Minimum Intensity for Audition

Minimum intensity of sound for audition for frequencies of 128 to 4,096 p.p.s.—(1) Thermophone as source. The receiver was held to the ear like a telephone receiver and was actuated by current from an oscillator, the threshold intensity being calculated from the corresponding current by Wente's formula. Tests of 14 ears for frequencies an octave apart show logarithmic sensitivities lying between the results of Wien and those of Fletcher and Wegel, the mean values increasing from 5 for 128 p.p.s. to a practicallv constant sensitivity of about 8.5 for the range 512 to 4,096 p.p.s. (2) Telephone receiver as source was then tried to see whether results nearer Wien's would be obtained. Two methods of calibration of diaphragm amplitude as a function of current were used, one involving the use of a microscope to magnify the motion of a quartz fiber attached to the diaphragm, the other amplifying the motion by means of a rocker which rotated a light mirror, both, however, involving extrapolation from relatively large amplitudes to those of the order of ${10}^{-9\mathrm{}}$ cm. The results for two listeners agreed in showing a somewhat lower sensitivity with the telephone than with the thermophone as source, the energy ratio increasing from about 2 for higher frequencies to 7 for 128 p.p.s. This difference may be due to the extrapolation used in reducing the readings. The results of the two methods show a much better agreement with each other than with the results of Wien.