Numerical investigation of the fundamental low frequency mechanisms of the objective occlusion effect: focus on the earcanal wall vibration

The occlusion of the human earcanal is commonly associated with the so-called occlusion effect. This phenomenon is particularly uncomfortable at low frequency when wearing an intra-aural occlusion device in shallow insertion. In low frequency, the occlusion effect is objectively quantified as the acoustic pressure increase in the occluded earcanal compared to the open one. Fundamentally, the occlusion effect is governed by the change of earcanal acoustic impedance seen by its wall due to its occlusion. However, several factors influence the occlusion effect such as the occlusion device (e.g., type, fit, position in the earcanal), the stimulation (e.g., position and nature) and the earcanal anatomy (e.g., geometry and material properties of earcanal surrounding tissues). Compared to the first two aforementioned aspects, the latter has not been investigated yet because of experimental difficulty. However, it is important since the earcanal anatomy influences the earcanal wall vibration, which is the origin of the acoustic pressure generated in the earcanal. In this work, the influence of the earcanal surrounding tissues geometry on its vibration distribution is numerically studied. For this purpose, a full factorial design of experiment is conducted using a 2D axi-symmetric finite element model of an open and occluded outer ear. The occlusion is simplified to an infinite impedance defined at the ear-canal entrance. The influence of the earcanal vibration distribution on the occlusion effect and its fundamental mechanism is then investigated. In conjunction, an eletro-acoustic model is proposed to explain in a simplified way the trend of the finite element model results.