Duration and peak level as co-factors in hearing loss from exposure to impact noise

The Equal Energy Hypothesis (EEH) is based on the assumption that hearing loss from a given exposure is proportional to the total energy of the exposure. A corollary of this assumption is the power of the exposure and the duration of the exposure are interchangeable. Studies of impulse and impact noise show that hearing loss does not follow the prediction of the EEH (Ward, 1986; Danielson et al., 1991). Thus, the following experiments were designed to assess the relative importance of the duration and peak level of impact noise in the production of hearing loss. Monaural chinchillas served as subjects. Their quiet thresholds were estimated before and after exposures using the evoked potential recorded from a chronic electrode in the inferior colliculus. The four groups of the "intensity" series were exposed for 7.5 hours to 200 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 each dB increase in the "duration" series, there was approximately 1.7 dB of increase in hearing loss. For each dB increase in peak level above 125 dB, there was an average of 6.6 dB increase in hearing loss. The 125 dB exposure is just below the "critical level" where the mode of cochlear damage shifts to mechanical failure. The results are discussed in terms of "critical" level and implication for noise standards.