Matthias Blau

Reciprocal measurement of acoustic feedback paths in hearing aids

A reciprocal measurement procedure to measure the acoustic feedback path in hearing aids is investigated. The advantage of the reciprocal measurement compared to the direct measurement is a significantly reduced sound pressure in the ear. The direct and reciprocal measurements are compared using measurements on a dummy head with adjustable ear canals, different earmolds, and variations in the outer sound field. The results show that the reciprocal measurement procedure can be used to obtain plausible feedback paths, while reducing the sound pressure in the ear canal by 30 to 40 dB.

Smoothing individual head-related transfer functions in the frequency and spatial domains

When re-synthesizing individual head related transfer functions (HRTFs) with a microphone array, smoothing HRTFs spectrally and/or spatially prior to the computation of appropriate microphone filters may improve the synthesis accuracy. In this study, the limits of the associated HRTF modifications, until which no perceptual degradations occur, are explored. First, complex spectral smoothing of HRTFs into constant relative bandwidths was considered. As a prerequisite to complex smoothing, the HRTF phase spectra were substituted by linear phases, either for the whole frequency range or above a certain cut-off frequency only. The results indicate that a broadband phase linearization of HRTFs can be perceived for certain directions/subjects and that the thresholds can be predicted by a simple model. HRTF phase spectra can be linearized above 1 kHz without being detectable. After substituting the original phase by a linear phase above 5 kHz, HRTFs may be smoothed complexly into constant relative bandwidths of 1/5 octave, without introducing noticeable artifacts. Second, spatially smoother HRTF directivity patterns were obtained by levelling out spatial notches. It turned out that spatial notches do not have to be retained if they are less than 29 dB below the maximum level in the directivity pattern.

Force spectra identification by FRF matrix inversion: A sensor placement criterion

Excitation force spectra are a key quantity in structure‐borne sound characterization. In this paper, the identification of multiple broadband force spectra by FRF matrix inversion is considered. Because of the notorious sensitivity of inverse methods to small errors in measured data, such measurements must be carried out with extreme care. One major decision that could heavily affect the accuracy of the final results concerns the placement of the vibration response sensors. To date, this is often accomplished by just putting them as close as possible to the force input points, or by subjective judgment, or by considering the condition number of the FRF matrix as a criterion. In this paper, an alternative criterion is proposed which takes into account that, especially in the frequency region up to a few hundred Hz, the resulting errors are often dominated by measurement noise‐induced bias on response spectra estimates.

Prediction of the Sound Pressure at the Ear Drum in Occluded Human Ears

In this paper, methods of predicting the sound pressure at the ear drum are compared to probe tube measurements in 30 human ears occluded by closed ear molds. The methods compared fall into two categories: in one of them, the acoustical source (receiver and tubing) is modeled independently from the individual ear, in the other one it is modeled jointly with the individual ear. Models of the individual ear include modified variants of the phase reflectance and the pressure minima methods and a model of an ear simulator. As a result, it appears that the modified phase reflectance method works best for independent models of source and ear, whereas the modified pressure minima method works best for joint models of source and ear. With these methods, acceptable accuracies can be obtained in the frequency range from 100 Hz to 10 kHz, except for a few extremely large ear canals.

Regularization approaches for synthesizing HRTF directivity patterns

As an alternative to traditional artificial heads, it is possible to synthesize individual head-related transfer functions (HRTFs) using a so-called virtual artificial head (VAH), consisting of a microphone array with an appropriate topology and filter coefficients optimized using a narrowband least squares cost function. The resulting spatial directivity pattern of such a VAH is known to be sensitive to small deviations of the assumed microphone characteristics, e.g., gain, phase and/or the positions of the microphones. In many beamformer design procedures, this sensitivity is reduced by imposing a white noise gain (WNG) constraint on the filter coefficients for a single desired look direction. In this paper, this constraint is shown to be inappropriate for regularizing the HRTF synthesis with multiple desired directions and three alternative different regularization approaches are proposed and evaluated. In the first approach, the measured deviations of the microphone characteristics are taken into account in the filter design. In the second approach, the filter coefficients are regularized using the mean WNG for all directions. The third approach additionally takes into account several frequency bins into both the optimization and the regularization. The different proposed regularization approaches are compared using analytic and measured transfer functions, including random deviations. Experimental results show that the approach using multiple frequency bands mimicking the spectral resolution of the human auditory system yields the best robustness among the considered regularization approaches.

Using inter-individual standard deviation of hearing thresholds as a criterion to compare methods aimed at quantifying the acoustic input to the human auditory system in occluded ear scenarios

Occluded ear scenarios are found in many applications, e.g., hearing aids or insert ear phones. Unfortunately, the correct quantification of the acoustic input delivered to the auditory system in such a scenario is complicated by the individual character of our outer ear anatomy. For instance, one can easily observe inter-individual differences in ear drum pressure level of up to 30 dB at 10 kHz with one and the same sound source. We may thus ask: (1) Is the sensitivity of our auditory system at threshold adapted to our outer ear anatomy? and (2) what is the best method to quantify the acoustic input? We propose to use the inter-individual standard deviation of hearing thresholds as a means to answer these questions: The quantity that is best suited to describe the input to the auditory system should result in the lowest inter-individual standard deviation of thresholds. Preliminary results based on tests with custom ear shells and with foam ear plugs show that up to 6 kHz, there are no significant differ...

Prediction of the Sound Pressure at the Ear Drum in Occluded Human Cadaver Ears

In this paper, a method of predicting the sound pressure at the ear drum, based on an optimized variant of the reflectance phase method and on a method of estimating individual drum impedances is presented and validated by probe tube measurements in human cadaver ears occluded by closed ear molds. It appears that the proposed method works with acceptable accuracy (roughly ±5 dB) in the frequency range from 1 kHz to 10 kHz. At lower frequencies, leakage is responsible for a high variability (up to 20 dB) of the sound pressure generated in the ear canal (and thus at the ear drum as well in this frequency range). In addition, it was observed that predictions based on ear canal models obtained from computer tomography scans were less accurate than predictions based on the method proposed in this paper. As the residual ear canals of the temporal bones used in this study were rather short, these results should be confirmed in vivo.

Model based prediction of sound pressure at the ear drum for an open earpiece equipped with two receivers and three microphones

In future hearing systems, one or more microphones and one or more receivers located within the earmold or ear canal are feasible. In order to predict the sound pressure at the ear drum in such a scenario, a one-dimensional electro-acoustic model of a prototype open earpiece with two integrated receivers and three integrated microphones was developed. The transducers were experimentally characterized by their (frequency-dependent) sensitivity (microphones) and Norton equivalents (receivers). The remaining acoustical system was modeled by 12 frequency-independent parameters which were fitted using a training set-up with well-defined loads at both sides of the ear piece. Put on an individual subject, the model could then be used to determine the acoustic impedance at the medial end of the earpiece, based on measured transfer functions between the integrated components. Subsequently, a model of the ear canal and its termination was estimated from the measured ear canal impedance, which could eventually be us...

Measurements of the acoustic feedback path of hearing aids on human subjects

Feedback is a problem in hearing aids which will cause signal degradation and reduce the maximum applicable gain. More specifically, the advantages of open fittings (e.g., minimizing the occlusion effect) are limited by acoustic feedback. Feedback cancelation algorithms are used to overcome these limitations. For the development of such algorithms, the acoustic feedback path of the hearing aid must be known. The acoustic feedback path is not only affected by the outer sound field but by the individual anatomy and physiology as well. In order to quantify these different influences, feedback path measurements were performed on 20 human subjects. The measurements included different static conditions as well as dynamic ones (i.e., repetitive movements were performed during the measurement). Since the sound pressure level must be limited in measurements on human subjects, a valid identification of the feedback path is difficult in many cases, due to a low signal to noise ratio. Therefore, all measurements were...

Static and dynamic measurements of the acoustic feedback path of hearing aids on human subjects

Feedback is a problem in hearing aids which will cause signal degradation and reduce the maximum applicable gain. More specifically, the advantages of open fittings (e.g. minimizing the occlusion effect) are limited by acoustic feedback. Feedback cancellation algorithms are used to overcome these limitations. For the development of such algorithms, the acoustic feedback path of the hearing aid must be known. The acoustic feedback path is not only affected by the outer sound field but by the individual anatomy and physiology as well. In order to quantify these different influences, feedback path measurements were performed on 20 human subjects. The measurements included different static conditions as well as dynamic ones (i.e. repetitive movements were performed during the measurement). Since the sound pressure level must be limited in measurements on human subjects, a valid identification of the feedback path is difficult in many cases, due to a low signal to noise ratio. Therefore, all measurements were performed reciprocally in addition to the direct measurements. Results show that yaw movements only have a small influence on the acoustic feedback path, whereas changes of the outer sound field can have a substantial impact on the feedback path.Feedback is a problem in hearing aids which will cause signal degradation and reduce the maximum applicable gain. More specifically, the advantages of open fittings (e.g. minimizing the occlusion effect) are limited by acoustic feedback. Feedback cancellation algorithms are used to overcome these limitations. For the development of such algorithms, the acoustic feedback path of the hearing aid must be known. The acoustic feedback path is not only affected by the outer sound field but by the individual anatomy and physiology as well. In order to quantify these different influences, feedback path measurements were performed on 20 human subjects. The measurements included different static conditions as well as dynamic ones (i.e. repetitive movements were performed during the measurement). Since the sound pressure level must be limited in measurements on human subjects, a valid identification of the feedback path is difficult in many cases, due to a low signal to noise ratio. Therefore, all measurements were ...