Animal model of the 4-kHz tonal dip in humans.

IT has long been observed that when humans are exposed to occupational or recreational noise, permanent hearing losses for high-frequency tones develop regardless of the frequency content of the exposure. The term "4-kHz tonal dip" has been coined to describe the audiogram of a typical noise-exposed person, although the dip has been found to occur anywhere from 3 to 6 kHz. As exposure to noise continues, permanent hearing losses begin to appear at frequencies immediately above and below 4-kHz. Generally, only after long histories of exposure do hearing losses begin to occur in the frequencies considered most important for speech discrimination. Since humans cannot be exposed experimentally to noises that may produce permanent hearing losses, we have been working to develop an animal model of the 4-kHz notch to determine the parameters of exposure that will produce this notch and to define the relation between hearing loss produced by low-frequency noise exposure and cochlear damage.