Methods for estimating the sound pressure at the eardrum

The problem of estimating the sound pressure generated at individual eardrums is systematically investigated. In audiometry, the reference to the pressure at the eardrum is usually realized by using a coupler such as the IEC 711 ear simulator, which is intended to approximate an average ear. The errors caused by individually shaped ear canals are calculated for a typical audiometric earphone (Beyer DT 48) in combination with the IEC 711 ear simulator and with an "ideal" coupler. These errors can reach 15 dB and are clearly more important than deviations of the ear simulator from an average ear. In order to obtain correct estimations, the chain matrices of individual ear canals have to be determined. Best estimates are obtained using the "reflectance phase method," but the "pressure minima method" also provides surprisingly good results, except in narrow frequency ranges. The reflectance phase method is checked using a physical model of the ear canal and the middle ear. The resulting errors of estimation remain within a limit of 3 dB up to more than 10 kHz.