Reflectance measurement validation using acoustic horns

Abstract

Variability in wideband acoustic reflectance (and absorbance) measurements adversely affects the clinical utility of reflectance for diagnosis of middle-ear disorders. A reflectance standard would encourage consistency across different measurement systems and help identify calibration related issues. Theoretical equations exist for the reflectance of finite-length exponential, conical, and parabolic acoustic horns. Reflectance measurements were repeatedly made in each of these three horn shapes and the results were compared to the corresponding theoretical reflectance. A method is described of adjusting acoustic impedance measurements to compensate for spreading of the wave front that propagates from the small diameter sound port of the probe to the larger diameter of the acoustic cavity. Agreement between measured and theoretical reflectance was less than 1 dB at most frequencies in the range from 0.2 to 10 kHz. Pearson correlation coefficients were greater than 0.95 between measured and theoretical time-domain reflectance within the flare region of the horns. The agreement suggests that the distributed reflectance of acoustic horns may be useful for validating reflectance measurements made in human ear canals; however, refinements to reflectance measurement methods may still be needed.

Sound pressure distribution within human ear canals: II. Reverse mechanical stimulation

“…Several authors have developed schemes to detect or predict and correct for longitudinal variations, including inferring the EC cross-section area function from reflectance measurements and developing a mathematical model of individual ECs (Rasetshwane et al, 2015;Scheperle et al, 2011). Simple tube or more complicated models have been used to extract the forward (stimulus) pressure wave from the sound field (Souza et al, 2014), and similar techniques have been used to extract the backward-traveling OAE wave from the EC sound pressure (Charaziak and Shera, 2017).…”

“…Of course, transverse nonuniformities in sound pressure can also come from longitudinal variations in the EC crosssection area (Rabbitt and Friedrich, 1991;Farmer-Fedor and Rabbitt, 2002). These nonuniformities generated within the ear canal will also propagate small finite distances, but the locations where they are maximal will depend strongly on individual anatomy (e.g., Rasetshwane et al, 2015), and it will be harder to define a location where OAEs will be less affected by ear-canal related non-uniformities in transverse sound pressure.…”