Damage to the cochlea following interrupted exposure to low frequency noise

This study determines how the magnitude and pattern of cochlear damage is altered when exposure to noise is interrupted by regularly spaced rest periods. Groups of chinchillas were exposed for six hours per session to an octave band of noise with a center frequency of 0.5 kHz. The rest interval between successive exposures varied from 18 to 162 hours. The total energy in these exposures was equal to that in a nine-day continuous exposure at 95 dB sound pressure level. The ears of all animals were prepared for histological study so that the extent of cochlear damage could be determined. The pattern of damage produced by interrupted exposure to low frequency noise was the same as that found with continuous exposure, while the magnitude of damage was usually reduced. The amount of protection provided by a particular rest period was found to be different for the low and high frequency regions of the cochlea. The significance of these findings with regard to the mechanisms of noise damage is discussed.