Otoacoustic emission and auditory brain-stem response as measures of physiological load imposed by impulsive noise on hearing system.

In the project, auditory brain-stem responses (ABR) and otoacoustic emission (OAE) were used to study the effects of continuous and impulsive noise on the hearing system. The purpose of this work was twofold: to assess the suitability of electrophysiological methods for the prediction of physiological load imposed on the hearing system by continuous and impulsive noise, and to evaluate the effective attenuation of hearing protector devices (HPD) with the use of electrophysiological methods. The ABR recordings were obtained for clicks and 4-kHz tone pips, and the otoacoustic emission measurements were made using the CEOAE and DPOAE methods. In data interpretation, it was assumed that equal levels of electrophysiological response correspond to equal effects of noise on the hearing system. The CEOAE and DPOAE measurements showed that noise exposure at safe stimuli levels was not sufficient to obtain an effective comparison of the effects of continuous and impulsive noise on the hearing system. The ABR method made it possible to determine the equivalent levels of both type of noise and evaluate the HPD attenuation. The ABR responses were obtained under simultaneous and forward masking protocols for bands of noise centered at frequencies from 250 Hz to 4 kHz, and for click trains comprising different number of impulses, with various impulse amplitudes, durations and repetition rates. Significant increase in wave V latency was observed in the ABR recordings for a change in the impulse repetition rate from 10 to 100 impulses per second, an increase in impulse duration from 0.1 to 5 ms, and an increase in masker duration from 201 to 501 ms. For band-pass continuous noise, wave V latency largely depended of the noise center frequency with minimal effect at lowest center frequencies. To compare the effects of continuous and impulsive noise, the so called isolatency curves were determined, defined as the masker level causing identical increase in latency for both kinds of noise. The effect of using an HPD on the wave V latency showed that such an electrophysiological measure is equivalent to 25-35-dB attenuation of the HPD, a higher value that that determined by standard methods.