The Frequency—Sensitivity of Normal Ears

Minimum Audible Pressure Variation for Tones of 60 to 4,000 Cycles.—The rather discordant results obtained by other investigators are briefly reviewed and are summarized in Fig. 1. In the present research an attempt was made to get results with a definite dynamical significance. A special air-damped telephone receiver held tightly to the ear, was excited by an alternating current of variable frequency from a vacuum tube oscillator provided with special low pass filters to eliminate upper harmonics, and the current strength was changed logarithmically by means of a special attenuator until the threshold was reached. The observations were made in a special sound proof room whose construction is described. The probable error is about 25 per cent. To reduce the results to absolute units, the system was calibrated in two ways, both involving the substitution of a condenser transmitter for the ear, the source of sound being a telephone receiver in one case and in the other a small thermal receiver inserted in the ear meatus or in a similar cavity in front of the condenser transmitter. Mechanical analogues of the vibrating systems involved in these measurements are described in an Appendix to help make the dynamics clear. While the unknown mechanical constants of the inner ear introduce some uncertainty, the agreement of the two calibrations indicates that the error is not large. Frequency-sensitivity curves were obtained for approximately 100 normal and 20 abnormal ears. So-called normal ears were found to vary widely in relative frequency sensitivity and in absolute sensitivity, and some audiograms show interesting individual peculiarities. But on the average, the minimum audible pressure variation increases regularly from about 0.15 dyne/${\mathrm{cm}.\mathrm{}}^2$ at 60 cycles to 0.001 dyne/${\mathrm{cm}.\mathrm{}}^2$ at 1,000 cycles and is then approximately constant up to at least 4,000 cycles.