The effects of variability in ear canal geometry and middle ear impedance on noise exposure level

Abstract

If a group of industrial workers is uniformly exposed to a particular noise level, each person develops a different amount of hearing loss. The reasons for individual differences in susceptibility to noise are not entirely clear, but might relate, in part, to variability in ear canal acoustics and middle ear impedance. This variability alters the sound field to eardrum SPL transformation, effectively changing the magnitude of the exposure, and potentially producing differences in hearing loss. To study these effects, a computer simulation was created from an electrical analog model of the middle ear and a network representation of the ear canal. The model predicts how variation in ear canal geometry (length and diameter) and middle ear load impedance influences the eardrum SPL, both for individuals wearing hearing protector devices (‘‘leaky’’ earplugs) and for the unprotected ear. The model also estimates the amount of attenuation offered by the earplug and the amount of conductive hearing loss created by changes in the middle ear. Therefore, it was possible to evaluate whether changes in exposure levels would likely result in hearing loss, given the amount of protection offered by either earplug attenuation or conductive hearing loss.