Safe level for noise exposure?

The safety of long, continuous exposure to several different levels of noise was determined. Chinchillas were exposed for two or nine days to an octave band of noise with a center frequency of .5 kHz or 4 kHz. By examining the inner ear for evidence of acute cellular degeneration, exposures which produced minimal permanent damage could be identified easily. Differences were found between the two bands of noise in the level required to produce injury, and the type, location and extent of damage which occurred as the level was increased. The findings help explain why exposure to industrial noise, which is generally low frequency, first produces measurable auditory changes for high frequency tones.