Compensating for evanescent modes and estimating characteristic impedance in waveguide acoustic impedance measurements

摘要

The ear-canal acoustic impedance and reflectance are useful for assessing conductive hearing disordersand calibrating stimulus levels in situ. However, such probe-based measurements are affectedby errors due to the presence of evanescent modes and incorrect estimates or assumptions regardingcharacteristic impedance. This paper proposes a method to compensate for evanescent modes inmeasurements of acoustic impedance, reflectance, and sound pressure in waveguides, as well asestimating the characteristic impedance immediately in front of the probe. This is achieved byadjusting the characteristic impedance and subtracting an acoustic inertance from the measuredimpedance such that the non-causality in the reflectance is minimized in the frequency domainusing the Hilbert transform. The method is thus capable of estimating plane-wave quantities of thesought-for parameters by supplying only an arbitrary initial value for the characteristic impedance.From a comparison with a simulated waveguide, it is shown that this method can accurately estimatethese quantities in a waveguide that is uniform at the position of the probe. Finally, it is demonstratedhow evanescent modes, characteristic impedance, and the proposed methodology canaffect the measured acoustic impedance and reflectance of an occluded-ear simulator.