Acoustic squeal with hearing aids occurs when a portion of the amplified sounds (usually the higher frequencies) escape from the ear canal, reach the microphone of the hearing aid, and get re-amplified. This action begins the feedback cycle of amplification and re-amplification of the same signals, resulting in the acoustic squeal. This is a familiar phenomenon, occurring often with Public Address (PA) systems when the gain of the system is too high or the microphone is located too close to a loudspeaker. For hearing aids, too, feedback is more likely to occur when high gain is required. For some children, however, a high gain hearing aid may be necessary. Indeed, some of the newer hearing aids, those which incorporate Wide Dynamic Range Compression (WDRC), may be more prone to acoustic feedback since they provide more gain at the lower input levels (and less gain with higher inputs). But there may be listening advantages to such a hearing aid for a particular child that we’d like to provide; we should not be limited in our ability to provide these advantages by the onset of acoustic squeal.

Over the years, we have tried to control acoustic feedback in a number of different ways. The traditional solution has been to focus on the earmold, to try to seal the amplified sound in the ear canal. So the first thing that is checked with children is to determine if the earmold is seated properly in the ear. For young children in particular, the earmolds often seem to be half in and half out of the ear. Is the earmold too loose? Has the child outgrown the earmold and it is time to secure another? Or, has the earmold itself shrunk with time? However, the tighter the earmold, the more uncomfortable it may be for a child and there are limits to our ability to seal the amplified sound within the ear canal. We need another way of accomplishing this purpose. Electronic means to reduce acoustic feedback have been recommended for years, but were not really feasible until the advent of digital hearing aids. Recently, several hearing aid companies have introduced hearing aids that incorporate realistic electronic means of reducing acoustic feedback. In user terms, this means that a person can obtain more gain from a hearing aid before the onset of acoustic squeal.

Electronic solutions have included the reduction in the high frequency gain of the hearing aid when feedback is sensed. This may indeed minimize acoustic feedback, but at the same time audibility at the high frequencies is also reduced. A variation on this method is the use of a notch filter. In a notch filter, only a narrow band of frequencies is reduced in gain. Such a system requires that the hearing aid include some kind of sensor that can detect and measure the frequency of the squeal and then reduce the gain in a narrow band just around the offending frequency. Some hearing aids can do this adaptively, that is continually sample the system for the presence of acoustic feedback and create a notch filter whenever this occurs. However, this method also requires modifying the hearing aid’s frequency response when feedback occurs. While such modifications may be minimal, audibility is still reduced somewhat. Still, this method of controlling feedback is likely to have much less of a negative effect than the signal distortions caused by acoustic feedback.

The most optimal method would be one in which feedback can be reduced electronically without any modifications in the basic response of the hearing aid. Several years ago, such a method was developed and incorporated in a wearable hearing aid. This type of circuit also depends upon a sensor circuit that can continually detect and monitor the occurrence of acoustic feedback. However, rather than using a notch filter to reduce the feedback, in this method a signal is created within the hearing aid which is equal to but opposite in phase to the feedback signal. When the two signals are added, the feedback signal is cancelled. It is as if two people of equal strength were simultaneously pushing on opposite sides of a swinging door. The door would remain in the central position as each person’s effort cancels the others.

Up to now, the only evidence attesting to the effectiveness of this type of system has been the reports issued by the manufacturers. While these can be quite convincing, still it is always desirable that such claims be independently confirmed. In a study conducted at the University of Iowa, Dr. Ruth Bentler and her colleagues evaluated the effectiveness of one such system. In this study, the amount of gain that could be achieved before the onset of feedback was measured for twenty-five subjects with and without the automatic feedback control (AFC) activated. The majority of the subjects were able to receive at least an extra 10 dB of gain without any effect on sound quality. Some subjects were able to achieve much more. The results of this study provide convincing evidence that it is now possible to reduce acoustic feedback via electronic means. Hopefully, visitors to classrooms of hearing-impaired children in the future will no longer be greeted by a discordant symphony of acoustic squeals.