Acoustic Feedback Control

Just about anybody who has ever worn a hearing aid has experienced this annoying sound, variously described as “whistling”, “howling”, “screeching” and various other adjectives denoting an unpleasant auditory experience. It is not a trivial problem; industry estimates that 10 to 15 percent of in-the-ear hearing aids are returned within the first 30 days because of feedback problems, while surveys of hearing aid users implicate the presence of feedback of being one of their primary problem areas. Often, the hearing aid users may not even be aware of its presence, since their hearing loss may prevent them from hearing the squeal, but when everyone else in the vicinity is complaining of an annoying squeal it’s time to check one’s hearing aid! Recently, an entire issue of “Trends in Amplification” was devoted to this topic. While this monograph, by Jerry Agnew of Starkey Laboratories, provides a comprehensive description of acoustic feedback (and other audible artifacts in hearing aids), I will only review the most common type here, the “recycling” of amplified sound escaping from the ear canal, i.e., acoustic feedback.

The tonal squeal, the most common sound produced by feedback, occurs because some of the sound amplified by the hearing aid is radiated outwards from the hearing aid receiver (the “loudspeaker”) and picked up again by the hearing aid microphone. This sound is then re-amplified, radiated outward once more, and again picked up by the microphone. As this cycle continues, the amplification system becomes unstable and the audible oscillation effect occurs. Even those people who do not wear hearing aids, or who do not experience acoustic feedback with hearing aids, are familiar with the feedback phenomenon: this is essentially what happens with a P.A. system when the person using a microphone stands too close to the loudspeaker. One usual solution is for the person talking to move further away from the loudspeaker. The corollary situation with hearing aids is that those aids whose microphones and receivers are closest together would be, other factors being equal, more likely to produce acoustic feedback. Thus, BTE hearing aids would be less likely to feedback than ITE or CIC hearing aids. But lots of other factors are also involved in increasing the likelihood that acoustic feedback will occur.

Because more sound will escape from the ear canal with more powerful hearing aids, it will be the stronger aids that exhibit the most acoustic feedback. For the most powerful hearing aids, feedback may occur no matter how carefully we seal the ear to limit the amount of radiated sound (when the gain – the amount of amplification – reaches about 60-65 dB). The tonality of the feedback usually reflects the peak in the amplification pattern of the hearing aid, since this is sound that is more likely to escape from the ear. There are certain conditions that will increase the chances of feedback for any acoustic hearing aid, no matter how weak or powerful. These include:

  • Improperly seating an earmold (or hearing aid built into an earmold) into the bowl of the ear and ear canal. Not only will this produce feedback because of a poor acoustic seal, but the improper insertion of an earmold or hearing aid can produce “pressure” points which eventually will become very painful.
  • Loose earmolds, because of shrinkage, an improper initial ear impression, or long-term changes (expansion) in the diameter of the ear canal.
  • Increasing reflections off an eardrum (when the eardrum becomes stiffer because of a cold or middle ear infection) or when wax is lodged down the ear canal. In these cases, more sound would be reflected backwards than would normally occur.
  • When the earmold is vented, usually for very appropriate acoustical or comfort reasons, the vent itself becomes a channel for the sound to escape from the ear and be picked up by the microphone.
  • The orientation of the earmold in the ear. If the sound bore is pointed to the wall of the ear canal rather than to the eardrum, the likelihood of reflections and thus the presence of feedback is increased.
  • Tubing cracks or loose tone-hooks (for BTE) hearing aids. Thin wall tubing may facilitate feedback for people wearing powerful BTE hearing aids.
  • Anything that facilitates the feedback cycle will increase the chances of feedback, such as placing one’s hands next to the hearing aid (often while adjusting the volume control), raising one’s coat collar or pulling down a stocking cap on a cold day, standing close to a wall or resting one’s head on a pillow, and using a telephone without a telephone coil. In these cases, the aid may be set just below the feedback point, but with the addition of these enhancement factors enough sound is reflected back into the microphone for the feedback cycle to commence.

Just as there are lots of reasons for acoustic feedback, so are there many ways to eliminate or reduce its occurrence. Some solutions are implicit in the causation, while others are not so self-evident. The time to worry about finding solutions is when you are unable to wear the aid at a comfortable loudness level because of the onset of feedback, and/or when those around you complain of the squeal. In most cases, your hearing aid dispensers can help you eliminate, or at least reduce, the occurrence of feedback. They will first, of course, have to determine what’s causing it.

The audiologist will check to see that the earmold is seated properly in the bowl of the ear and the ear canal, that the earmold is not too loose either because of an initial poor impression, later shrinkage of the mold itself, or an increase in the diameter of the ear canal in time. Re-casing in-the-ear hearing aids (of all types) is a frequent occurrence during the initial period a hearing aid is used, when the presence of feedback suggests the earmold doesn’t fit right. For behind-the-ear (BTE) hearing aids, the audiologist can trace the possibility that tubing problems (from the tone hook to the earmold) is causing the feedback. One solution may be the use of thick wall rather than thin wall tubing. Because it reduces and flattens the peaks of the amplification curve, an acoustic damper (a little insert that fits in the end of the tone-hook) can sometimes help reduce feedback with BTE hearing aids.

When persistent feedback occurs, audiologists will examine the ear canal to make sure that impacted wax is not the cause of the feedback. They may also do an “immitance” test, which assesses the mobility of the eardrum. A stiff eardrum (because of a cold or some middle ear problem) will also increase the likelihood of feedback. In these cases, solving the underlying condition is the solution to the feedback problem. Occasionally, the onset of acoustic feedback may be the first indication of impacted wax or an incipient middle ear problem.

Feedback caused by the presence of a vent in the earmold may require that electroacoustic targets be slightly modified. If, for example, for acoustic reasons the audiologist has recommended a large diameter vent in the earmold, but this causes feedback, then the diameter size may have to be reduced (or the vent completely plugged up) in order to eliminate the feedback. This will change the “prescribed” characteristics of the hearing, perhaps slightly, perhaps more. At this point, decisions would have to be made whether a more feasible alternative exists, such as a different hearing aid/earmold combination, or whether the electroacoustic changes due to the reduction or elimination of the vent is the best realistic compromise that can be achieved.

For the most powerful aids, or when a person absolutely requires a large diameter vent or open earmold for one reason or another, there may be no good acoustical solutions for feedback. That is, even after all has been done that can be done with earmolds and tubing, feedback may still be occurring before a person can reach his or her preferred listening level. Several current “solutions” to this situation has been to reduce either or both the gain of the hearing aid or its high frequency response. These “solutions” actually deprive people of the potential benefits of sound amplification and, generally, I find them objectionable. Of course, if someone has no usable high frequency hearing, and the squeal is being produced by the feeding back of these high frequencies, then reducing them could be advantageous. But there’s got to be a better way.

One better way was reviewed in the July/August 1994 issue. The electronic circuit of the “Genius” hearing aid senses the frequency of the acoustic feedback and generates the same frequency within the hearing aid, but in opposite phase. This helps cancel out some of the feedback; reportedly 10 to 12 dB more amplification is possible with this circuit before feedback occurs. This is on the market now. Many other potential electronic methods are reviewed in the “Trends in Amplification” monograph, but as far as I know these are not now commercially available. I suspect that we’ll soon see some of these methods, and perhaps others as well, incorporated in the new digital hearing aids coming on the market. For consumers, the “millennium” will be when they can wear hearing aids “prescribed” to their listening needs, without limitations as to power and the amplification pattern across frequencies, and without the necessity for wearing tightly fitting uncomfortable earmolds. Maybe, when the millennium does come in a few short years, we’ll actually see this development!

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