standing at the front hatch during travel over concrete road at 55 km/h (see Fig. 11). Higher values for the commander’s standing position are thought to be due to the microphone being ‘outside’ of the vehicle and therefore greater exposure to external sources from the vehicle (such as the engine,exhaust, track, etc.) and wind noise. The highest noise value obtained in the rear cab was 99 dB(A) for the same road surface at a nominal speed of 60 km/h (seat 2, Fig. 12). Noise levels were significantly higher at the commander’s standing position compared with all other crew locations and this was consistent for the different speed and road surface operations.Similar noise levels were measured in the front and rear cab of the Viking during transit over track and concrete road at a nominal speed of 35 km/h;although lower noise levels were measured in the rear cab for low speed travel over track(20 km/h)and cross-country compared with the front cab seats. Overall, noise levels were greater when the vehicle travelled at high speed compared with slower speeds and the spread of noise values measured at the various seat locations appeared greater in the rear cab seats for the different operations compared with the front cab.That is,the differences in terrain and speed had a greater effect on noise in the rear cab compared with the front cab.

Fig.6. Frequency-weighted vibration magnitudes for repeat runs in the Viking vehicle during travel over cross-country (individual value +; median ) and concrete road(individual value ; median ).

Table 7 Median and interquartile range of frequency-weighted vibration magnitudes for the six repeat measurements in Viking.

Fig. 7. A-weighted equivalent continuous sound pressure levels (LAeq) for varying speed for crew in the front cab in Viking during travel over concrete road.

Fig. 8. A-weighted equivalent continuous sound pressure levels (LAeq) for varying speed for crew in the rear cab in Viking during travel over concrete road.

Fig. 9. Frequency weighted vibration magnitudes (m/s2 r.m.s.) for varying speed for crew in the front cab in the Viking during travel over concrete road.

Fig.10. Frequency weighted vibration magnitudes (m/s2 r.m.s.) for varying speed for crew in the rear cab in the Viking during travel over concrete road.

Fig. 11. Average A-weighted equivalent continuous sound pressure levels (LAeq)measured in the front cab of the Viking during travel over different types of terrain.

Fig. 12. Average A-weighted equivalent continuous sound pressure levels (LAeq)measured in the rear cab of the Viking during travel over different types of terrain.

Fig.13. Frequency weighted vibration magnitudes (m/s2 r.m.s.) measured in the front(driver’s seat, commander’s floor, commander’s seat, and crew right seat) and rear(seat 1 and seat 4) cabs in the Viking vehicle.

Fig. 14. Vibration dose values (10 min, m/s1.75) measured in the front (driver’s seat,commander’s floor,commander’s seat,and crew right seat)and rear(seat 1 and seat 4)cabs in the Viking vehicle.

Fig.13 shows average frequency-weighted vibration magnitudes(of two measurement runs)measured at four locations in the front cab(driver’s seat,commander’s floor,commander’s seat,and crew right seat) and two seats in the rear cab (seat 1 and seat 4) in the Viking vehicle. (Measurements in the rear cab, seats 1 and 4,correspond to the vehicle travelling at 60 km/h over concrete road.)Fig. 14 shows the corresponding vibration dose values, VDV, for these locations. The VDV measure emphasises the importance of impacts and shocks in the vibration exposure. Since VDV is a cumulative measure of the exposure to vibration, an indication of the measurement duration must be considered together with the VDV; the VDVs presented in Fig. 14 correspond to a common duration of 10 min.(Note that multiplying factors of 1.4 have been used for measurements in the horizontal axes.) Acceleration measurements were made in the three orthogonal axes on the seat(and floor); the highest values (frequency-weighted vibration magnitude or VDV) are shown in Figs. 13 and 14. The highest frequency-weighted vibration magnitudes would be required to assess the health effects of exposure to vibration.Vibrations in the vertical axis were the highest for the front cab (apart from one measurement: front cab, crew right seat, cross-country, x-axis(0.65 m/s2r.m.s.) was the highest). However, for most of the measurements in the rear cab, the highest vibrations occurred in the fore-and-aft axis of the seat (the seats in the rear cab were positioned such that the y-axis for the seat occupants corresponded to the fore-and-aft axis of the vehicle (see Fig. 2)). The data for all measurement locations show that vibration levels (vibration magnitudes and VDVs) increased with an increase in speed of the vehicle.The highest vibrations occurred on the two seats in the rear cab: seat 1 showed the highest vibration magnitude (1.52 m/s2r.m.s.,travel over track at 35 km/h)and seat 4 showed the highest VDV (15.1 m/s1.75for 10 min, travel over track at 35 km/h). Also,higher vibrations occurred during travel over track compared to travel over road at the same speed (35 km/h).

4. Discussion

The variation seen in sound pressure levels and vibration magnitudes during repeat measures could be used to determine whether the effect of different parameters on the noise and vibration occurring in the Viking vehicle was likely to be significant or within the bounds of variation.This can be demonstrated using the data in Table 7,which shows that the interquartile range for the vibration data varies between axes and seats.Vibration magnitudes measured during travel over concrete road were more consistent(lower interquartile range) compared with travel over crosscountry. This is thought to be due to a combination of the driver being able to maintain the speed of the vehicle over concrete road than travel over cross-country terrain, and the differences on terrain whilst traversing over cross-country compared to the consistent surface over concrete road. For example, the vibration magnitudes on the driver’s floor during travel over cross-country terrain are seen as 0.30 m/s2r.m.s., 0.37 m/s2r.m.s. and 0.94 m/s2r.m.s. for the x-, y- and z-axes, respectively. The corresponding absolute interquartile range for the vibration magnitudes were 0.03 m/s2r.m.s., 0.03 m/s2r.m.s. and 0.11 m/s2r.m.s., respectively.However, the vibration magnitude for the z-axis is about three times greater than that measured for the x- and y-axes. The corresponding median vibration magnitudes for travel over concrete road at 40 km/h were 0.10 m/s2r.m.s.,0.10 m/s2r.m.s.and 0.67 m/s2r.m.s.for the x-,y-and z-axes with interquartile ranges of 0.00 m/s2r.m.s., 0.00 m/s2r.m.s. and 0.01 m/s2r.m.s. for the three axes,respectively.(Numerically,the interquartile ranges for both x-and y-axes were 0.002 m/s2r.m.s.) A quantitative measure of the variation, normalised variability Nv, in the measured vibration magnitudes can be determined using Eq. (4) (Paddan and Griffin, 1994[10]).

The normalised variability, Nv, for cross-country travel for the three axes is calculated as 10.0%, 8.0% and 11.7%, respectively. This shows that the normalised variation is similar for the three axes.This information can be used to estimate the variation in vibration that should be expected during cross-country travel assuming that the variation measured in these data can be used for other speeds and operating conditions. Fig.15 re-presents the vibration magnitudes measured at the driver’s floor during travel over concrete road(shown in Fig.9)with the variation as the interquartile range(taken from Table 7). This shows the variation that should be expected, and accepted, when measuring vibration magnitudes in a Viking vehicle.If the effect of a parameter were to be investigated,then, for the effect of that parameter to be significant, the effect would have to be greater than the variation measured over repeat measures.Therefore,based on these data,the vibration magnitudes presented in Fig.13 are more similar between the seats rather than they are different. Furthermore, the differences in vibration magnitudes between speeds are also smaller than those illustrated in Fig.13.

Noise measurements at the commander’s and other crew members’positions were made in accordance with BS EN ISO 9612(2009) [11]. The standard states many sources of error and uncertainty that would apply when making noise measurements: these include location of measurement, rubbing of the microphone (for example, against clothing or skin), impacts on the microphone,wind and air flows. Measurements made in the safe confines of a controlled laboratory would eliminate or ameliorate the effects of other factors.

Fig.15. Estimated variation in frequency weighted vibration magnitudes (m/s2 r.m.s.)measured on the driver’s floor for varying speed in the front cab in the Viking during travel over concrete road.

It is seen that the noise levels measured in the front cab increased with an increase in speed of the vehicle(Fig.7):the rate of increase ranged from 1.3 dB/(km/h)for the driver to 2.1 dB/(km/h)for the commander in a standing position.Sound pressure levels at the standing commander were higher than those for the crew within the vehicle (see Fig. 7). Although the vibration magnitudes increased with increasing speed,the increase was not constant(see Figs. 9 and 10). As an example, the increase is approximated to 0.05 m/s2r.m.s./(km/h),as shown in Fig.10 for seat 4 in the rear cab(vertical axis). The importance of measuring noise and vibration during differing speeds of the vehicle is demonstrated in Fig. 8(noise) and Fig.10 (vibration) for the rear cab. The elevated vibration magnitude on the vehicle floor with the vehicle travelling at 15 km/h might be related to the sudden increase in sound pressure levels in the cab at the same speed.This result would not have been evident if measurements had not been made while travelling at different speeds.

The sound pressure levels (Figs.11 and 12) and vibration magnitudes(Figs.13 and 14)correspond to the vehicle travelling at one speed and over one type of terrain. A typical manoeuvre in this vehicle would not necessarily comprise a single type of terrain and speed: a combination of speeds and types of terrain would be traversed. The data presented could be used to estimate a crew member’s exposure to noise and vibration assuming similar terrain and speeds were traversed.For example,it could be assumed that a typical day in the vehicle could comprise 6 h of driving based on the following speeds and terrains:

2 h of concrete road at 35 km/h

30 min of concrete road at 55 km/h

1 h of track at 20 km/h

1 h of track at 35 km/h

1 h 30 min of cross-country

Then the daily personal noise exposure level,LEP,d,and the daily vibration exposure level of the crew in the vehicle could be estimated as shown in Table 8.(It is noted that the speeds of the vehicle were different during travel over concrete road (see Table 2):60 km/h during measurements in the rear cab and 55 km/h during measurements in the front cab. Also, no sound pressure level measurements were made in the commander’s seat location during travel over track at 20 km/h.) These daily exposures could be assessed using guidance suggested in, for example, the European Directives on Physical Agents noise, PA(N)D (European Commission, 2002) [12] and vibration, PA(V)D (European Commission,2003)[13].The two Directives show the following exposure values:

Therefore,based on the above simulated daily manoeuvres,the exposure of the driver would exceed the upper exposure action level for noise (LEP,dof 85 dB(A)) and exceed the exposure action value for vibration of 0.5 m/s2A(8), and also exceed the exposure action value corresponding a VDV of 9.1 m/s1.75.Data for the other crew members can be interpreted similarly.This has demonstrated the method that could be employed to estimate exposures of the crew members in the Viking vehicle.It is accepted that the speed of the vehicle may differ from that assessed in this study, but the assessment would not warrant a greater precision than that presented above due to the variation that will occur, and should be expected, during such an assessment.

The commander in the vehicle would normally be standing and looking out through the hatch so that guidance and direction can be provided for the driver; the commander was standing during the measurements.The commander would be standing on the seat that would be used as the squab when seated;it is understood that the preferred stance for the commander is to stand. However, there would be conditions which would require the commander to sit on the seat with the vehicle hatch closed. In such a case, the driver would also be in a ‘closed hatch’ position and both crew members would be using the vehicle optics to observe the terrain.When the commander is in a standing stance, the exposure to noise and vibration would be different to that when in a seated posture: the source of vibration exposure would be through the feet rather than the buttocks and the backrest; and noise exposure would include other extraneous factors such as wind noise. The epidemiological and adverse effects of exposure to vibration in a standing posture will be different-the posture and stance of the legs and the knees would attenuate vibration transmitted to the body (Paddan and Griffin, 1993 [14]). Furthermore, there is no specific guidance for assessing the effects of vibration on the standing human body;ISO 2631-1 (1997) [9] offers health guidance for seated persons only.Notwithstanding that the criteria for assessing the health effects of exposure to whole-body vibration are intended for seated occupants and,whilst vibration magnitudes measured at the floor have been used for this study, in the absence of specific guidance, it is acknowledged that a further study in the assessment of standing persons in tracked vehicles is required.

The seat in a vehicle can have a high influence on the seat occupant’s exposure to vibration since the seat is in between theperson and the vehicle chassis.Furthermore,seats in some vehicles have not always been optimised for the vibration occurring in the vehicle. This has been demonstrated in a study conducted by Paddan and Griffin (2002b) [15] where vibration measurements were made in 100 different work vehicles (comprising cars, vans,lift trucks,lorries,tractors,buses,dumpers,excavators,helicopters,armoured vehicles, mobile cranes, grass rollers, mowers and milk floats).The seats were of both conventional and suspension type.A predictive procedure used in the study showed that 94 (out of the 100 vehicles)might have benefited from changing the current seat in the vehicle to a seat from one of the other vehicles investigated.This change of seat would have a direct impact on the exposure to vibration of the seat occupant: a reduction in the vibration exposure.

Table 8 Estimated noise and vibration daily exposure levels based on a simulated daily manoeuvre in the Viking vehicle.

Seats in some military vehicles,for example the driver’s seat in an armoured fighting vehicle which has a low vertical profile,may have an inclined backrest, thus the driver could be in a semireclined position. Also, the seat squab might be angled in a nonhorizontal position. This is mostly the case for the driver who might be expected to drive with the hatch closed and this being a requirement within the restricted vertical dimensions of the vehicle. Also, the commander could be standing (on the seat) and looking out of the hatch.The effect of sitting in reclined seats on the vibration discomfort experienced by the crew would be expected to be different to that in a seat with a vertical backrest;this has been discussed elsewhere (Basri and Griffin, 2012 [16]; Paddan et al.,2012 [17]).

5. Conclusions

Noise measurements made in the Viking vehicle indicate that the highest LAeqof 104 dB(A) occurred in the front cab at the commander’s location when standing and travelling over concrete road at high speed (about 55 km/h). Higher levels of noise were measured with the commander standing compared with when sitting with their head inside of the vehicle.Slightly higher levels of noise were measured in the front cab compared with the rear cab when travelling in the Viking.Marginally lower levels of noise were measured in the vehicle during travel over gravel track compared with travel over concrete road. Travel over cross-country terrain showed the lowest levels of noise compared to the other types of terrain.Noise levels in the Viking vehicle increased with an increase in vehicle speed at a rate of approximately 1.3 dB/(km/h).

Highest vibration magnitude in the front cab of 0.96 m/s2r.m.s.was measured in the vertical axis on the driver’s seat during travel over gravel track at about 35 km/h. Generally, similar vibration magnitudes were measured in the front and the rear cabs in the Viking apart from one seat in the rear cab during travel at 35 km/h over the track surface.Higher vibration magnitudes were measured during travel over gravel track compared to travel over concrete road.Vibration magnitudes in the Viking vehicle increased with an increase in vehicle speed.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.