This series of blogs has focused on the topic of acoustic feedback as it applies to hearing aids and attempts to manage feedback over the years. This series is designed to introduce the basic concepts. The serious reader should consult more detailed references.
Acoustic feedback solutions that have been presented up to this point include:
- Overall gain reduction
- High-frequency gain reduction
- Electronic damping of high-frequency peaks
- Bandpass filtering
- Notch filtering
Three additional feedback solution considerations will be discussed in this blog; frequency shifting, phase shifting, and frequency warbling. The final approach, adaptive feedback, in its various forms, will be covered in following blogs.
Additional Insight into Feedback
Every typical sound reinforcement system has two responses:
- Open loop – the microphone is isolated from the loudspeaker (receiver)
- Closed loop – the microphone is acoustically coupled with the loudspeaker (receiver)
Figure 15. Closed loop response. Feedback between speaker and microphone.
Figure 14. Open loop response. No feedback between speaker and microphone.
The measured response of the output of an amplifying system relative to its input is called its transfer function. The transfer function is important in the discussion of feedback because if the measured open loop response of a system has constant magnitude across the frequency range of interest, the system can be modeled using various kinds of level control followed by some delay. Looking at the transfer function of a simple level change and delay element can provide insight into the behavior of acoustic feedback in actual situations. In the open loop and closed loop diagrams, DSP (digital signal processing) has been added. The main advantage of this is the ability of DSP to analyze the signal and perform intelligent and/or non-linear operations. Acoustic feedback solutions that reduce the overall gain, reduce the high-frequency gain, or dampen the high-frequency peaks apply primarily to analog circuits, and when used as such, were able to provide only a limited solution that ended up as either non-satisfactory or not feasible in particular conditions. It will be obvious later that the current attempts at feedback management all involve DSP.
Figure 16. The transfer function of a hearing aid is the measured response of the output relative to its input.
Acoustic Feedback Solution #6 – Frequency Shifting
Another strategy at feedback reduction emerged also from public address systems as far back as the 1960s. The approach is to shift the feedback frequency upward or downward by a small amount as it passes through the amplifier. This actually prevents feedback from occurring because a microphone-speaker-microphone loop on any particular frequency cannot be created because each frequency is constantly shifted up or down. This means that the frequency of the signal being fed back and re-amplified is different each time it is processed through the amplifier. The basic idea is that as feedback gets generated in one area of the response it eventually gets attenuated by another area. The frequency shifter continues to move the generated feedback frequency along the transfer function until it reaches a section that effectively attenuates the feedback. A shift of 5 to 10 Hz has been considered sufficient to eliminate the feedback.
Using this general approach with hearing aids, Bennett et. al., in 1980 {{1}}[[1]] Bennett MJ, Srikandan, and Brown,LM, A controlled feedback hearing aid, Hearing Aid Journal, 33(7) 12,42 [[1]] used a variation that increased the shift to 30 Hz because they felt that although the shift of 5 Hz was fine for public address systems, it was not sufficient for hearing aids. Feedback was suppressed, but the approach introduced audible distortions because this is not a “musical transformation” and the ratio between the signal’s harmonics is not preserved by the frequency shift. For example, for voiced sounds in speech, all constituting frequencies are multiples of a fundamental frequency. So shifting frequency by a certain amount results in the harmonic structure of the signal being violated. As a result, a person’s voice begins to sound mechanical as the amount of the shift increases.
Egolf {{2}}[[2]] Egolf, D. (1982) Review of the acoustic feedback literature from a controlled systems point of view, in Studebaker J. and Bess F (Eds). The Vanderbilt Hearing Aid Report. Monographs in Contemporary Audiology, Upper Darby, PA pp 94-103 [[2]] reported that Bennett, et. al. attempted to test a prototype hearing aid that used progressive amounts of frequency shift. At intensity levels where acoustic feedback was likely to occur, the frequency shift was small, but at higher input levels, the frequency shift was greater. The results showed that under these conditions the usable gain increased from 6 to 15 dB, but at the higher levels, a warbling noise was detected in the output signal.
This approach to feedback management in hearing aids seems not to have progressed further.
Acoustic Feedback Solution #7 – Phase Shifting
A laboratory experiment of inserting a phase shifter into the forward signal path of a hearing aid was described by Preves {{3}}[[3]] Preves, D. (1985) Evaluation of phase compensation for enhancing the signal processing capabilities of hearing aids in situ, Doctoral dissertation, University of Minnesota.[[3]] His experiments showed an increase in available gain of 15 dB over the uncompensated condition. And, when the gain was adjusted to a level where sub-oscillatory feedback started, the transient response was improved and ringing was reduced. However, when gain was adjusted to a level below sub-oscillatory feedback, the transient response was the same both with and without phase compensation. No evidence has been found that this was used in a commercial hearing aid.
Acoustic Feedback Solution #8 – Frequency Warbling
Agnew {{4}}[[4]] Agnew, J. (1996). Acoustic feedback and other audible artifacts in hearing aids, Trends in Amplification, Vol. 1 No 2[[4]] reported that warbling a potential feedback frequency at a warble frequency of 5 Hz had been suggested by Nishinomiya in 1968 {{5}}[[5]] Nishinomiya, G. (1968). Improvement of acoustic feedback stability of public address system by warbling, Proceedings of the Sixth International Congress of Acoustics, 3:93-96.[[5]] Nishinomiya reported that listeners commented about annoyance with this approach, and also that perhaps it would have been better to warble the specific frequency where feedback is liable to occur. Again, while such a technique might lend itself to a public address system, it is unlikely to function in hearing aids because of in-situ and changing ambient conditions leading to feedback in a number of frequencies.
Future blogs: Adaptive Feedback in its various forms.
