CAUSALITY CONSTRAINTS ON EAR-CANAL TESTS OF ACOUSTIC REFLECTION FUNCTION AND REFLECTANCE

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PROJECT SUMMARY/ABSTRACT This project develops and evaluates procedures to estimate the ear-canal sound field near the tympanic membrane (TM). The underlying framework of these procedures is to model the ear canal as an acoustic waveguide. Spatial variation in the cross-sectional area of the ear canal is described acoustically by quantifying forward and reverse sound waves traveling between a probe inserted into the ear canal and a location near the TM. Describing the sound field near the TM in terms of sound pressure and its reflected and transmitted components more accurately quantifies the acoustical high frequency (HF) input to the middle ear. Such a description improves HF calibration of the stimulus near the TM for physiological tests such as otoacoustic emission and auditory brainstem responses, and HF behavioral tests such as extended audiometry and tests of spatial processing of sound. The specification also improves the ability to describe forward and reverse otoacoustic emission responses at HFs, which non-invasively encode information on outer-hair cell function within the cochlea. The project outcomes will benefit acoustic diagnostic tests of middle-ear and cochlear function. The goals of Aim 1 are to: (i) implement and evaluate a causal, time-domain measurement at the probe tip of the acoustic reflection function (RF) of the ear, such that reflectance is the Fourier transform of the RF, (ii) use the RF to calculate area-distance functions of the ear canal between the probe tip and TM, and (iii) use RF and area data to estimate the sound field near the TM (i.e., to within 3.6 mm). Current frequency- domain tests to measure reflectance are not constrained to obey causality, which limits their accuracy in clinical applications, especially at frequencies above 8 kHz. Causality requires that sound cannot be reflected prior to its incidence. The RF of the ear is a key response for subsequent area-distance calculations within the ear canal. RF data will be acquired in Aim 1 experiments in tubing systems with varying areas, molds of ear canals and an artificial ear simulator (IEC711 coupler). Present approaches to describe the area-distance function in the ear canal largely use the Webster, or plane-wave, horn equation, which is of insufficient accuracy for the area variations in the ear canal. This project will measure area-distance functions using a novel spherical-wave horn model that controls for changes in the taper of the ear canal. These area-distance algorithms include a novel time-domain description of losses at the ear-canal walls. The goals of Aim 2 are to: (i) acquire RF data in groups of 5-year-old children and adults with normal hearing, (ii) calculate area-distance functions in their canal, (iii) use a digital scanner to measure the ear-canal geometry, (iv) compare acoustic and scanned measurements of the area-distance function, (v) estimate the sound field pressure, and transmission and reflection of sound, near the TM, and (vi) test for maturational differences in ear-canal and middle-ear function in children relative to adults. The approach makes maximal use of the acoustic data provided in a probe measurement within the ear canal.