Estimation of acoustic test fixture tilting compensation in front of high-level impulsive sounds

Abstract

Acoustic Test Fixtures (ATFs) are widely used to evaluate the performance of Hearing Protection Devices (HPDs) against high-level impulsive sounds. The attenuation conferred by HPDs is partially limited by inertial and compressive transmissions, which could manifest as earmuff or earplug movements. The external longitudinal displacements of an earplug inserted in the ear canal of an ATF have previously been measured using a laser Doppler vibrometer in order to correlate them with the protected ear canal acoustic pressure. However, at high impulsive levels, a tilting motion of the ATF can combine with the earplug's displacements. To quantify this effect and assess its impact on the earplug displacements measurements, the displacements of a modified ATF across levels ranging from 156 to 180 dB peak have been evaluated using two different sensors. First, the protocol employing a laser-vibrometer to assess earplug displacement has been replicated. Second, an accelerometer combined with various signal processing techniques has been tested. Both sensors consistently detected the tilting motion of the ATF, with peak displacement amplitudes ranging in average from 1 μm at 156 dB peak to 100 μm at 180 dB peak. These results demonstrate that the maximum amplitude of ATF displacement increases exponentially with impulse peak level. However, due to a time delay in the occurrence of this phenomenon, limited to no compensation for ATF tilting is required. These findings support an enhanced experimental protocol for investigating transmission characteristics of HPDs when evaluated with an ATF exposed to high-level impulsive sounds.