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Measurement of acoustic stress of vehicle occupants with vehicle blasting

Armoured vehicles must guarantee that passengers are protected in the event of blasting. This also applies for hearing. As a high level of sound pressure can occur in the interior of a vehicle, a check is performed to ensure that this does not exceed a defined limit value. The armasuisse S+T test centre has developed a new measuring procedure for the first time to determine the impact of the sound pressure on hearing.

19.06.2020 | armasuisse Science and Technology, Test centre, Mario Clausen

Experiment  im Hallraum bei der EMPA in Dübendorf

In order for a system such as an armoured vehicle to be declared suitable for use, it is checked with regard to various criteria before a troop deployment. One of the central goals of a protected system is to offer the members of the armed forces protection in the event of an emergency. One of the respective criteria for this concerns the protection of hearing. Thus very high sound pressure values can occur in the interior of the vehicle in the event of a blast by a mine or a booby trap. In an extreme case, this could lead to a blast trauma, making the AdAs unfit for combat.

Experts from the armasuisse test centre (WTT) want to determine this impact on hearing using two different experiments. For this purpose, they have equipped a helmet with special microphones which can measure the sound waves generated with a blast.

Experiment 1 in a diffuse field

The first experiment was performed in an almost undamped echo chamber at the Swiss Federal Laboratories for Materials Testing and Research (EMPA) in Dübendorf. An echo chamber is a special acoustics room that is constructed such that the sound level is approximately the same at every point in the room. For the first noise level measurement, an artificial head created by WTT was placed in the echo chamber and the helmet with integrated microphone fitted on it, in order to measure and record the occurring sound level. Loudspeaker impulses were then generated in the audible human range, so that a diffuse a sound field which does not contain any more directional information was created in the chamber. The second measurement only took place with microphones that were placed in the room such that they were located at the positions of the two auditory canals. From the comparison of the two measurements, the transmission function from the helmet to the ear, and thus to hearing, can be determined in the diffuse field.

Experiment 2 in the vehicle sound field

As the sound field in the interior of a vehicle deviates from the ideal diffuse field, the experiment described above will be repeated for the armoured car MOWAG EAGLE 6x6 TASys in a second step. For this purpose, two dummies with fitted helmets and integrated microphones are used in the vehicle. As in experiment 1, noise levels are generated from various different loudspeaker positions and the impulse responses measured. The dummies with the helmets are then removed from the vehicle, sound pressure microphones installed at the positions of the auditory canals and the impulse responses once again measured. Vehicle- and passenger-specific transmission functions can be determined by comparing the two impulse responses. This function essentially describes the impact of the helmet and the head on the recorded signal and thus on hearing.

Acoustic examinations of blasting tests

These transmission functions ultimately form the basis for the acoustic analyses of subsequent blasting tests. Using these functions, sound pressure values that have been measured with the helmets equipped with microphones can be converted to the sound pressure values acting on the ears of an AdA. The comparison of the transmission function obtained in the vehicle with that from the echo chamber enable conclusions to be drawn on sensitivity regarding the location of the sound field and thus the necessity to measure another vehicle individually.