Method to measure ear canal impedance and reflectance

The accuracy of the single microphone method of measuring impedance depends upon a calibration using a least‐mean‐squares fit of the Thevenin source characteristics to measured data from a set of closed, cylindrical tubes [Allen, in Peripheral Auditory Mechanisms, edited by Allen et al., pp. 44–51 (1985)]. Using a 2048‐point chirp stimulus, the Thevenin parameters are determined at each frequency using the lengths of the closed tubes. These lengths, which depend upon the insertion distance of the foam eartip suitable for use in human ear canals, are optimally chosen using Brent's method. Results on the calibration sensitivity using four versus six closed tubes, of varying lengths and diameters will be presented. The area distance functions of the calibration tubes and ear canal enable calculation of the energy reflectance from impedance data at arbitrary positions in the tubes [Keefe and Ling, J. Acoust. Soc. Am. Suppl. 1 85, S67 (1989)], but the area at the eartip is sufficient to calculate reflectance a...