Physical Characteristics of Normal and Abnormal Ears

Abstract

Sensitivity Curves for Normal and Abnormal Ears.---Previous investigators have worked only with normal ears and their results are not as a rule corrected for the resonance properties of the diaphragms used. Lord Rayleigh's curves are more reliable but do not extend above 768 double vibrations. In the present work a telephone receiver was used with a natural frequency above the range of the tests, 5,200 d.v., and its vibrational and electrical characteristics were determined so that the vibrational energy of the diaphragm could be calculated for any current of any frequency used. The currents were generated with an audion oscillating circuit, and a Wheatstone bridge was used in measuring the minimum audible current for both normal and abnormal ears; the readings in the case of normal ears were then converted into minimum vibrational energy and plotted as a function of the frequency up to 4,000 d.v. Curves are given (1) for normal ears; (2) for ears diseased only in the middle ear; (3) for ears diseased only in the internal ear; (4) for ears having a combination of middle and internal ear trouble. The normal ear curve shows a number of characteristic maxima with subsidiary inflections. In middle ear deafness the hearing is most deficient for low tones and more nearly normal for high ones, while inner ear deafness begins for high pitches and may not affect the hearing for low pitches. Sets of curves show the development of the diseases with time. In true nerve deafness cases have been studied in which total deafness appears to exist for a part of the sound spectrum; in one case on record, for instance, 200 billion times the energy required to stimulate the normal ear failed to produce the sensation of sound. Some conclusions regarding the mechanism of audition are drawn. The normal ear acts like a complex mechanical or electrical vibrating system whose response depends on many factors including the tympanum, bones, muscles, ear cavities and the internal ear. No resonant frequency can be assigned to any one member of it. When the middle ear mechanism is absent, the internal ear has resonant frequencies different from those of normal ears. The receiving mechanism in the internal ear is arranged in a linear manner beginning with the basal part which probably responds to the high pitched sounds, and each section is sensitive to only a narrow range of frequencies. The probable fact that the high frequency fibers are under greater tension than the lower frequency ones may account for the characteristics of internal ear deafness.Discussion of Theories of Audition.---The difficulties confronting each theory are briefly pointed out. In some ways the above results tend to confirm the Helmholtz theory, but it is hard to see how any mechanical theory can explain the absolute insensitiveness to a limited range of pitch which has been observed in cases of nerve deafness.