Acoustic Feedback and Other Audible Artifacts in Hearing Aids

Abstract

A coustic feedback has been described as "whistling," "howling," "screeching," "screaming," 'squealing,'" "whining,'" "ringing, " "humming," "buzzing." "oscillating" and by various other names. The high-pitched whistling of a hearing aid experiencing acoustic feedback is an irritating sound for the hearing aid wearer and for nearby individuals. Suppressing these irritating squealing noises is not easy. Thus dealing with acoustic feedback is still a prevalent problem that plagues clinicians and wearers alike. Though specific figures are considered by manufacturers to be proprietary information, industry experts estimate that as many as 10% to 15% of in-the-ear hearing aid products are likely to be returned to the factory within the first 90 days after manufacture for feedback-related problems. Obviously this adds to the overall cost of hearing aids to the dispenser, and anything that can be done to help reduce these returns will ultimately benefit the wearer.This issue is intended to provide comprehensive information on the origins and characteristics of acoustic feedback in hearing aids and to discuss its minimization or prevention. Although it will primarily discuss acoustic feedback, for completeness the discussion will also include the audible symptoms of electrical and electromagnetic feedback and pickup. These also occur in hearing aids and are frequently confused by the wearer with acoustic feedback. Some discussion of the symptoms of both may help the clinician to understand and resolve complaints of oscillations and other audible artifacts in a troublesome hearing aid fitting. It is useful for the clinician to be able to distinguish between various manifestations of oscillation and other audible sounds in order to be able to logically identify the problem and counsel the wearer appropriately.Before starting a detailed discussion of feedback, an important point should be made concerning terminology. Acoustic feedback in a hearing aid fitting produces a form of instability and the resulting audible oscillation. It is caused by a sound wave from the output leaking back to the input. Though all acoustic feedback of the correct phase and magnitude produces an undesired form of oscillation in a hearing aid, not all oscillation is due to acoustic feedback. In precise terms the objectionable audible sound produced by a hearing aid due to acoustic feedback should be called audible oscillation due to acoustic feedback. Through common usage, this more accurate term has generally been abbreviated simply to acoustic feedback, though in reality acoustic feedback is the cause of the problem and not the audible effect. However, to comply with common usage the term acoustic feedback will be used consistently throughout the text to refer to the unpleasant and undesired squealing and screeching that occurs in a hearing aid and which is caused by the leakage of amplified output sound back to the microphone.A second point should also be made about terminology. For simplicity, hearing aids will be discussed in two general ca...

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