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The variation of the sound pressure along the auditory canal was determined experimentally on a number of subjects, male and female, placed in a progressive sound field. This was accomplished by insertion of a small flexible probe microphone at various positions along the auditory canal. The subjects were placed in front of a loudspeaker in a room free from acoustic wall reflections. The free sound field at the subjects' location was essentially that of a plane progressive wave. The measurements were carried out over the significant range of audiofrequencies for various orientations in azimuth of the subjects with respect to the sound source. The sound pressure at the eardrum is found to be greater than the free-field pressure. The average ratio of these two quantities is a function of frequency, and reaches values of about 20 db in the vicinity of 3000 c.p.s. The human ear is thus an effective acoustic “amplifier.” The increase in sound pressure at the eardrum over the free-field pressure is caused by a combination effect of diffraction by the head and pinna and resonance in the auditory canal. The measurements of the sound pressures at several other positions along the auditory canal serve to separate these two phenomena to a certain extent and to furnish additional information about the pressure distribution. Most of the data were obtained with a group of male subjects, but measurements on a few women did not show any marked discrepancies.
Wiener et al. (Tue,) studied this question.