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 proc edure is developed fur estimating Y-TM in intact ears. The method consists of (1) measuring the admittance in the ear canal Y-EC with a commercially a vailable earphone-and-microphone system, and (2) estimating Y-TM via unifor m-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-estimatio n algorithm [Keefe et al., J. Acoust. Sec. Am. 91, 470 (1992)] and measurem ents made with controlled static pressures in the canal. Measurements in ar tificial loads are used to test the accuracy of the measurement system and to determine sources of error. For accurate admittance measurements: (1) ex tension of the microphone tube medially beyond the earphone's port is neces sary for frequencies above 2 kHz: (2) the acoustic system must be calibrate d in known loads with diameters within 15% of the canal diameter, because t he source's output characteristics vary with load diameter. The method is a pplied 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 th ose of previous measurements made at the cat TM. Sources of error include n onuniform waves generated at the earphone's narrow port, inaccuracy in esti mation of canal dimensions, irregular geometry of the canal, and earphone-m icrophone cross talk. (C) 2000 Acoustical Society of America. [S0001-4466(0 0)00409-4].