On the objective occlusion effect induced by in-ear devices under bone-conducted stimulation: a theoretical investigation of the influence of the earcanal wall vibration and its spatial distribution

Abstract

The occlusion effect is commonly experienced as the altered perception of one’s own physiological noise when the earcanal entrance is blocked. By affecting the acoustic comfort of hearing protectors and hearing aids’ users, the occlusion effect participates to their inconsistent or incorrect use. In this thesis, the objective occlusion effect caused by in-ear devices under bone-conducted stimulation is investigated in order to ultimately mitigate the phenomenon. For this purpose, this thesis focuses on (i) the explanation of the fundamental mechanism of the objective occlusion effect and its multiple interpretations, (ii) the influence of the earcanal wall vibration and its spatial distribution, and (iii) the mechanism(s) of contribution of in-ear devices to the occlusion effect. In a first part, the vibro-acoustic behavior of the earcanal open and occluded by an infinite impedance and submitted to a bone-conducted stimulation is thoroughly analyzed, illustrated and interpreted using a 3D finite element model of a realistic outer ear in conjunction with an associated electro-acoustic model. The two models are very complementary to dissect physical phenomena and to highlight the influence of the earcanal wall normal vibration distribution on the vibro-acoustic behavior of the open earcanal. This distribution is here characterized by its centroid position along the earcanal curvilinear axis. In particular, it is shown that this centroid position corresponds to the location of an equivalent source representing the normal vibration of the earcanal wall in electro-acoustic model. In addition, common interpretations of the occlusion effect in terms of “leak” and “trap” are shown to misrepresent the fundamental mechanism of the phenomenon related to the earcanal impedance increase. In a second part, a widespread theory of the objective occlusion effect provided by Tonndorf in 1964 is revisited to clarify its ambiguous points from which several misinterpretations of the phenomenon could have been derived. Investigating the electro-acoustic model associated with the theory, a second order high-pass filter effect for the volume velocity transferred between the earcanal wall and the eardrum is highlighted. This filter remains for partial occlusion but vanishes for perfect occlusion. In the latter case, the volume velocity transferred from the earcanal cavity to the middle ear through the eardrum drastically increases, which explains the predominance of the occluded outer ear pathway on the hearing by bone-conduction at low frequencies. In a third part, the principle of an indirect method to estimate the centroid position of the earcanal wall normal velocity is presented. This method consists in measuring the acoustic pressure transfer function between the earcanal open and occluded by an external capped duct coupled to the earcanal entrance under bone-conducted stimulation. The centroid position is then estimated at the lowest antiresonance frequency of the earcanal coupled to the external duct using an electro-acoustic model associated to the coupled system. This indirect method is evaluated and investigated using a 3D outer ear finite element model. In a last part, the mechanism(s) of contribution of earplugs to the occlusion effect is investigated using a 2D axi-symmetric finite element model of the outer ear in conjunction with an associated electro-acoustic model. Two mechanisms are highlighted: (i) a Poisson effect induced by the normal component of the earcanal wall vibration and (ii) a longitudinal motion caused by the tangential component of the earcanal wall vibration. By varying the geometry of the earcanal surrounding tissues, the spatial distribution of the earcanal wall vibration, in both normal and tangential directions, is shown to influence the contribution of earplugs to the occlusion effect. Also, it is shown that the contribution of earplugs to the occlusion effect dominates that of the earcanal wall which is not covered by them from medium to deep insertion depth. Overall, this thesis constitutes an additional effort in the understanding of the occlusion effect induced by in-ear devices and ends at the beginning of the development of new concepts mitigating the phenomenon using their own passive vibro-acoustic behavior.

Date	28 Jul 2021
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Olivier Doutres (Supervisor) & Franck Sgard (Co-supervisor)