Cochlear Microphonic Response to Low-Frequency Noise.

The results of studies of the transfer of acoustic information from outside the ear to the auditory nerve fibers are described. The target of the studies was to determine whether physiological correlates of a behaviorally-demonstrated permanent threshold shiift in speech frequency regions following exposure to high intensity, low frequency simulation could be found in cochlear potentials. The incident stimulus defined in the behavioral studies was 63.5 Ha octave band noise presented at 120 dB SPL in the free field. Our first measurements were to determine whether the external ear modified the acoustic waveform delivered to the tympanic membrane. The results showed that the pinna and external auditory canal altered the spectrum of a 5 kHz low-pass noise due to resonances above 2 kHz. Removal of the pinna and the auditory canal to leave but 1-2 mm of tissue around the bulla opening to the tympanic membrane eliminated the resonances, and a nearly flat spectrum was obtained. There was no evidence that the spectrum of the low-frequency noise contained a significant component at the frequencies corresponding to the threshold shift defined behaviorally. The acoustic spectra of the low-frequency noise measured near the tympanic membrane were compared with the spectra of the cochlear microphonic recorded from the inner ear.