Custom molded silicon earplugs: effect of material properties on acoustic attenuation and mechanical skin contact

Abstract

Silicone is one of the most common used material for custom earplug manufacturing. Nevertheless, the influence of its mechanical properties on both the sound attenuation and the mechanical skin contact are based on empirical models. The knowledge of the silicon properties is frequently limited to its shore value which is necessary but not sufficient to fully describe the vibro-acoustic behavior of the earplug. In this work, various sensitivity analyses based on 2D-axisymmetric finite element model of the occluded auditory canal coupled to mechanical properties measurements of silicon commonly used for custom earplugs are performed. Complementary attenuation measurements with acoustical tests fixtures are made in a means of model validation. The achievable values around realistic material properties variations are presented from the two perspectives of sound attenuation and mechanical impedance modulus at the contact between the earplug and the auditory canal. The rigidity or mass-frequency law effects are retrieved and mostly quantified. Unexpected or contradictory effects are also highlighted, for example a sign change around the mean attenuation value at two consecutive frequencies or an unexpected coupling between two parameters. These results contribute to filling the knowledge gap in terms of complex mechanical behavior of silicone earplugs and could benefit to earplug manufacturers for product optimization.