DURATION AND PEAK LEVEL AS COFACTORS IN HEARING-LOSS FROM EXPOSURE TOIMPACT NOISE

The Equal Energy Hypothesis (EEH) is based on the assumption that hear ing loss from a given exposure is proportional to the total energy of the exposure. A corollary of this assumption is the power of the expos ure and the duration of the exposure are interchangeable. Studies of i mpulse and impact noise show that hearing loss does not follow the pre diction of the EEH (Ward, 1986; Danielson et al., 1991). Thus, the fol lowing experiments were designed to assess the relative importance of the duration and peak level of impact noise in the production of heari ng loss. Monaural chinchillas served as subjects. Their quiet threshol ds were estimated before and after exposures using the evoked potentia l recorded from a chronic electrode in the inferior colliculus. The fo ur groups of the ''intensity'' series were exposed for 7.5 hours to 20 0 msec impacts at 1 per second at levels of 113, 119, 125, and 131 dB peak equivalent SPL. The three groups of the ''duration'' series were exposed to the 125 dB impacts for either 1.9, 7.5, or 30 hours. With e ach dB increase in the ''duration'' series, there was approximately 1. 7 dB of increase in hearing loss. For each dB increase in peak level a bove 125 dB, there was an average 6.6 dB increase in hearing loss. The 125 dB exposure is just below the ''critical level'' where the mode o f cochlear damage shifts to mechanical failure. The results are discus sed in terms of ''critical'' level and implication for noise standards .