The effects of impulse noise level on the magnitude of asymptotic threshold shift (ATS) in the chinchilla

Four groups of monaural chinchillas were exposed to either 99, 106, 113, or 120 dB peak SPL reverberant impulse noise for 10 days. The impulses had a “B” duration of approximately 160 ms and were presented at the rate of 1 impulse/s. Hearing thresholds were monitored before, during, and for 40 days after exposure. After 24 h in noise, all four levels produced a stable ATS. The level of ATS at 0.5, 2.0, and 8.0 kHz grew at the rate of 2.5 dB for each dB increase in the impulse noise level. The PTS function at these frequencies suggests a threshold effect, i.e., the two lowest levels produced no PTS, while at the two highest levels PTS began to increase rapidly. The relation between ATS and PTS is complex. Both the 106 and 113 dB exposures produced similar levels of ATS but the 106 dB group showed no PTS while the 113 dB group developed as much as 30 dB PTS at 2.0 kHz. Although the data are limited, it appears that after some criterion level has been reached, PTS also grows at the rate of 2.5 dB/dB. The cochleograms for the four groups of animals show an orderly increase in the severity of damage. However, an orderly relation between individual cochleograms and audiograms was not found. [Work supported by NIOSH and NIEHS.]