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

Preliminary results of classifying otosclerosis and disarticulation using a convolutional neural network trained with simulated wideband acoustic immittance data

Current noninvasive methods of clinical practice often do not identify the causes of conductive hearing loss due to pathologic changes in the middle ear with sufficient certainty. Wideband acoustic immittance (WAI) measurement is noninvasive, inexpensive and objective. It is very sensitive to pathologic changes in the middle ear and therefore promising for diagnosis. However, evaluation of the data is difficult because of large interindividual variations. Machine learning methods like Convolutional neural networks (CNN) which might be able to deal with this overlaying pattern require a large amount of labeled measurement data for training and validation. This is difficult to provide given the low prevalence of many middle-ear pathologies. Therefore, this study proposes an approach in which the WAI training data of the CNN are simulated with a finite-element ear model and the Monte-Carlo method. With this approach, virtual populations of normal, otosclerotic, and disarticulated ears were generated, consistent with the averaged data of measured populations and well representing the qualitative characteristics of individuals. The CNN trained with the virtual data achieved for otosclerosis an AUC of 91.1 %, a sensitivity of 85.7 %, and a specificity of 85.2 %. For disarticulation, an AUC of 99.5 %, sensitivity of 100 %, and specificity of 93.1 % was achieved. Furthermore, it was estimated that specificity could potentially be increased to about 99 % in both pathological cases if stapes reflex threshold measurements were used to confirm the diagnosis. Thus, the procedures' performance is comparable to classifiers from other studies trained with real measurement data, and therefore the procedure offers great potential for the diagnosis of rare pathologies or early-stages pathologies. The clinical potential of these preliminary results remains to be evaluated on more measurement data and additional pathologies.

Domain-adaptation method between acoustic-response data using different insert earphones

Classifying acoustic responses captured through earphones offers valuable insights into nearby environments, such as whether the earphones are in or out of the ear. However, the performances of classification algorithms often suffer when applied to other devices due to domain mismatches. This study proposes a domain-adaptation method tailored for acoustic-response data from two distinct insert earphone models. The method trains a domain-adaptation function using a pair of datasets obtained from a set of acoustic loads, yielding a domain-adapted dataset suitable for training classification algorithms in a target domain. The effectiveness of this approach is validated through assessments of domain adaptation quality and resulting performance enhancements in the classification algorithm tasked with discerning whether an earphone is positioned inside or outside the ear. Importantly, our method requires significantly fewer measurements than the original dataset, reducing data collection time while providing a suitable training dataset for the target domain. Additionally, the method's reusability across future devices streamlines data collection time and efforts for the future devices.

The influence of tympanic-membrane orientation on acoustic ear-canal quantities: A finite-element analysis

Assuming plane waves, ear-canal acoustic quantities, collectively known as wideband acoustic immittance (WAI), are frequently used in research and in the clinic to assess the conductive status of the middle ear. Secondary applications include compensating for the ear-canal acoustics when delivering stimuli to the ear and measuring otoacoustic emissions. However, the ear canal is inherently non-uniform and terminated at an oblique angle by the conical-shaped tympanic membrane (TM), thus potentially confounding the ability of WAI quantities in characterizing the middle-ear status. This paper studies the isolated possible confounding effects of TM orientation and shape on characterizing the middle ear using WAI in human ears. That is, the non-uniform geometry of the ear canal is not considered except for that resulting from the TM orientation and shape. This is achieved using finite-element models of uniform ear canals terminated by both lumped-element and finite-element middle-ear models. In addition, the effects on stimulation and reverse-transmission quantities are investigated, including the physical significance of quantities seeking to approximate the sound pressure at the TM. The results show a relatively small effect of the TM orientation on WAI quantities, except for a distinct delay above 10 kHz, further affecting some stimulation and reverse-transmission quantities.

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.

Measurements of ear-canal geometry from high-resolution CT scans of human adult ears

Description of the ear canal's geometry is essential for describing peripheral sound flow, yet physical measurements of the canal's geometry are lacking and recent measurements suggest that older-adult-canal areas are systematically larger than previously assumed. Methods to measure ear-canal geometry from multi-planar reconstructions of high-resolution CT images were developed and applied to 66 ears from 47 subjects, ages 18-90 years. The canal's termination, central axis, entrance, and first bend were identified based on objective definitions, and the canal's cross-sectional area was measured along its canal's central axis in 1-2 mm increments. In general, left and right ears from a given subject were far more similar than measurements across subjects, where areas varied by factors of 2-3 at many locations. The canal areas varied systematically with age cohort at the first-bend location, where canal-based measurement probes likely sit; young adults (18-30 years) had an average area of 44mm2 whereas older adults (61-90 years) had a significantly larger average area of 69mm2. Across all subjects ages 18-90, measured means ± standard deviations included: canals termination area at the tympanic annulus 56±8mm2; area at the canal's first bend 53±18mm2; area at the canal's entrance 97±24mm2; and canal length 31.4±3.1mm2.

Comments on forward pressure and other reflectance-based quantities for delivering stimuli to the ear

The forward pressure has been proposed as an "optimal" reflectance-based quantity for delivering stimuli to the ear during evoked otoacoustic-emission measurements and audiometry. It is motivated by and avoids detrimental stimulus-level errors near standing-wave antiresonance frequencies when levels are adjusted in situ. While enjoying widespread popularity within research, the forward pressure possesses certain undesirable properties, some of which complicate its implementation into commercial otoacoustic-emission instruments conforming to existing international standards. These properties include its inability to approximate the total sound pressure anywhere in the ear canal and its discrepancy from the sound pressure at the tympanic membrane, which depends directly on the reflectance. This paper summarizes and comments on such properties of the forward pressure. Further, based on previous published data, alternative reflectance-based quantities that do not share these properties are investigated. A complex integrated pressure, with magnitude identical to the previously proposed scalar integrated pressure, is suggested as a suitable quantity for avoiding standing-wave errors when delivering stimuli to the ear. This complex integrated pressure approximates the magnitude and phase of the sound pressure at the tympanic membrane and can immediately be implemented into standardized commercial instruments to take advantage of improved stimulus-level accuracy and reproducibility in the clinic.

Three-dimensional wideband absorbance immittance findings in young adults with large vestibular aqueduct syndrome

Objective

To investigate the effect of large vestibular aqueduct syndrome (LVAS) on middle ear sound transmission using wideband absorbance immittance (WAI).

Methods

WAI results from young adult LVAS patients and normal adults were compared.

Results

Averaged energy absorbance (EA) at ambient and peak pressure in the LVAS group showed differences to the normal group. Under ambient pressure, the average EA of the LVAS group was significantly higher than the normal group at frequencies 472-866 Hz and 6169-8000 Hz (p < .05) and lower at frequencies 1122-2520 Hz (p < .05). Under peak pressure, absorbance was increased at frequencies 515-728, 841, and 6169-8000 Hz (p < .05) and decreased at 1122-1374 Hz and 1587-2448 Hz (p < .05). An investigation into the effect of external auditory canal pressure on EA across frequencies in the pressure-frequency domain, showed that EA differed significantly in the low-frequency region of 707and 1000 Hz from 0 to 200 daPa and 500 Hz at 50 daPa (p < .05). There was also a significant difference in EA between the two groups at 8000 Hz (p < .05) in the pressure range -200-300 daPa.

Conclusion

WAI is a valuable tool to measure the effect of LVAS on middle ear sound transmission. LVAS has a significant effect on EA at low and mid frequencies under ambient pressure, while the frequencies affected are mainly at low frequencies when positive pressure is presented.

Level of Evidence

Level 3a.

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.

A systematic study on effects of calibration-waveguide geometry and least-squares formulation on ear-probe source calibrations

Measuring ear-canal absorbance and compensating for effects of the ear-canal acoustics on otoacoustic-emission measurements using an ear probe rely on accurately determining its acoustic source parameters. Using pressure measurements made in several rigid waveguides and models of their input impedances, a conventional calibration method estimates the ear-probe Thévenin-equivalent source parameters via a least-squares fit to an over-determined system of equations. Such a calibration procedure involves critical considerations on the geometry and number of utilized calibration waveguides. This paper studies the effects of calibration-waveguide geometry on achieving accurate ear-probe calibrations and measurements by systematically varying the lengths, length ratios, radii, and number of waveguides. For calibration-waveguide lengths in the range of 10-60 mm, accurate calibrations were generally obtained with absorbance measurement errors of approximately 0.02. Longer waveguides resulted in calibration errors, mainly due to coincident resonance frequencies among waveguides in the presence of mismatches between their assumed and actual geometries. The accuracy of calibrations was independent of the calibration-waveguide radius, except for an increased sensitivity of wider waveguides to noise. Finally, it is demonstrated how reformulating the over-determined system of equations to return the least-squares reflectance source parameters substantially reduces calibration and measurement errors.

On causality and aural impulse responses synthesized using the inverse discrete Fourier transform

Causality is a fundamental property of physical systems and dictates that a time impulse response characterizing any causal system must be one-sided. However, when synthesized using the inverse discrete Fourier transform (IDFT) of a corresponding band-limited numerical frequency transfer function, several papers have reported two-sided IDFT impulse responses of ear-canal reflectance and ear-probe source parameters. Judging from the literature on ear-canal reflectance, the significance and source of these seemingly non-physical negative-time components appear largely unclear. This paper summarizes and clarifies different sources of negative-time components through ideal and practical examples and illustrates the implications of constraining aural IDFT impulse responses to be one-sided. Two-sided IDFT impulse responses, derived from frequency-domain measurements of physical systems, normally occur due to the two-sided properties of the discrete Fourier transform. Still, reflectance IDFT impulse responses may serve a number of practical and diagnostic purposes.

Improved multimodal formulation of the wave propagation in a 3D waveguide with varying cross-section and curvature

An efficient method is proposed to solve the multimodal wave propagation within a three-dimensional waveguide bounded by a hard wall with varying cross section and curvature. This is achieved by first turning the original problem, in a complex-shaped waveguide, into a cylindrical waveguide with unit radius, by means of an adapted and flexible geometrical transformation. Then supplementary modes are defined to enrich the standard modal basis that is usually considered in such methods and to help restore the right boundary condition. It is shown through various numerical applications that the introduction of these supplementary modes, whatever the complexity of the waveguide geometry, significantly enhances the multimodal method, notably by increasing its convergence rate, whether one's aim is to solve the wavefield or the scattering problem.

Measurements of ear-canal cross-sectional areas from live human ears with implications for wideband acoustic immittance measurements

Wideband acoustic immittance (WAI) measures are noninvasive diagnostic measurements that require an estimate of the ear canal's area at the measurement location. Yet, physical measurements of the area at WAI probe locations are lacking. Methods to measure ear-canal areas from silicone molds were developed and applied to 169 subjects, ages 18-75 years. The average areas at the canal's first bend and at 12 mm insertion depth, which are likely WAI probe locations, were 63.4 ± 13.5 and 61.6 ± 13.5 mm², respectively. These areas are substantially larger than those assumed by current FDA-approved WAI measurement devices as well as areas estimated with acoustical methods or measured on cadaver ears. Left and right ears from the same subject had similar areas. Sex, height, and weight were not significant factors in predicting area. Age cohort was a significant predictor of area, with area increasing with decade of life. A subset of areas from the youngest female subjects did not show an effect of race on area (White or Chinese). Areas were also measured as a function of insertion depth of 4.8-13.2 mm from the canal entrance; area was largest closest to the canal entrance and systematically decreased with insertion depth.

Reproducing ear-canal reflectance using two measurement techniques in adult ears

Clinical diagnostic applications of ear-canal reflectance have been researched extensively in the literature, however, the measurement uncertainty associated with the conventional measurement technique using an insert ear probe is unknown in human ear canals. Ear-canal reflectance measured using an ear probe is affected by multiple sources of error, including incorrect estimates of the ear-canal cross-sectional area and oblique ear-probe insertions. In this paper, ear-canal reflectance measurements are reproduced in an occluded-ear simulator and in 54 adult ear canals using two different measurement techniques: a conventional ear probe and a two-microphone probe that enables the separation of reverse- and forward-propagating plane waves. The two-microphone probe is inserted directly into test subjects' ear canals, and the two-microphone method is distinguished by not requiring the ear-canal cross-sectional area to calculate the ear-canal reflectance. The results show a reasonable agreement between the two measurement techniques. The paper further examines the influence of oblique ear-probe insertions and the compensation for such oblique insertions, which results in an improved agreement between the two measurement techniques.

Cochlear Reflectance and Otoacoustic Emission Predictions of Hearing Loss

OBJECTIVES

Cochlear reflectance (CR) is the cochlear contribution to ear-canal reflectance. CR is a type of otoacoustic emission (OAE) that is calculated as a transfer function between forward pressure and reflected pressure. The purpose of this study was to compare wideband CR to distortion-product (DP) OAEs in two ways: (1) in a clinical-screening paradigm where the task is to determine whether an ear is normal or has hearing loss and (2) in the prediction of audiometric thresholds. The goal of the study was to assess the clinical utility of CR.

DESIGN

Data were collected from 32 normal-hearing and 124 hearing-impaired participants. A wideband noise stimulus presented at 3 stimulus levels (30, 40, 50 dB sound pressure level) was used to elicit the CR. DPOAEs were elicited using primary tones spanning a wide frequency range (1 to 16 kHz). Predictions of auditory status (i.e., hearing-threshold category) and predictions of audiometric threshold were based on regression analysis. Test performance (identification of normal versus impaired hearing) was evaluated using clinical decision theory.

RESULTS

When regressions were based only on physiological measurements near the audiometric frequency, the accuracy of CR predictions of auditory status and audiometric threshold was less than reported in previous studies using DPOAE measurements. CR predictions were improved when regressions were based on measurements obtained at many frequencies. CR predictions were further improved when regressions were performed on males and females separately.

CONCLUSIONS

Compared with CR measurements, DPOAE measurements have the advantages in a screening paradigm of better test performance and shorter test time. The full potential of CR measurements to predict audiometric thresholds may require further improvements in signal-processing methods to increase its signal to noise ratio. CR measurements have theoretical significance in revealing the number of cycles of delay at each frequency that is most sensitive to hearing loss.

Causality-constrained measurements of aural acoustic reflectance and reflection functions

Causality-constrained procedures are described to measure acoustic pressure reflectance and reflection function (RF) in the ear canal or unknown waveguide, in which reflectance is the Fourier transform of the RF. Reflectance calibration is reformulated to generate causal outputs, with results described for a calibration based on a reflectance waveguide equation to calculate incident pressure and source reflectance in the frequency domain or source RF in the time domain. The viscothermal model RF of each tube is band-limited to the stimulus bandwidth. Results are described in which incident pressure is either known from long-tube measurements or calculated as a calibration output. Calibrations based on constrained nonlinear optimizations are simpler and more accurate when incident pressure is known. Outputs measured by causality-constrained procedures differ at higher frequencies from those using standard procedures with non-causal outputs. Evanescent-mode effects formulated in the time domain and incorporated into frequency-domain calibrations are negligible for long-tube calibrations. Causal reflectance and RFs are evaluated in an adult ear canal and time- and frequency-domain results are contrasted using forward and inverse Fourier transforms. These results contribute to the long-term goals of improving applications to calibrate sound stimuli in the ear canal at high frequencies and diagnose conductive hearing impairments.

On the calculation of reflectance in non-uniform ear canals

Ear-canal reflectance is useful for quantifying the conductive status of the middle ear because it can be measured non-invasively at a distance from the tympanic membrane. Deriving the ear-canal reflectance requires decomposing the total acoustic pressure into its forward- and reverse-propagating components. This decomposition is conveniently achieved using formulas that involve the input and characteristic impedances of the ear canal. The characteristic impedance is defined as the ratio of sound pressure to volume flow of a propagating wave and, for uniform waveguides, the plane-wave characteristic impedance is a real-valued constant. However, in non-uniform waveguides, the characteristic impedances are complex-valued quantities, depend on the direction of propagation, and more accurately characterize a propagating wave in a non-uniform ear canal. In this paper, relevant properties of the plane-wave and spherical-wave characteristic impedances are reviewed. In addition, the utility of the plane-wave and spherical-wave reflectances in representing the reflection occurring due to the middle ear, calibrating stimulus levels, and characterizing the emitted pressure in simulated non-uniform ear canals is investigated and compared.

A comparison of ear-canal-reflectance measurement methods in an ear simulator

Ear-canal reflectance has been researched extensively for diagnosing conductive hearing disorders and compensating for the ear-canal acoustics in non-invasive measurements of the auditory system. Little emphasis, however, has been placed on assessing measurement accuracy and variability. In this paper, a number of ear-canal-reflectance measurement methods reported in the literature are utilized and compared. Measurement variation seems to arise chiefly from three factors: the residual ear-canal length, the ear-probe insertion angle, and the measurement frequency bandwidth. Calculation of the ear-canal reflectance from the measured ear-canal impedance requires estimating the ear-canal characteristic impedance in situ. The variability in ear-canal estimated characteristic impedance and reflectance due to these principal factors is assessed in an idealized controlled setup using a uniform occluded-ear simulator. In addition, the influence of this measurement variability on reflectance-based methods for calibrating stimulus levels is evaluated and, by operating the condenser microphone of the occluded-ear simulator as an electro-static speaker, the variability in estimating the emitted pressure from the ear is determined. The various measurement methods differ widely in their robustness to variations in the three principal factors influencing the accuracy and variability of ear-canal reflectance.

Compensating for oblique ear-probe insertions in ear-canal reflectance measurements

Measurements of the ear-canal reflectance using an ear probe require estimating the characteristic impedance of the ear canal in situ. However, an oblique insertion of the ear probe into a uniform waveguide prevents accurately estimating its characteristic impedance using existing time-domain methods. This is caused by the non-uniformity immediately in front of the ear probe when inserted at an oblique angle, resembling a short horn loading, and introduces errors into the ear-canal reflectance. This paper gives an overview of the influence of oblique ear-probe insertions and shows how they can be detected and quantified by estimating the characteristic impedance using multiple truncation frequencies, i.e., limiting the utilized frequency range. Additionally, a method is proposed to compensate for the effects on reflectance of an oblique ear-probe insertion into a uniform waveguide. The incident impedance of the horn loading is estimated, i.e., were the uniform waveguide anechoic, which replaces the characteristic impedance when calculating reflectance. The method can compensate for an oblique ear-probe insertion into a uniform occluded-ear simulator and decrease the dependency of reflectance on insertion depth in an ear canal. However, more research is required to further assess the method in ear canals.

Evanescent waves in simulated ear canals: Experimental demonstration and method for compensation

Evanescent waves emerge from a small sound source that radiates into a waveguide with a larger cross-sectional area, but unlike planar waves, do not propagate far from the source. Evanescent waves thus contaminate in-ear calibration of acoustic stimuli. Measurements with an otoacoustic-emission (OAE) probe inserted at the entrance of long tubes of various diameters show a decline in the evanescent wave with distance from the source when advancing a probe tube through the OAE probe and into the long tube. The amplitude of the evanescent pressure increases with frequency and depends strongly on the diameter of the long tube. Modifying the shape of the aperture of the probe's sound source, thus effectively enlarging its diameter and redirecting acoustic flow, greatly reduced evanescent waves. The reduction in evanescent-wave pressure was observed in calibration cavities used to determine the Thévenin-equivalent source pressure and impedance of the probe. Errors in source calibrations were considerably larger in the unmodified configuration. An alternative method is proposed for calculation of acoustic source parameters that models the evanescent-wave pressure and reduces its influence on the calculation. This reduction greatly improves the quality of source calibrations, which should improve the accuracy of ear-canal impedance measurements and related quantities.

Quantifying undesired parallel components in Thévenin-equivalent acoustic source parameters

The calibration of an ear probe to determine its Thévenin-equivalent acoustic source parameters facilitates the measurement of ear-canal impedance and reflectance. Existing calibration error metrics, used to evaluate the quality of a calibration, are unable to reveal undesired parallel components in the source parameters. Such parallel components can result from, e.g., a leak in the ear tip or improperly accounting for evanescent modes, and introduce errors into subsequent measurements of impedance and reflectance. This paper proposes a set of additional error metrics that are capable of detecting such parallel components by examining the causality of the source admittance in the frequency domain and estimating the source pressure in the time domain. The proposed and existing error metrics are applied to four different calibrations using two existing calibration methods, representing typical use cases and introducing deliberate parallel components. The results demonstrate the capability of the proposed error metrics in identifying various undesired components in the source parameters that might otherwise go undetected.