Experimental evaluation of earplug behavior in front of high-level impulse noise using laser Doppler vibrometer

Hearing protection devices facing high-level impulse noises provide an attenuation, generally, between 20 and 40 dB. One reason for this limitation is the direct interactions between the protection device and the impulse waves. In the case of earplugs, direct transmissions through the earplug occur. These direct transmissions combine with the already well-studied indirect transmissions arising from wave propagation in the external ear's tissues (skin, cartilage, and bone). To evaluate the transmission induced directly by the earplug, an experimental protocol using a laser Doppler vibrometer was developed. Thus, the earplug's outer lateral face (OLF) displacements and acoustic pressure at the eardrum were measured simultaneously. Two earplugs (polyurethane foam and acrylonitrile butadiene styrene) inserted in an acoustic test fixture were stimulated with impulses ranging from 137 to 180 dB-peak. A slight earplug OLF movement in the ear canal varying from 1 μm to 0.1 mm could be observed, which is likely related to ear canal longitudinal compression. The earplug's OLF displacement and acoustic pressure variation at the eardrum strongly depended on the earplug type. These direct transmissions and underlying consequences considerably alter the protection efficiency.