Effects of the Hardness of Acoustic Test Fixtures' Ears on the Evaluation of Earplug’s Direct Transmissions Facing High-Level Impulse Noises.
AbstractApart from secondary propagation paths, the performance of Hearing Protection Devices (HPD) facing high-level impulse noises is also limited by the acoustic transmission through the protector. In the earplug's case, part of these transmissions can be described as the protection's displacement along the ear canal axis. It has been demonstrated that the peak amplitude of this displacement is directly proportional to the peak acoustic pressure measured behind the protector when Acoustic Test Fixtures (ATF) are used. In order to better understand these results and the consequences for the human ear, it is now required to evaluate the influence of the material properties involved in the ATF ear canal and HPD coupling. For this purpose, a laser-vibrometer is used to measure the earplug's peak displacements when the latter is inserted in two ATF ear canals of different materials (soft and medium hardness). The impulse waves employed to excite the earplug ranged from 150 dB to 180 dB peak. The experiment highlights that for the same earplug, the measured peak displacements were significantly different (200 µm and 66 µm respectively, for a soft and a medium hardness at 177 dB peak) with correlated consequences on the peak noise reduction of the HPD (19 dB peak and 29 dB peak respectively, for a soft and a medium hardness at 177 dB peak). These results suggest new perspectives for HPD evaluation through the choice of judicious ATF materials that conform to biological tissues.
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