TAM acoustic and vibration environment paper (ICA version)-Rev Simoncelli.doc

1. Introduction

This paper presents an analysis of noise and vibration environment produced by a track combat vehicle, specifically an Argentine Medium Tank (TAM) unit of the Army.

The sound levels perceived inside and exposure to whole-body vibration could cause injuries to the hearing organ, affects the performance and other health problems due to the time of exposure. Although TAM crews use helmets with hearing protection and intercoms, noises produced inside affect the communication intelligibility during the mission. Under these considerations, and with no references in Argentina for this kind of evaluations, measurements of noise and vibration were made to determine the current emission levels and evaluate the impact of the tank for the military crew.

For evaluation purpose, International standard for whole-body vibration, ISO 2631-1[9] and ISO 2631-5[10], National standard for the assessment of the noise emissions of motor vehicles, IRAM-AITA 9C [1] and IRAM-AITA 9C-1 [2], and locally Argentinean laws [4] [5] [6] were used. 2. Noise MeasurementS and EvaluationS2.1. Measurement Procedure

The assessment of the TAMs noise emissions characteristics was conducted using the following standards: IRAM-AITA 9C [1], for the assessment of the noise emissions produced by accelerating motor vehicles, and IRAM-AITA 9C-1 [2], for the verification of the noise emissions of motor vehicles in use, detained. Also, a verification of the emission levels in the crew compartment was conducted in order to analyze the operators exposure levels.

Whilst the purpose of application of the AITA IRAM-9C is the verification of noise emission from civilian vehicles, whose mechanical, structural and mobility are very different from those of a military vehicle, these rules are the only approved in Argentina that provide one possible way of measuring and verifying such noise.

The detained TAM measurements were carried out on the test track of the Arsenals battalion Boulogne Sur Mer on Saturday, August 25th of 2012. In addition, acceleration measurements and interior measurements were conducted on one of the streets of the firing field of the military garrison of Campo de Mayo on October 25th of 2012.

It was used a Class 1 sound level meter Svantek 959, according to the requirements of the IRAM 4074-1 standard [3]. To verify the accuracy of the measurements a calibrator of the same brand and class was employed. All measurements were carried out using a windshield for the microphone, such as required for this type of tests.

The standard recommends to carry out the measurements in a place free of reflective surfaces within a radius of 50 meters from the intersection of the center of the test track and the axis drawn between the measurement points. Therefore, it was selected a track section which allows to develop the speed recommended by the standard. This section has only vegetation as adjacent elements, without presenting reflective characteristics for the wavelengths in analysis.

2.1.1 Assessment of Noise Emissions with Vehicle in Use, Detained

The measurements of the noise emissions of the tank in use, but detained, were realized as state by the IRAM AITA 9C-1 [2] standard in Annex A. Since the vehicle has a front engine, the measurement was set at 50 cm of height and 50 cm of distance to the front wheel axe. As indicated by the standard it was conducted three measurements.From the condition of the vehicle engine idling it was accelerated up to 1800 rpm, of the maximum power. Considering that the noise was stable enough, it was decided to measure with an integration period of 15 seconds, obtaining weighted sound levels (dBA) as required by the regulations. Also unweighted sound levels (dB) were considered, which contemplates the exposure to low frequencies, and the sound exposure level (SEL), whose value has better consistence to the subjective annoyance perceived.

2.1.2 Assessment of noise emissions with vehicle in accelerationMeasurements of the noise emissions of the tank in acceleration following the guidelines of the IRAM-AITA 9C [2] standard were conducted. Two measurements positions were selected, each on opposite sides of the track, at 7,5 meters of the center of the path and 1,5 meters of height, and with the sound level meters parallel to the ground. The standard indicates that the vehicle speed must correspond to of the maximum power. Being TAM maximum speed of approximately 60 km / h (3300 rpm), it was proceeded to carry out the measurements at a speed of approximately 40 km / h (2500 rpm).

The test area, track and points of measurement are shown in the Figures 1 and 2.

FIGURE 1. Scheme of the test area and the measurements positions.

FIGURE 2. Picture of the noise measurement site.

2.1.3 Assessment of Interior Noise

The measurements were conducted at the tank driver position, being this position the closest to the engine. Due to the confined space on the tanks cabin, it was decided to fix the sound level meter to a rigid structure at the side of the driver seat. Figure 3 shows the drivers cabin and the measurement point.

FIGURE 3. Picture of driver cockpit and noise measurement point.

The TAM was assessed in a field test at variable velocity, through a test track of the Argentinean Army with diverse obstacles as slopes, mud, water, and others; normal operating conditions of a unit of this kind of tank. The entire circuit required 4 minutes and 32 seconds to be completed.2.2. ResultsThe results obtained for each procedure of measurement are presented below.2.1.4 Assessment of Noise Emissions with Vehicle in Use, DetainedIs presented in the following table the results of the measurements with the detained tank, with the engine idling at 1800 rpm (Table 1).

TABLE 1. Noise emissions levels with the vehicle detained, in use.

Leq(A) [dBA]Leq(Z) [dB]

S-188.9104.9

S-290.2105.6

Energy Average89.6105.2

It can be observed that the sound level is higher at the left side of the tank (S-2), the side of the driver, leaving the driver exposed to higher sound energy.

Moreover, the Resolution N 293/2003, determines that the noise exposure in occupational environments must not be higher than 85 dBA for a workday of 8 hours. Having been registered values around 90 dBA outside the tank and assuming a higher value inside, as a result of reverberation and vibration transmission, it could be supposed that the occupants are being exposed to levels that could affect their hearing health.

Also, the large difference between sound pressure levels obtained with and without A weighting suggests the existence of noise with low frequency components and probably vibrations. Is shown in the Figure 4 a third octave band spectrum of the noise emissions with the vehicle detained.

FIGURE 4. Frequency analysis of the engine noise registered with the vehicle detained.

It can be observed that is a wide band spectrum, with tonal components at the bands of 50 and 100 Hz, due to a difference of 10 dB between these bands and adjacent. It should be considered then, that the presence of tonal noise would cause a much higher subjective annoyance than it would result due to a noise with the same amount of energy but without tonal components.

Considering that the tank has a six-cylinder engine and running at 1800 rpm, it executes three explosions per revolution, so the spectral components should have a higher presence harmonics number 1,5 and 3 respectively. These harmonics frequencies can be calculated from,

(1)

(2)which would result in 45 and 90 Hz. Since lower frequencies of 50 and 100 Hz bands are 45 and 89 respectively, previously calculated harmonics would fall in these bands. Therefore, it is demonstrated that the engine harmonics would be responsible for the tonal behavior observed in the tanks noise emissions.

2.1.5 Assessment of Noise Emissions with Vehicle in Acceleration

Is shown in the following table (Table 2) the results of the measurements of noise emissions of the tank in acceleration, taking the arithmetic average of the four registries as required by the standard. It is verified that, due to a difference between registries no higher than 3 dB, the arithmetic average does not have a remarkable deviation in comparison with the energy average.TABLE 2. Noise emissions levels with the vehicle in acceleration.

Leq(A) [dBA]Leq(Z) [dB]LAE [dBA]SEL [dB]Peak [dB]

Measurement 1S-194,4106,5102,2114,3120,4

S-294,1106,5100,1112,5120,9

Measurement 2S-191,0102,598,0109,5116,3

S-291,4103,397,4109,3116,5

Arithmetic Average92,7104,799,4111,4118,5

Energy Average93.0105.199.8111.9119.0

As in the test results obtained with the vehicle detained, the results of Table 2 show a significant difference between levels with and without weighting, which implies a high energy present in the low frequency components.

Worth noting that the difference between Leq(A) and SEL is almost of 20 dB so, assessing tanks passage noise by measuring at a fixed point we could deduce that the energy value presented by the SEL indicator has a higher correlation with the subjective annoyance than the Leq(A), suggested by the standard. This is because the SEL corresponds to the total energy of the event presented in its equivalent in a time interval of 1 second, while Leq corresponds to the equivalent energy of the event in the same time. In this way the Leq allows background noise before and after the vehicle passage to reject and "dilute" the highest values that have short duration and are more responsible, to our knowledge, of the subjective annoyance perceived.

So, although 93 dBA obtained represent a high annoyance and could cause hearing damage with extended exposure permanence, 111 dB measured for sound exposure level (SEL) indicate extremely annoying values according to the short time duration of the vehicular passage.

2.1.6 Assessment of Interior NoiseIs shown in the Table 3 the results of the noise emissions levels measured on the TAMs cabin. TABLE 3. Noise emissions levels measured in the TAMs cabin.

Leq(A) [dBA]Leq(Z) [dB]LAE [dBA]SEL [dB]Peak [dB]LAmax Fast [dBA]

104.6117.3129.3142136.2116.1

According to the WHO report [7], when the exposure level exceeds 70 dBA in 24 hours and maximum levels of 110 dBA, hearing damage may occur. Therefore, the results presented in Table 3 confirm and reinforce the claim that the conditions to which the crew of the TAM is exposed would be harming their auditory health significantly and progressively with each exposure; unless the hearing protection used is sufficiently effective. However, in the presence of such high levels, continuous and instantaneous, noise effects can also generate extra-auditory conditions, especially with high low frequency components.

As observed in Figure 5, the noise within the cabin presents significant low frequency components, which tend to increase below 20 Hz, indicating the presence of vibration.

FIGURE 5. Frequency analysis of the noise emissions in driver cockpit.

2.3. Discussion of ResultsPursuant to Resolution 293/2003 [4] and Regulatory Decree 351/1979 [5] complementary to Law 19.587/1972 [6], Health and Safety in the Workplace, the maximum sound exposure level should not exceed 85 dBA for an exposure of eight work hours with 3 dB of exchange rate. That is, for an exposure to a sound pressure level that adds 3 dB to the allowable 85 dBA (resulting 88 dBA), the time of exposure should be reduced by half (4 hours).

The law on Health and Safety at Workspace has been considered as a reference to qualify the military crew exposure, despite the peculiarity of the case. Therefore, according to the Figure 8, the levels with the vehicle detained only comply with the regulations for an exposure that does not exceed 2 hours in proximity of the tank, decreasing to just 5 minutes for the interior according to the levels recorded in the driving position.

FIGURE 6. Maximum allowable exposure level to avoid permanent shift of the audibility threshold [3].

Also, the tonal components and instantaneous peak levels recorded would further increase the perceived level of subjective annoyance and the possibility of hearing impairment.It is also noticeable the level difference obtained, on the order of 20 dB, between the SEL descriptor analysis and Leq(A). Being the passage an event of short duration and with high noise amplitude, on the integration for obtaining Leq(A) these levels are dismissed, while the SEL is able to reflect the total energy of the event. Therefore, for this type of analysis this descriptor should be used instead of Leq(A). The current legislation would not be giving an analysis procedure according to the event.

3. VIBRATION MEASUREMENTs AND EVALUATIONs3.1. Measurement ProcedureThe measurement was carried out on a cross country circuit, in the area of Campo de Mayo. This kind of environment is the natural operating environment for which the vehicle was designed and is expected that develops the best result of their performance.

As exploratory work, within the possible options, the measurement was chosen in the seat of the gunner. Figure 7 shows a schematic of the measurement points selected.

FIGURE 7. Outline of crew enclosure and placement of the accelerometers.

The measurement lasted three minutes and fifty-six seconds. It was used a digital sound level meter Class 1 SVAN 959, which meets the requirements specified in ISO 8041:2005 and a triaxial accelerometer Dytran, model 3233A.

In order to meet the requirements set out in point 4 of the ISO 2631-5 [E], the data record was taken at a sampling rate of 160 data per second.

The values obtained were processed to tabulate the parameters set in the ISO 2631-1 [9] and ISO 2631-5 [10], these are:

a. The weighted root-mean-square acceleration (aw), according to Eq. (1) of ISO 2631-1:1997 (E).

b. The fourth power vibration dose method (VDV), according to Eq. (5) of ISO 2631-1:1997(E).

c. The equivalent static compressive stress (Se), according to Eq. (A.1) of ISO 2631-5:2004(E).

d. In order to determine the magnitude of recorded transient events, Crest Factor (CF) according to section 6.2 in ISO 2631-1, and coefficients defined in Eq. (7) and (8), were calculated from the measured data. Obtained values give more consistency to evaluate using alternative methods provided in the aforementioned standard.

3.2. ResultsIn Tables 4 and 5 show the results with regard to ISO 2631-1:1997 (E) and ISO 2631-5:2004 (E) respectively. Similarly, Figure 8 shows the weighted and unweighted frequency spectrum of the measured vibrations in the gunner seat (z axis). It can be seen the dominant frequency in the one-third octave of 50 Hz, which magnitude is six times greater than the range of the resonance frequencies of the body, from 4 to 8 Hz.

TABLE 4. Measurement results according to ISO 2631-1.

FactorUnitSeat (Z)Floor (Z)Backrest (X)

awm/s20.770.671.01

VDVm/s1.755.694.959.44

Peakm/s211.617.520.42

FC15.1411.2220.18

Eq. (7)8.227.248.13

Eq. (8)17.1813.0337.59

TABLE 5. Measurement results according to ISO 2631-5.

FactorUnitSeat (Z)Floor (Z)Backrest (X)

Dkm/s27.635.934.7

SeMPa0.240.190.07

FIGURE 8. Frequency spectrum, weighted and unweighted, of the measured vibrations in the seat (z axis) of the gunner.

To compare different obtained values they are normalized to their equivalent exposure time of daily 8 hours.

2.1.7 Effects on Health

As stated above, there are several criteria, set up in several different laws and regulations, national and international. This paper analyzed according to those specified in ISO 2631 and the Argentina law, prescribed in the Resolution 295/03. The evaluation takes the measured value in the seat of the gunner (z axis).

3.2.1.1. Assess Using the Basic Method: aw, ISO 2631-1:1997(E).According to what was stated in the previous paragraph the normalized value A(8) is 0.77 m/s2, which is within the caution zone set in Figure B.1 of ISO Standard.

3.2.1.2. Assess Using VDV, ISO 2631-1:1997(E).

The normalized value, VDV(8), gives a result of 18.8 m/s1.75, which exceeds the limits of the zone of caution and the risk should be assessed as "probable". The exposure time, with this value, to minimize the risk should be reduced to 5 hours and a half.

3.2.1.3. Assessment under Resolution 295/2003.

Although the allowable value of weighted acceleration stated in the law as 0.5 m/s2 is higher than the obtained, if it is analyzed by the method of individual frequencies, also set in the same rule, the measured acceleration levels are below the curve of daily 8 hours; aspect that can be seen in Figure 9.

FIGURE 9. Individual frequency spectrum analysis according to Resolution 295/03.Despite exceed 4% in the frequency of 2 Hz, the levels corresponding to 4 to 8 Hz (resonance frequency of the body) remains notoriously low.

3.2.1.4. Assessment according to Sed, corresponding to ISO 2631-5:2004(E).

Normalized values from Table 5, to express it in terms of daily dose values, are shown in Table 6:

TABLE 6. Normalized seat values.

UnitSeat (Z)

Dk8m/s216.95

Se8MPa0.54

It gets a Se8 value that is between the ranges of values considered "moderate probability of adverse health effects" according to the risk levels defined in ISO 2631-5.

3.3. Discussion of ResultsObtained values by different evaluation criteria do not generate comparable effects to the same situation. Although with the same set of data, the results impose to assume different postures regarding the effects that are predicted on health (Table 7.).

TABLE 7. Summary of values obtained.

INT. STD.METHODUNITVALUEEFFECTS ON HEALTH

ISO 2631-1:1997(E)A(8)m/s20.77INSIDE HEALTH GUIDANCE CAUTION ZONES

VDV(8)m/s1.7518.8ABOVE HEALTH GUIDANCE CAUTION ZONES

ISO 2631-5:2004(E)SedMPa0.54MODERATE PROBABILITY OF RISK

The prediction of the risks that causes whole-body vibration should be carried out with a set of results that can be qualify and labeled in various ways.

Observing ISO 2631-1, the evaluation with A(8) falls within the caution zone where there is potential health risk. However, the standard considers that may be applied additional method to evaluate, because the former underestimates the energy component of the peaks present in some vibrational events.

The VDV(8) value is more convenient in this case according to the peak factor measured. Evaluating with this amount places the assess in the "probable" risk area and should consider that a gunner can be exposed continuously without risk, for a day no more than five hours. This factor is more sensitive to actual shock built-in tank track whose presence is shown in the high values of crest factor, this latter as a clear evidences that the gunner is subject to certain levels of transient events.

In contrast, the values obtained with Sed, determined in ISO 2631-5, minimize health effects, placing the gunner vibration environment in a situation slightly above a low probability of risk. Specifically oriented to the biomechanical behavior of the column in the course of a period of lifetime, the values obtained are such that the gunner tolerates this vibration environment without time restrictions.

Finally, the legislation 295/03, locally applicable, shows two different results. On the one hand, making the analysis by means of individual frequencies, results fit the curve of 8 hours of exposure. It is contradictory to the limit value of the weighted acceleration for 8 hours/day, exceeded in the case studied, for which it would be suitable a reduced exposure time to 3.4 hours per day or implement corrective action to reduce or isolating the subject of the vibration source.

Among the options of taking a more conservative position where would limit operationally the time used, and another, more suitable to the specific case that minimizes the risk margin, the final evaluation, in unequivocal terms, is still open, without definitive answer. The vibration exposure of the combat vehicle crew is discrete, discontinuous and difficult to pin down in the course of a year or during the operating life of a crew member. Depends, among other factors, on operating requirements that may vary from vehicle to vehicle and from crew to crew.

Therefore, faced with the difficulty of establishing a priori exposure time for the evaluation, the above enumerated methods provide an approach that can give time margins, although extensive and dispersed, that could be use to design the time length of a combat vehicle operational profile.

4. CONCLUSIONS

The purpose of this paper does not seek to discuss the property and relevance of the different methods to assess the effects of noise and vibration with regard to health in a combat vehicle. Only, it was proposed to present and describe the results in light of one case, extreme and unusual, restrain to the boundary conditions imposed by the rules and regulations set forth above. Being a vehicle that moves on tracks, is expected a higher noise level and vibration with high presence of transients, for which, the evaluation criteria, place the case study in an indeterminate situation where cannot say unequivocally how long is the limitation on the exposure time of the crew.

Increase in size the samples, to get statistical value, including all crew positions and in different vehicles, is the way to conclude on the reliability of methods to assess the noise and vibration to which crews are exposed it in the armored vehicles.

In next studies and research how to protect or isolate to the man from ambient noise and vibration exposure will be determined. Likewise, the communication quality that allows the environment of the tank will be evaluated.

5. REFERENCES

[1] IRAM-AITA 9C. Medicin del ruido emitido por vehculos automotores en aceleracin. Mtodo de Ingeniera. Instituto Argentino de Normalizacin y Certificacin. 1994. Argentina.[2] IRAM-AITA 9C-1. Medicin del ruido emitido por vehculos en uso, detenidos. Mtodo de verificacin, 1994. Argentina.[3] IRAM 4074-1. Medidor de nivel sonoro. Especificaciones generales. 1994. Argentina.[4] Resolucin N 295/2003. Modificacin del Decreto 351/1979. Argentina.[5] Decreto N 351/1979. Decreto reglamentario de la Ley Nacional N 19.587/1972. Argentina.[6] Ley Nacional N 19.587/1972.Higiene y seguridad en el trabajo. Argentina.[7] World Health Organization. Guidelines for Comunity Noise, 1999.

[8] IRAM 4079-1. Ruidos. Niveles mximos admisibles en mbitos laborales para evitar deterioro auditivo. Relacin entre la exposicin y el desplazamiento permanente del umbral de audicin. 2006. Argentina.[9] International Standard ISO 2631-1:1997(E): Mechanical vibration and shock-Evaluation of human exposure to whole body vibration-Part 1: General requirements.

[10] International Standard ISO 2631-5:2004(E): Mechanical vibration and shock-Evaluation of human exposure to whole body vibration-Part 5: Method for evaluation of vibration containing multiple shocks.Measurement point

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