A noninvasive method for estimating acoustic admittance at the tympanic membrane.

The acoustic admittance at the tympanic membrane (TM), Y(TM), describes the linear acoustic properties of the ear. Here, a noninvasive measurement procedure is developed for estimating Y(TM) in intact ears. The method consists of (1) measuring the admittance in the ear canal Y(EC) with a commercially available earphone-and-microphone system, and (2) estimating Y(TM) via a uniform-tube approximation of the space between the measurement point and the TM. The dimensions of this space are estimated from Y(EC) via an area-estimation algorithm [Keefe et al., J. Acoust. Soc. Am. 91, 470 (1992)] and measurements made with controlled static pressures in the canal. Measurements in artificial loads are used to test the accuracy of the measurement system and to determine sources of error. For accurate admittance measurements: (1) extension of the microphone tube medially beyond the earphone's port is necessary for frequencies above 2 kHz; (2) the acoustic system must be calibrated in known loads with diameters within 15% of the canal diameter, because the source's output characteristics vary with load diameter. The method is applied to intact ears of anesthetized domestic cats; for frequencies below 5 kHz, the estimated Y(TM) in four ears have features that are similar to those of previous measurements made at the cat TM. Sources of error include nonuniform waves generated at the earphone's narrow port, inaccuracy in estimation of canal dimensions, irregular geometry of the canal, and earphone-microphone cross talk.