[Impulsive noise: PTS and anatomic correlations].

Abstract

The EEH (Equal Energy Hypothesis) postulates that permanent hearing loss (PTS) produced by exposure to noise is a function of sound energy of the exposure. The validity of EEH for continuous noise exposure was confirmed by large scale demographic studies and experiments in controlled laboratory setting. However, for impulse noise, EEH may not be as valid. This study was designed to test the applicability of EEH in impulse noise exposure in chinchillas. 9 groups of chinchillas were exposed to impulse noise. The intensity of the impulses (113, 125 and 137 dB SPL) and the rate (900, 3,600 and 14,400 impulses/hour) of the 9 exposure conditions were counterbalanced so that the 9 groups received the same total energy. PTS was obtained at 0.5, 2. and 8.0 kHz, by recording the evoked potentials from a chronic electrode implanted in the inferior colliculus. The cochleas were dissected and evaluated using conventional histological surface preparations and examined through scanning electron microscopy (SEM) employing a JEOL 35 microscopy. In our experimental conditions only the 113 dB SPL exposures were consistent with EEH (PTS less than 20 dB). The 125 dB SPL peak level is a critical value in that the validity of EEH also depends on exposure duration. As duration increases, so does PTS. At a 137 dB SPL peak level, EEH is not applicable. The amount of PTS is higher than 50 dB. It slightly increases when the impulse rate per hour becomes higher. In all cases examined PTS is less at 0.5 kHz than at 2.0 or 8 kHz. In these animals excellent informations concerning the sensory cell losses and cells damage was obtained with SEM.