A comparison of methods for measuring the acoustic input impedance of ear canals for hearing aid applications

In hearing aid fitting the sound pressure at the ear drum is a reference quantity, since all real ear characteristic values refer to it. Typically, the sound pressure at the ear drum is estimated by a model of an average ear canal (e.g., a coupler). Such a model cannot account for inter-individual differences. Alternatively, there are methods to predict the acoustics of the individual ear canal. Some of these methods make use of the acoustic input impedance of the ear canal. In general, the accuracy of the measured impedance depends on the effort that will be made. Therefore, different methods of impedance measurements were investigated concerning accuracy and effort. The methods differ in the number of calibration measurements (and calibration parameters). They were compared on the basis of impedance measurements on different model ear canals. Measurements were done with an impedance probe consisting of a typical hearing aid receiver and a hearing aid microphone. The measurements were compared to measurements with a reference impedance probe and method. As a result, it was observed that with a single calibration measurement the maximum absolute error of the transfer impedance was smaller than 3 dB up to 8 kHz.