A common result of noise exposure is a noise-induced threshold shift, defined as an increase in auditory threshold that persists after cessation of a noise exposure. If the hearing threshold returns to normal after some period of time, the threshold shift is called a temporary threshold shift (TTS). If the threshold does not return to normal, the remaining amount of threshold shift is called a permanent threshold shift (PTS). Observed PTS/TTS may be the result of a variety of mechanical and biochemical processes, including physical damage or distortion of the tympanic membrane and cochlear hair cell stereocilia, oxidative stress-related hair cell death, changes in cochlear blood flow, and swelling of cochlear nerve terminals resulting from glutamate excitotoxicity (Henderson et al. 2006; Kujawa and Liberman 2009). Although the outer hair cells are the most prominent target for noise effects, severe noise exposures may also result in inner hair cell death and loss of auditory nerve fibers (Henderson et al. 2006).

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