Reflectance measurement validation using acoustic horns

Effect of curvature on sound propagation in the ear canal

In this paper, we investigate the effect of the curvature and torsion of the ear canal on its resonance through a comparison between several ear canal models. Utilizing Stinson's ear canal geometries as a reference, we build and analyze several ear canal models using both transmission matrix and numerical methods for the purpose of comparative assessment. A conical transmission unit, which considers visco-thermal effects, is employed for the modeling of the human ear canal. While the transfer matrix and numerical method agree well for a straight axis model, this simplification results in up to 20% deviation from a curved canal. We propose the curve twist ratio as a metric to quantify the influence of curvature on the ear canal and find that our proposed metric can effectively express the error introduced by the simplified straight axis model. Upon this metric, an empirical equation is proposed for incorporating the curvature effect in the transmission matrix method, enabling it to generate comparable results to those of the numerical method, which considers the effect of the curvature and torsion, thus dramatically accelerating computation.

A reference for ear-canal absorbance based on semi-anechoic waveguides

Wideband acoustic immittance (WAI), in particular, ear-canal absorbance, is a useful clinical tool for assessing the middle-ear status and diagnosing conductive hearing disorders. However, little evidence documents the measurement accuracy of WAI in human ears, and, because its clinical adoption is still in its infancy, no international standards exist to define appropriate requirements for commercial instrumentation. A challenge from a standardization point of view is the lack of an absorbance reference, i.e., an acoustic load similar to the adult ear canal with a known absorbance. This paper explores various approaches to providing such an acoustic load to quantify WAI measurement accuracy. The approaches considered here include standardized and inexpensive occluded-ear simulators, and a family of semi-anechoic waveguides with different step discontinuities in cross-sectional area. These semi-anechoic waveguides could be included in a future WAI standard. In addition, a means of monitoring the stability of WAI calibrations over time is proposed, utilizing a single inexpensive occluded-ear simulator.

A reciprocity method for validating acoustic ear-probe source calibrations

Measurements of wideband acoustic immittance (WAI) rely on the calibration of an ear probe to obtain its acoustic source parameters. The clinical use of WAI and instruments offering the functionality are steadily growing, however, no international standard exists to ensure a certain reliability of the hardware and methods underlying such measurements. This paper describes a reciprocity method that can evaluate the accuracy of and identify errors in ear-probe source calibrations. By placing the ear probes of two calibrated WAI instruments face-to-face at opposite ends of a short waveguide, the source parameters of each ear probe can be measured using the opposite calibrated ear probe. The calibrated and measured source parameters of each ear probe can then be compared directly, and the influence of possible calibration errors on WAI measurements may be approximated. In various exemplary ear-probe calibrations presented here, the reciprocity method accurately identifies errors that would otherwise remain undetected and result in measurement errors in real ears. The method is likely unsuitable for routine calibration of WAI instruments but may be considered for conformance testing as part of a potential future WAI standard.

Sound field estimation near the tympanic membrane using area-distance measurements in the ear canal

The sound field near the tympanic membrane (TM) is estimated based on acoustic data measured at the tip of a probe inserted into the ear canal, from which the area-distance function of the ear canal is calculated. Such information has the potential to quantify the sound input to the middle ear at high frequencies. Spatial variation in the ear-canal cross-sectional area is described acoustically by quantifying forward and reverse sound waves between the probe tip and a near-TM location. A causal acoustic reflection function (RF) measured in the time domain at the probe tip is used to calculate area-distance functions of the ear canal. Area-distance functions are compared with plane-wave methods based on layer peeling and Ware-Aki algorithms. A time-domain model of viscothermal wall loss is devised and applied to ear-canal data, with area-distance functions compared between loss-less and lossy methods. This model is applicable to time-domain RF calibrations using measured data in short tubes. Specification of the near-TM sound field may benefit the interpretation of data from physiological tests such as otoacoustic emission and auditory brainstem responses and high-frequency behavioral tests such as extended audiometry and tests of spatial processing of sound.

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

This work is part of a study of the interactions of ear canal (EC) sound with tympanic membrane (TM) surface displacements. In human temporal bones, the ossicles were stimulated mechanically "in reverse" to mimic otoacoustic emissions (OAEs), and the sound field within the ear canal was sampled with 0.5-2 mm spacing near the TM surface and at more distal locations within the EC, including along the longitudinal EC axis. Sound fields were measured with the EC open or occluded. The reverse-driven sound field near the TM had larger and more irregular spatial variations below 10 kHz than with forward sound stimulation, consistent with a significant contribution of nonuniform sound modes. These variations generally did not propagate more than ∼4 mm laterally from the TM. Longitudinal sound field variations with the EC open or blocked were consistent with standing-wave patterns in tubes with open or closed ends. Relative contributions of the nonuniform components to the total sound pressure near the TM were largest at EC natural frequencies where the longitudinal component was small. Transverse variations in EC sound pressure can be reduced by reducing longitudinal EC sound pressure variations, e.g., via reducing reflections from occluding earplugs.

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.