Contributions to high-frequency reflectance from the change in diameter between sound-delivery tube and ear canal

Abstract

Measurement of wideband reflectance requires stimuli to be delivered to the ear canal through a tube with diameter seven-times smaller than that of the average canal. This discontinuity may contribute to the measurement-location sensitivity of reflectance above 5 kHz [Lewis and Neely. (2015). J. Acoust. Soc. Am. 138, 977–993]. To test this hypothesis, reflectance was measured at different locations in a set of artificial ear canals (cylindrical tubes with diameters of 8, 9, and 10 mm) terminated by an IEC711 coupler, and compared to the theoretical reflectance. The measured reflectance exhibited a high-frequency notch that (1) decreased in frequency as distance between the probe and coupler increased and (2) increased in magnitude as canal diameter increased. The theoretical reflectance lacked the high-frequency notch of the measured reflectance; however, it was possible to simulate the measured data by adding an inductance in series with the canal model [Karal. (1953). J. Acoust. Soc. Am. 25, 327–334]. Transforming the impedance measured in the artificial canals to remove the estimated inductance decreased the notch depth and measurement-location sensitivity of the measured reflectance, especially for the larger diameter canals. Additionally, the measured reflectance more-closely approximated the theoretical reflectance. Findings have implications for in-situ sound-level calibrations.