Determination of eardrum reflectance using measurement of phase

Abstract

At frequencies greater than 8 kHz the acoustic energy reflectance at the eardrum is not well established in living human subjects. This quantity is of interest, though, in high‐frequency audiometry and in network modeling of the middle ear. In this paper a technique for measuring high‐frequency reflectances is discussed that makes use of the phase component of a pressure signal, rather than its amplitude. In a section of ear canal having a constant cross section, or having a constant taper, the maximum rate change of phase with position is simply related to the energy reflectance at the eardrum. Experimentally the phase of the pressure signal is determined at several positions over a 2‐mm distance, centered about an interference minimum. By choosing appropriate sound frequencies the measurements can be confined to a 6‐ or 7‐mm region near the ear canal entrance; this avoids the deep penetration of probe microphones required by other techniques, and avoids the large variations in ear canal cross section that occur close to the eardrum. Preliminary results have been obtained, suggesting an eardrum energy reflectance of about 80% (25‐dB equivalent standing wave ratio) at 15 kHz.