Ear canal deformations by various earplugs: An in situ investigation using MRI

Abstract

To be efficient, an Hearing Protection Device (HPD) should be worn during the time the workers are exposed to high level noises in order to protect them against hearing losses or deafness hazards. Several reasons like the difficulty to communicate, aesthetic considerations or discomfort issues prevent the workers from wearing HPDs properly. In the past, some indicators of physical (static pressure on the ear canal walls) and auditory (insertion loss, occlusion effect) discomfort induced by earplugs have been assessed using numerical models. However, most of these models rely on the assumption that the shapes of the open and occluded ear canal are identical which has never been demonstrated experimentally. As the recent rise of medical imaging and numerical computing technologies allows for reconstructing the 3D geometry of the outer ear, this study proposes to investigate the validity of this assumption. The shapes of the open and occluded ear canal of a human subject were obtained using high-resolution magnetic resonance imaging. Several kinds of earplugs were tested. Their non deformed shapes were obtained thanks to preliminary measurements. In each case the qualitative (from a radiologist assessment) and quantitative (from deformation measurements) comparison between the reconstructed occluded and open ear canal was carried out to evaluate whether and how both earplugs and ear canal deform. The deformation of earplugs and tissues of the ear canal provides useful information to investigate how each earplug and skin should be modelled and characterized mechanically in order to predict the earplug sound attenuation and occlusion effect.