Everyone who works with hearing aids or wears a hearing aid is familiar with feedback. It’s that nasty screech you get when you put your hand up to your ear or when a friend gives you a warm hug. Hearing aid practitioners spend a lot of time dealing with feedback.
Today, I want to discuss the “seeds” of feedback, a condition I call “incipient feedback.” Incipient feedback degrades the quality of amplification and, if left unchecked, will eventually result in full-blown feedback that makes you want to pull out your hair.
Incipient feedback peaks can be seen on real-ear curves as little peaks in the frequency response that grow into huge spikes when you put your hand near the patient’s ear (causing the amplified sound to re-enter the microphone and enhance the feedback loop).
REDUCING FEEDBACK BY REDUCING GAIN
If you use a feedback-detection protocol like the Critical Gain measurement in the Siemens fitting software, you see two curves: a feedback curve and a use-gain curve. Feedback results when these two curves get too close to each other. The software functions by reducing the gain in the zone where the two curves approach each other.
This “solution” to feedback, i.e., reducing gain, may or may not be desirable. Without question you want to reduce feedback, but if the patient needs the gain in the zone you reduce, you are simply trading one problem for another.
PREVENTION IS BEST
We need to remember that prevention is the best cure for feedback. That involves the usually laborious task of selecting an appropriate model of hearing aid and/or making a “perfectly” fitted earmold or custom shell for the aid. In the case of stubborn feedback problems, you may have to make several earmolds to get one that works well.
If you are working with BTE instruments and have a serious feedback problem, I suggest that you make three impressions for three completely different earmolds: a half-shell mold made out of clear acrylic plastic, a full-shell earmold made out of silicone and treated with the new “slick seal” coating, and an experimental earmold suggested by the oldest, most experienced factory advisor at the earmold lab you use.
When I’m talking to patient about a feedback problem, I say, “This wonderful hearing aid is worthless without a perfectly fitted earmold. We can go slowly and work to slay this “demon” or we can pull out all the stops and put our best ideas to work quickly.“
Why is my number one suggestion a non-vented, long-canal, half-shell, acrylic mold? There are several reasons: This mold goes in and out of the ear easily; it can be easily “wiggled” more tightly into the ear if needed; it can be easily modified on a standard drill/sander; it is almost always very comfortable to wear; and most patients love it.
While this is my favorite earmold, I will quickly agree that it is not perfect for eliminating feedback. However, keeping the patient happy with an almost perfect earmold is a big step in the right direction.
The new “super slick” treated silicone earmolds are making a name for themselves. From what I have seen they have the greatest potential of any earmold to eliminate feedback.
They do have their drawbacks. For one thing, they are difficult to modify in the office. I have tried all my tricks–the little blue pumice stones, the special sanders—but without success. So I send them back to the factory if they need adjusting.
They also take a little work to insert and remove, but so does the stopper in a bottle of champagne that holds in high pressure. And that doesn’t make us stop drinking champagne.
Feedback is one of the realities of hearing aids. Sooner or later your patients are going to have problems with feedback. It pays to watch out for incipient feedback and fix it before it grows into a real “monster.”
By far the best cure for feedback is a body-worn aid. Microphone far away from the earphone. BTE is OK for low air-to-air gain but it’s really not appropriate for high-gain applications. Feedback elminators are just attempts to defy the laws of physics.