Feedback path variability modeling for robust hearing aids

Study on the applicability of instrumental measures for black-box evaluation of static feedback control in hearing aids

Presented is a report on black-box evaluation of feedback control systems for commercial hearing aids. The aim of the study is to examine the ability of existing instrumental measures to quantify the performance of the feedback control system in black-box settings and on realistic signals, when more than one element of the signal processing chain may be active (compression, noise suppression, microphone directionality, etc.). The evaluation is carried out on 6 different hearing aids and for 10 measures. Thereby it is possible to see which measure is best suited to measuring which specific characteristic of the feedback control system, and serves as a beginning for conducting perceptual tests. The study uses static (but variable) feedback paths and is based on signals recorded from the in-ear microphone of an artificial head, on which the hearing instruments are mounted.

Extracting the invariant model from the feedback paths of digital hearing aids

Feedback whistling is a severe problem with hearing aids. A typical acoustical feedback path represents a wave propagation path from the receiver to the microphone and includes many complicated effects among which some are invariant or nearly invariant for all users and in all acoustical environments given a specific type of hearing aids. Based on this observation, a feedback path model that consists of an invariant model and a variant model is proposed. A common-acoustical-pole and zero model-based approach and an iterative least-square search-based approach are used to extract the invariant model from a set of impulse responses of the feedback paths. A hybrid approach combining the two methods is also proposed. The general properties of the three methods are studied using artificial datasets, and the methods are cross-validated using the measured feedback paths. The results show that the proposed hybrid method gives the best overall performance, and the extracted invariant model is effective in modeling the feedback path.

Evaluation of feedback reduction techniques in hearing aids based on physical performance measures

This paper presents a physical evaluation of four feedback cancellation techniques in commercial hearing aids and two implementations of a recently developed feedback cancellation algorithm. Based on physical measures for detecting instability, oscillations and distortion, three performance aspects were measured: 1) the added stable gain compared to the hearing aid operating without feedback reduction for white noise as well as for spectrally colored input signals in two static acoustic conditions, 2) the amount of feedback, oscillations and distortion at gain values below the maximum stable gain, 3) the ability to track feedback path changes. Added stable gains between 3 dB and 26 dB were identified. Five of the six techniques achieve worse feedback reduction for a tonal opera input signal than for a speech input signal. Preventing the feedback canceller to drift away from an initial feedback path measurement results in improved performance for tonal signals at the expense of a worse feedback reduction in the acoustic conditions that differ from the condition for which the initialization was performed, as well as a worse tracking of feedback path changes. Repeated measures indicated that the reproducibility of the test set-up is crucial, in particular when the hearing aid operates close to instability.

Using a reflection model for modeling the dynamic feedback path of digital hearing aids

Feedback whistling is one of the severe problems with hearing aids, especially in dynamic situations when the users hug, pick up a telephone, etc. This paper investigates the properties of the dynamic feedback paths of digital hearing aids and proposes a model based on a reflection assumption. The model is compared with two existing models: a direct model and an initialization model, using the measured dynamic feedback paths. The comparison shows that the proposed approach is able to model the dynamic feedback paths more efficiently and accurately in terms of mean-square error and maximum stable gain. The method is also extended to dual-microphone hearing aids to assess the possibility of relating the two dynamic feedback paths through the reflection model. However, it is found that in a complicated acoustic environment, the relation between the two feedback paths can be very intricate and difficult to exploit to yield better modeling of the dynamic feedback paths.

[New aspects of hearing aid fitting in noise-induced hearing loss]

In the past hearing aid fitting frequently turned out to be a problem in patients with noise-induced hearing loss. Selective amplification in the high frequency range and at the same time natural sound and appropriate wearing comfort (open fitting) could not be achieved in numerous cases. Today these problems can be tackled by modern hearing aid technology providing us with efficient feedback suppression algorithms making open fittings possible for many more patients. This development is particularly beneficial for patients with noise-induced hearing loss. Unfortunately, open fitting is in opposition to wearing hearing aids at noisy workplaces. Tight fittings, however, can be used at work if a special listening program for noisy conditions is available. This dilemma is discussed and possible solutions are pointed out.

Challenges and recent developments in hearing aids. Part II. Feedback and occlusion effect reduction strategies, laser shell manufacturing processes, and other signal processing technologies

This is the second part of a review on the challenges and recent developments in hearing aids. Feedback and the occlusion effect pose great challenges in hearing aid design and usage. Yet, conventional solutions to feedback and the occlusion effect often create a dilemma: the solution to one often leads to the other. This review discusses the advanced signal processing strategies to reduce feedback and some new approaches to reduce the occlusion effect. Specifically, the causes of three types of feedback (acoustic, mechanical, and electromagnetic) are discussed. The strategies currently used to reduce acoustic feedback (i.e., adaptive feedback reduction algorithms using adaptive gain reduction, notch filtering, and phase cancellation strategies) and the design of new receivers that are built to reduce mechanical and electromagnetic feedback are explained. In addition, various new strategies (i.e., redesigned sound delivery devices and receiver-in-the-ear-canal hearing aid configuration) to reduce the occlusion effect are reviewed. Many manufacturers have recently adopted laser shell-manufacturing technologies to overcome problems associated with manufacturing custom hearing aid shells. The mechanisms of selected laser sintering and stereo lithographic apparatus and the properties of custom shells produced by these two processes are reviewed. Further, various new developments in hearing aid transducers, telecoils, channel-free amplification, open-platform programming options, rechargeable hearing aids, ear-level frequency modulated (FM) receivers, wireless Bluetooth FM systems, and wireless programming options are briefly explained and discussed. Finally, the applications of advanced hearing aid technologies to enhance other devices such as cochlear implants, hearing protectors, and cellular phones are discussed.

Evaluation of feedback-reduction algorithms for hearing aids

Three adaptive feedback-reduction algorithms were implemented in a laboratory-based digital hearing aid system and evaluated with dynamic feedback paths and hearing-impaired subjects. The evaluation included measurements of maximum stable gain and subjective quality ratings. The continuously adapting CNN algorithm (Closed-loop processing with No probe Noise) provided the best performance: 8.5 dB of added stable gain (ASG) relative to a reference algorithm averaged over all subjects, ears, and vent conditions. Two intermittently adapting algorithms, ONO (Open-loop with Noise when Oscillation detected) and ONQ (Open-loop with Noise when Quiet detected), provided an average of 5 dB of ASG. Subjects with more severe hearing losses received greater benefits: 13 dB average ASG for the CNN algorithm and 7-8 dB average ASG for the ONO and ONQ algorithms. These values are conservative estimates of ASG because the fitting procedure produced a frequency-gain characteristic that already included precautions against feedback. Speech quality ratings showed no substantial algorithm effect on pleasantness or intelligibility, although subjects informally expressed strong objections to the probe noise used by the ONO and ONQ algorithms. This objection was not reflected in the speech quality ratings because of limitations of the experimental procedure. The results clearly indicate that the CNN algorithm is the most promising choice for adaptive feedback reduction in hearing aids.