A finite element model of a human head and a corresponding acoustic test fixture to assess the objective occlusion effect induced by earplugs under bone-conducted stimulation

Abstract

The occlusion effect refers to the increased perception of bone-conducted sounds (e.g., physiological noises like one’s own voice, breathing, chewing, heartbeat) when one wears hearing protection devices, and it constitutes a major acoustic discomfort factor. This phenomenon occurs mainly at low frequencies (below 1.0 kHz) and is particularly pronounced for earplug-type hearing protection devices. This thesis is interested in assessing the occlusion effect of earplugs using virtual and physical testers both based on the same real head geometry of a living participant. The virtual tester consists of a finite-element head model whereas the physical one corresponds to an augmented acoustic test fixture (i.e., a realistic artificial head). The finite-element head model is evaluated by comparing numerical results with experimental data (i) available in the literature obtained on groups of participants, (ii) obtained on the augmented acoustic test fixture, and (iii) measured on the participant whose head is used for constructing the geometrical head model. The augmented acoustic test fixture is evaluated itself by comparing the occlusion effect of a foam earplug measured on it and on the participant. In fact, several numerical models for studying the occlusion effect are available in the literature. However, it is found that boundary and loading conditions greatly affect the simulation results of these models due to the consideration of truncated outer ears. On the contrary, the finite-element head model developed in this thesis is found to be only sensitive to the boundary conditions on the artificial boundaries created by the truncation (at the head base in this case) at very low frequencies. It thus avoids choosing boundary conditions so that the corresponding simulation results match the experimental data at the best. Moreover, the finite-element model includes an external air domain surrounding the entire head. A perfectly absorbing condition (i.e., perfectly matched layer) is applied to the boundaries of the air domain, which simulates a free sound field. This allows for taking into account the radiation from the soft tissues of the head into the open earcanal. The present numerical model thus simulates the occlusion effect that better matches experimental data at low frequencies. Thirdly, the finite-element head model allows for investigating the effect on the occlusion effect of the stimulation position. The sensitivity of the occlusion effect to the stimulation position for a given part of the head (e.g., the ipsilateral mastoid) suggests that this factor contributes to the variability of the experimental data obtained on groups of participants in the literature. In this study, the present model is also used to investigate the variability of the occlusion effect induced by the material properties of the earcanal surrounding tissues (i.e., soft tissues, cartilage, and bone). The stiffness parameters (i.e., Young’s modulus and Poisson’s ratio) of the soft tissues and the cartilage are found to affect the occlusion effect the most compared with the other parameters. Besides, the sound fields in the open and occluded earcanals under bone-conducted stimulation are explored using the finite-element head model. Results suggest that the occlusion effect measured at the eardrum position is similar to that assessed at the position of the earplug medial surface. The latter position is safer and more comfortable for the human subjects involved. Finally, the experimental evaluation of the augmented acoustic test fixture shows that in contrast to the measurement on the participant, the occlusion effect of earplugs does not decrease with frequency, which is mainly due to the mechanical properties of the materials used for its fabrication. In the long term, after robust validation and calibration, the augmented acoustic test fixture developed during this project could serve as a powerful tool for evaluating the acoustic performance of earplugs (e.g., occlusion effect and sound attenuation).

Date	21 Dec 2022
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Jacques A. de Guise (Supervisor), Franck Sgard (Co-supervisor) & Éric Wagnac (Co-supervisor)