The Diffraction of a Progressive Sound Wave by the Human Head
Francis M. Wiener · The Journal of the Acoustical Society of America · 1947
The increase in sound pressure at the eardrum over the free-field pressure in a progressive sound field is caused by a combination effect of diffraction by the head and pinna and resonance in the auditory canal. In an earlier paper the pressure distribution in the auditory canal was determined on a number of subjects in a plane progressive sound field. Expressed in terms of the ratio of the sound pressure at the eardrum to that at the entrance of the ear canal, it was found to be essentially independent of the orientation of the observer with respect to the sound source, as might be expected from theory. This fact makes it possible to separate the two effects and to measure the diffraction by the head by inserting a probe microphone near the entrance of the ear canal and comparing the sound pressure measured there with the free-field pressure. To the obstacle effect of head and pinna, so determined, the pressure increase due to resonance in the auditory canal is to be added. The subjects were placed in front of a loudspeaker in a room free from acoustic wall reflections. The measurements were carried out in an essentially plane progressive wave over the frequency range extending from 200–6000 c.p.s. and covered a range of azimuths of 360 degrees. From these data the average ratio of the sound pressures at the two eardrums for six observers was determined as a function of frequency and azimuth. This quantity, important in the mechanism of binaural localization, was found to be in fair agreement with the values obtained by Sivian and White from threshold measurements.