Abstract
Above 8 kHz the sound pressure transformation along the human ear canal (a central issue in the development of high-frequency audiometry) depends very much on the details of the canal geometry. In addition to canal volume and length, which provide an adequate description at lower frequencies, one must account for the curvature of the ear canal and the variation of cross-sectional area along the canal length. In particular, calculation of the sound pressure transformation requires specification of cross-sectional areas perpendicular to a curved axis that follows the “center” of the canal. A photogrammetric technique has been developed that accurately determines the geometry of human ear canals and specifies it in this format. Stereoscopic photographs of cadaver ear canal castings are analyzed to define a large number (up to 1000) of coordinate points on the surface of each casting. A numerical procedure extracts the curved, center axis that passes through the points and calculates cross sections normal to this axis. The geometry of one ear canal has been sucessfully obtained using the technique, and measurements on several more are under way.
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