Principle of an acoustical method for estimating the centroid position of the earcanal wall normal velocity induced by bone-conducted stimulation: Numerical evaluation

Abstract

The occlusion effect is commonly experienced as the altered perception of one’s own physiological noise when the earcanal entrance is blocked. Objectively, this phenomenon corresponds to an acoustic pressure increase in the occluded earcanal. The occlusion effect originates from the earcanal wall normal vibration and depends on the spatial distribution of the latter. At low frequencies, this spatial distribution can be characterized by the position of its centroid along the earcanal middle axis. This paper describes the principle of an acoustical method for estimating this centroid position at low frequencies. The proposed method consists in measuring the eardrum acoustic pressure transfer function between the earcanal open and occluded by an external capped duct coupled to the earcanal entrance of a subject submitted to a bone-conducted stimulation. The centroid position is then estimated at the antiresonance frequency of the coupled system using an associated electro-acoustic model. The proposed method is evaluated and investigated numerically using a 3D finite element model of an outer ear. The sensitivity of the method is shown to increase with frequency. To maximize the method accuracy, the radius of the coupling duct must be as large as possible (in the limits of the earcanal entrance dimension) and any incomplete seal between the duct and the earcanal entrance must be avoided. Also, the coupling position of the duct and its temperature must be known as precisely as possible. On the contrary, the proposed method does not require the knowledge of the eardrum acoustic impedance.