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

Kren Rahbek Nørgaard, Efren Fernandez Grande

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The ear-canal acoustic impedance and reflectance are useful for assessing conductive hearing disorders
and calibrating stimulus levels in situ. However, such probe-based measurements are affected
by errors due to the presence of evanescent modes and incorrect estimates or assumptions regarding
characteristic impedance. This paper proposes a method to compensate for evanescent modes in
measurements of acoustic impedance, reflectance, and sound pressure in waveguides, as well as
estimating the characteristic impedance immediately in front of the probe. This is achieved by
adjusting the characteristic impedance and subtracting an acoustic inertance from the measured
impedance such that the non-causality in the reflectance is minimized in the frequency domain
using the Hilbert transform. The method is thus capable of estimating plane-wave quantities of the
sought-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 estimate
these quantities in a waveguide that is uniform at the position of the probe. Finally, it is demonstrated
how evanescent modes, characteristic impedance, and the proposed methodology can
affect the measured acoustic impedance and reflectance of an occluded-ear simulator.
Original languageEnglish
JournalJournal of the Acoustical Society of America
Issue number6
Pages (from-to)3497–3509
Publication statusPublished - 2017


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