Tests of some common assumptions of ear-canal acoustics in cats

The accuracy of ear-canal admittance and reflectance as measures of the ear 's properties depends on the acoustic effects of the canal. Here, measureme nts of acoustic admittance at different canal locations in domestic cats ar e used to test three common assumptions. (I) Can a uniform-tube model of th e canal represent spatial variations in admittance? Data from cats support this assumption for frequencies below 3 kHz, where the admittance inferred at the tympanic membrane (TM) based on a uniform-tube model differs by less than 3 dB in magnitude and 0.07 periods in angle from the admittance measu red at the TM; for higher frequencies greater differences occur. (2) Do lar ge static air pressures in the canal make the middle ear rigid without affe cting the properties of the canal space? The measurements reported indicate that large negative static pressures reduce the low-frequency compliance o f the cat middle ear to about 10% of the compliance of the canal air volume . Static displacements of the: acoustic probe, TM, and canal walls with sta tic pressure may affect estimates of the canal volume and middle-ear compli ance by as much as 15% to 20%. (3) Is the acoustic-reflectance magnitude co nstant with position along the canal? Reflectance data from cat ear canals generally support this idea, except within a frequency region near 0.5 kHz for which there is evidence of energy loss. These results demonstrate that noninvasive measurements in the canal describe middle-ear acoustic properti es to within tolerances that depend on the effects of the canal. (C) 2000 A coustical Society of America. [S0001 -4966(00)00509-9].