The area discontinuity between probe and ear canal as a source of power-reflectance measurement-location variability.

This study examined the effect of the area discontinuity between the measurement-probe sound source and ear canal on the plane-wave approximation of power reflectance. The area discontinuity was hypothesized to introduce measurement-location sensitivity to the power reflectance, especially above 5 kHz. Measurements were made in human and artificial ear canals (tubes coupled to an IEC711 ear simulator). In both cases, the power reflectance exhibited a high-frequency notch that decreased in frequency as the residual canal length increased. The area discontinuity between probe and canal was modeled as an inductance in series with the canal's acoustic impedance. To compensate for the effects of the discontinuity, the discontinuity's impedance was subtracted from the measured load impedance of the canal. In the artificial ears, compensation for the estimated area discontinuity removed the high-frequency notch and reduced the position dependence of the power reflectance. Subtracting the estimated discontinuity impedance from the load impedance in the human ears had a minimal effect on the power-reflectance measurement-location variability and magnitude of the high-frequency notch. The area-discontinuity between probe and ear canal is not supported as the primary source of measurement-variability in the plane-wave approximation of the power reflectance in human ears.