A Quantitative Study of the Cochlear Response.

Abstract

Quantitative measurements of the cochlear response in animals have been part of a program concerned with investigations on hearing problems during the past year and one-half in these laboratories. The methods and apparatus used for the detection of the responses was similar to that described by Garceau and Davis. An accurate system for the measurement and generation of pure stimulation tones was essential. An audio-frequency oscillator generated electrical voltages for the production of these tones. The voltages were amplified, measured, attenuated to the desired level and finally transformed into sound by a Western Electric 555 speaker unit. The harmonic content of the sound wave as measured by means of a Brush crystal microphone, amplifier and General Radio Distortion Factor meter was never more than one per cent for tones below 4000 cycles per second. The total power of the harmonics was therefore 40 decibels below that of the fundamental in this range. A fabric reinforced rubber tube 6 feet in length conveyed the sound to the animal's ear. The stimulus was calibrated in terms of microwatts of acoustical energy. This was accomplished by measuring the power level to the speaker necessary to produce threshold stimuli for each ear of 6 humans. The mean value of these thresholds was considered identical to the “minimum audible pressure” threshold values as determined by Sivian and White. The electrical voltages generated in the cochlea of the animal's ear were amplified and recorded with an electrode placed on the round window and an indifferent electrode buried in the muscles of the neck. The low level amplifier consisted of 4 stages which were resistance capacity coupled.