Acoustic Feedback and Other Audible Artifacts in Hearing Aids (1996)

Acoustic 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 categories, unless the text specifically addresses a certain type of hearing aid. The term in-the-ear hearing aid will be used to generically refer to ITE (in-the-ear), ITC (in-the-canal) and CIC (completely-in-the-canal) hearing aids, unless a specific one of the three types is stated and discussed. This general term is intended to contrast to BTE (behind-the-ear) hearing aids. This distinction is made because there are two significant differences in construction between the generic categories of in-the-ear and behind-the-ear hearing aids. These differences affect the causes of feedback, the symptoms, and possible solutions. Also, note the use of lower-case letters for in-the-ear to denote the generic connotation. The upper-case initials ITE will be used to specifically refer to the full-concha “In-The-Ear” hearing aid. Italicized type will be used in various places in the text to draw attention to certain important concepts and to terminology which is not typical for audiological use.

The first major difference in construction between custom in-the-ear hearing aids and BTE hearing aids is the obvious difference between the two types in the methods of housing the electronic amplifiers and the transducers, such as microphones, receiver and telecoils. BTE hearing aids are generally manufactured on a production line and have identical internal construction for a specific model of hearing aid. Frequently, BTE hearing aids have cavities in the case for the microphone and receiver in order to provide acoustic isolation to minimize internal acoustic feedback. Custom in-the-ear hearing aids, on the other hand, are individually handcrafted at final assembly, though most of the faceplate circuit subassemblies are made on a production line with standardized layouts. This means that transducers for in-the-ear hearing aids are fitted into the shell wherever space allows. The close proximity of transducers makes a custom in-the-ear hearing aid more prone to feedback than a BTE hearing aid, unless considerable care is taken during construction. The effects of these differences will be described later in this issue.

The second significant difference in construction is that BTE hearing aids use an earhook, acoustic tubing and an external earmold to provide the acoustic coupling from the hearing aid to the ear. In-the-ear hearing aids, on the other hand, are built to fit directly into the ear. Thus, while generic comments can be made about the two type of hearing aids concerning items such as venting and canal length, there may be some differences in approaches to feedback control which will need to be described in more detail. Finally, there will be instances in which it is important to specifically discuss differences between in-the-ear products (i.e. ITE versus ITC versus CIC), and these instances will be made clear in the text.