Wideband acoustic immittance: Suitable quantities for assessing middle-ear status

This paper investigates the physical significance of ear-canal wave quantities—such as absorbance—that are commonly measured in wideband-acoustic-immittance tests and evaluates whether these quantities uniquely characterize middle-ear function. This is explored using simulations of lumped-element and transmission-line ear canals, along with measurements in three-dimensional-printed uniform and anatomical ear canals of varying geometry, terminated by the same middle-ear simulator. The results demonstrate the following points: (1) Wave quantities computed using estimated or geometrical cross-sectional areas do not uniquely characterize the middle ear because they describe the wave interaction between ear canal and middle ear and are therefore confounded by variations in ear-canal geometry. (2) At low frequencies, the reflection of sound waves in the ear canal plays a negligible role in the middle-ear transduction mechanism because of the long sound wavelength relative to ear-canal length, and wave quantities therefore do not provide a meaningful physical characterization of middle-ear function. (3) Wave quantities computed using an invariant cross-sectional area are likely most appropriate for clinical diagnostics, as they reduce the influence of ear-canal geometry up to approximately 3 kHz. However, this simplification means that wave quantities no longer characterize the reflection of sound waves in the ear canal at any frequency.