Middle Ear Nonlinearity in High Intensity Sound: Impact on Hearing Damage and Protection

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ABSTRACT: Our ears are vulnerable to intense sounds (> 130 dB SPL) that can damage middle- and inner-ear structures and produce temporary or permanent hearing loss. Prior research has focused on studying damage to the cochlea, but relatively little is known of how the middle ear transmits extremely high-level damaging sounds to the cochlea. While the middle ear muscle reflex elicited by high level steady state sounds can reduce sound transmission to the cochlea, it cannot protect the ear during brief impulsive sounds. Nonlinear mechanisms of the middle ear that can provide protection for impulsive sounds have been observed. However, there are few quantitative studies of middle ear nonlinear processes, and the contributions of nonlinear mechanisms to the frequency and level dependence of middle ear sound transfer are not well described. Furthermore, the etiologies of middle ear structural failures produced by intense sounds are poorly known, as are their effects on middle ear function. This lack of knowledge limits our ability to prevent such failures. There is also a lack of knowledge of how surgically reconstructed middle ears with a passive prosthesis or graft work and their failure rate at high level sounds. To tackle these questions, we propose three aims in this study. Aim 1 will use human cadaveric temporal bones to systematically characterize middle ear nonlinear processes in time and frequency domains at high intensity, in normal and reconstructed middle ears, to gain critical new knowledge on middle ear structure and function at high sound levels. Aim 2 will extend the sound intensity to levels that can rupture the tympanic membrane and other soft tissues to investigate the cause and time history of failures in normal and reconstructed middle ears. For Aim 3, we propose to develop finite element middle ear nonlinear models that incorporate multiple middle ear nonlinear mechanisms, including impulsive time-domain models and continuous tone frequency-domain models. This study will impact our ability to predict noise-induced hearing damage, development of new strategies for ear protection and improvements in the surgical repair of damaged middle ears.