At times hearing aids make an ear-splitting screech that will drive most people crazy. This horrific noise, called feedback, occurs in all audio systems, including public address systems.
In the hearing care profession, we have been fighting the feedback “monster” for many years. Possibly you may have heard that the incredible improvements in digital processing have eliminated feedback from hearing aids. But, I’m sorry to say, controlling feedback is still one of the major tasks of fitting hearing instruments. And, if the current popularity of open-fitted hearing aids continues—and it looks as if these instrument are here to stay—the number of patients dealing with problematic feedback will keep on increasing.
SO HOW DO WE TAME THIS MONSTER?
We should start by giving some serious thought to the difference between what the patient needs in terms of the amount and bandwidth of amplification, and what the patient wants in terms of models and venting.
Patients prefer the sensation of wearing an open-fitted hearing aid. It’s inherently more comfortable to have your ear canal feel “open” than “plugged up.” For this reason, patients find it much easier to get used to open-fitted instruments than fittings that occlude the ear canal. Also, some patients hate the idea of wearing one of the larger, old-fashioned BTE instruments, so they sometimes choose a style of instrument that tends to have substantially more feedback.
When I discuss hearing aids with patients, I give them my recommendations. But, I am no longer surprised when the patient chooses a hearing aid that I have clearly described as a “poor choice.” Years ago, when I was younger and more foolish, I would argue with the patient. No more. I would rather the patient wear a “poor choice” instrument than no instrument at all.
But, in these cases I make sure the patient and, especially, the patient’s family understand my reasoning. Otherwise the patient will return in a year saying, “Why did you sell me this hearing aid if you knew I was going to have problems with it?” It is important that the patient and hearing aid professional know and discuss the limits of amplification for each model. For example, open-fitted hearing aids cannot produce any significant gain in the lower frequencies. The amplified sound in the lower frequencies escapes the ear through the wide open ear and causes feedback.
Controlling feedback depends on the type of “plumbing” (canal length and vent) the hearing aids have. Earmolds and vents are critical items as these “plastics” hold the sound in the ear canal, thus preventing feedback from occurring. When feedback is an issue, use longer canal lengths and small vent; this decreases the amount of “leakage” of amplification and reduces feedback.
When I talk about feedback I like to compare sound pressure to water pressure: To avoid a water leak with a hose, the hose must be attached securely to the water outlet. Any leak in the hose results in a substantial loss of water. Following this analogy, any opening in the earmold allows a substantial loss of sound, and an increase risk of feedback.
Here are two other valuable suggestions.
When a hearing aid is fitted, the audiologist or hearing aid specialist uses the manufacturer’s software to set the sound level and adjust the hearing aids. Most hearing aids have a built-in feedback-reduction algorithm.
I use the Siemens system, and the feedback control in its software is called the Critical Gain adjustment. You activate “critical gain” and the software studies the feedback inherent in the hearing aids, then asks you whether or not you want to use these recommended adjustments. This type of “feedback elimination” can be extreme helpful and should be used whenever needed.
Finally, here’s one more way to fight the “monster.”
Make sure your hearing aid patients keep their ears clean and free from wax and dead skin. Any debris in the ear canal can reflect the sound back out of the ear and cause feedback. A thin piece of dead skin in the ear canal often causes the system to go into feedback. So, advise patients to have their physician or hearing aid professional regularly inspect their ear canals.