Study of the occlusion effect induced by an earplug : modelling and experimental validation

Abstract

Despite existing limits for occupational noise exposure, professional hearing loss remains a high priority problem both in Québec and worldwide. Several approaches exist to protect workers from harmful noise levels. The most frequently employed short term solution includes the distribution of hearing protection devices (HPD) such as earplugs and ear muffs. While HPDs offer an inexpensive (e.g. direct cost) and efficient means of protection workers often only tend to wear HPDs for limited amounts of time and, thus, remain at risk of developing professional hearing loss. Discomfort while using HPDs contributes to HPD underutilization and non-use. Two more general categories of discomfort can be distinguished. The category physical discomfort includes, for instance, problems such as heating of the ear and irritation of the ear canal that that occur upon earplug insertion. The category auditory discomfort refers to alterations in the auditory perception of sounds and one’s own voice as well as hindered workplace communications. One important auditory discomfort that promotes HPD non-use is the occlusion effect. The occlusion effect occurs upon earplug insertion and describes sound amplification phenomena in the occluded ear canal at the low frequencies. The sound amplification is both perceivable and measurable (e.g., open and occluded sound pressure levels, hearing threshold shift). Additionally, the occlusion effect causes the HPD wearer to perceive his/her own voice as being distorted (e.g. hollow sounding) and physiological noises (e.g. respiration, blood circulation) are amplified also subsequent to earplug insertion. Reducing the occlusion effect has the potential to increase the auditory comfort of HPDs and could help preventing occupational hearing loss in the future. In order to improve this and other shortcomings observed with currently existing HPDs a large research collaboration between the Robert-Sauvé research institute in occupational health and safety (IRSST) and the École de technologie supérieure (ÉTS) has been launched. The present study represents a part of this collaboration and aims at studying the occlusion effect of the system earplug – ear canal through the development of novel numerical models and experimental methods. A 3D (complex geometry) and an axisymmetric (simplified geometry) linear elasto-acoustic finite element model are presented in this study to simulate the objective bone conduction earplug occlusion effect. The 3D model of complex geometry is used to predict the occlusion effect induced by a silicone earplug at several insertion depths. Power balance computations are used to explain how the ear canal walls and the medial earplug surface contribute to observed occlusion effect magnitudes at varying occlusion depths. The numerical occlusion effect predictions are validated with experimental reference data that were retrieved from the literature. The 3D model is used to investigate two well established qualitative occlusion effect models using power balance computations. The axisymmetric occlusion effect model is used to predict open and occluded transfer function levels as well as the occlusion effect across three different excitation scenarios. First, only structure borne excitation is considered. Next, airborne noise is added incoherently to the structure borne excitation to study the effect of a mixed excitation. The mixed excitation is considered (i) for a leak free (perfect seal) earplug insertion and (ii) Under the presence of small earplug leaks. Each stimulation scenario is examined across four different boundary and load conditions. All predicted transfer function levels and occlusion effects are compared to experimental data. An adapted version of the axi-symmetric occlusion effect model is employed to investigate the contribution of the earplug type to the occlusion effect magnitude. First, the numerical model is validated with the help of experimental occlusion effect data which were measured in two independent human reference groups which each use a different earplug type (silicone earplug and foam earplug). Second, the numerical model is further validated through comparison with two gold standard lumped element occlusion effect models which were drawn from the literature. Third, power balance computations are employed to investigate the power flow inside the occluded ear canal cavity as well as the earplug body (coupled to the ear canal walls) of the numerical external ear model. The power balances are computed both for the foam and the silicone earplug models at medium and very shallow earplug insertion depths. A prototype of a novel artificial external ear test fixture for objective and standardized measurement of the occlusion effect is developed. Details on the implementation of the artificial external ear and the assembly of the occlusion effect test fixture are presented. The experimental test fixture is used to investigate the contribution of the structure and airborne sound transmission pathways. Experimental data is provided to demonstrate that the test fixture is functional and that it can be used to measure the occlusion effect of a foam earplug. The occlusion effect measurement is repeated for several mechanical stimulation levels to study the system’s linearity.

Date	19 Dec 2014
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Frédéric Laville (Supervisor), Yvan Petit (Co-supervisor) & Franck Sgard (Co-supervisor)