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Passive earplugs are commonly used to reduce workers' exposure to excessively high noise levels. Yet, they are associated with various discomforts of diverse origins (e.g., acoustic, physical, functional, or psychological). The occlusion effect, characterized by an increased perception of physiological sounds transmitted through bone conduction to the cochlea, presents a challenge, leading to acoustic discomfort, especially at low frequencies (0.1 to 1 kHz) and for shallow or moderate insertion depths of the earplug. Building upon acoustic meta-material principles, this study presents “meta-earplug” concepts designed to alleviate the occlusion effect. The approach focuses on reducing the input impedance of the earplug medial surface either to the characteristic impedance of air, using broadband perfect absorption, or to the acoustic impedance of the open ear canal, resulting in a zero objective occlusion effect. For these purposes, the proposed meta-earplug concepts are made of Helmholtz resonators arranged in parallel or series. Transfer matrix models are used in an optimization process to refine the geometry of the meta-earplugs. Although meta-earplug concepts involving multiple Helmholtz resonators have been preliminarily assessed using an artificial ear, a meta-earplug featuring a single resonator has undergone testing with human participants. The evaluation encompasses both objective measurements and subjective assessments.
Carillo et al. (Fri,) studied this question.