Reduction of the occlusion effect induced by earplugs using quasi perfect broadband absorption

Impact of the ear canal motion on the impedance boundary conditions in models of the occlusion effect

The occlusion effect (OE) denotes the increased low-frequency perception of bone-conducted sounds when the ear canal (EC) is occluded. Circuit and finite element (FE) models are commonly used to investigate the OE and improve its prediction, often applying acoustic impedances at the EC entrance and tympanic membrane (TM). This study investigates the sound generation caused by the structural motion of the EC. In addition to the EC wall vibration, it accounts for the motions of the EC entrance and TM, resulting from nondeforming motion of the surrounding structures. A model extension including these motions with the impedances is proposed. Related mechanisms are illustrated based on a circuit model. Implications are discussed by using an EC motion extracted from a FE model of a human head. The results demonstrate that the motions of the EC entrance and TM, addressed by the proposed extension, affects the TM sound pressure and may lead to a reduction of the OE at lower frequencies compared to solely considering the EC wall vibration. Accordingly, this phenomenon potentially reconciles differences between experimental data and OE simulations at frequencies below about 250 Hz, highlighting the importance to discern between multiple contributing mechanisms to the TM sound pressure.

Passive earplug including Helmholtz resonators arranged in series to achieve broadband near zero occlusion effect at low frequencies

The use of passive earplugs is often associated with the occlusion effect: a phenomenon described as the increased auditory perception of one's own physiological noise at low frequencies. As a notable acoustic discomfort, the occlusion effect penalizes the use and the efficiency of earplugs. This phenomenon is objectively characterized by the increase in sound pressure level in the occluded ear canal compared to the open ear canal. Taking inspiration from acoustic metamaterials, a new design of a three-dimensional printed "meta-earplug," made of four Helmholtz resonators arranged in series, is proposed for achieving near zero objective occlusion effect measured on artificial ear in a broadband frequency range (300 Hz to 1 kHz). For this purpose, the geometry of the meta-earplug is optimized to achieve a null occlusion effect target based on an analytical model of the phenomenon. It results from the optimization process that the input impedance of the meta-earplug medial surface approximately matches the input impedance of the open ear canal, weighted by the ratio of volume velocity imposed by the ear canal wall to the ear canal cavity between open and occluded cases. Acoustic properties of the meta-earplug are also shown to significantly improve its sound attenuation at the piston-like mode of the system.