Note

In Québec, approximately 360,000 workers are exposed daily to noise levels that can lead to hearing problems. Hearing protection devices (HPDs) are often used to mitigate this problem. However, the discomfort associated with their use limits how long they are worn and thus the effectiveness of the protection they provide. This research is being conducted in the context of developing acoustic design tools that minimize the sources of auditory and physical discomfort of these protectors while ensuring adequate attenuation.

The study focuses on two comfort indicators associated with wearing earplugs: attenuation and the occlusion effect. Attenuation quantifies the reduction in sound pressure levels on the eardrum. This indicator may be a source of discomfort if it is to high (making it impossible to communicate) or too low (excessive noise level). The occlusion effect is characterized by an increase in low frequency sound pressure levels at the eardrum, resulting from vibration of the earcanal walls created by sources internal to the human body or by external bone conduction. These two comfort indicators are dependent on numerous factors (such as the material that the earplugs are made of, positioning in the ear) which are still rarely taken into account in the experimental devices designed to study them. Today’s artificial ears use a cylindrical earcanal of constant circular cross-section, covered with a layer of silicone to imitate skin and ending in an acoustic coupler that simulates eardrum impedance. This coupler limits the study of attenuation because of its design, which restricts how deeply the earplugs can be inserted. The absence of temporal bone and cartilage surrounding the canal and the simplification of its geometry in these artificial ears considerably limits the study of the occlusion effect. In short, these artificial ears are poorly suited to measuring the attenuation of earplugs and do not enable objective measurement of the occlusion effect.

The objective of this study is to recommend methodological guidelines for the design of artificial ears adapted to these measures. The primary novel aspect of this study lies in reconstructing 3-D digital ear models using individual magnetic resonance images (MRI) obtained in vivo. The second novel aspect is the development of artificial ears based on these geometric models and adapted to the earplug comfort study by taking into account the geometry and tissues surrounding the earcanal. The mechanical properties of artificial ears were adjusted during the manufacturing stage to better simulate human tissue, such as bone, cartilage and soft tissue. A method to estimate the displacement field of the earcanal walls resulting from inserting the earplugs has been proposed and validated for the artificial ear and for the case of a human subject. Use of this method should make it possible to validate synthetic materials that simulate the mechanical behaviour of human tissues.

Finally, attenuation and occlusion effect measurements were carried out and compared to digital simulations. In the long-term, the tools developed in this study will make it possible to improve hearing protector effectiveness, thus reducing the risk of occupational hearing loss.