Screeching, chirping, squealing, ringing. Feedback. Everyone's heard it, no one likes it. Older hearing aids do it every time you hug your friends, chew food or get too close to a wall. Newer hearing aids often will have cut out features for feedback. Why does feedback exist? What causes feedback?
There are actually a few kinds of feedback. The most common one is acoustic feedback. This is the usual feedback you here when the hearing aid gets in the proximity of another person or a wall. Have you ever been to a meeting that is set up with a PA system, and someone covers the microphone and it screeches through the whole system? This is what is going on with the hearing aids, just larger scale. When a microphone picks up the sound of itself, it begins to loop its own mechanical noise. The looping of that noise picks up, gets louder and louder, and then after a moment, becomes that horrible squeal we all love to hate.
I like to think of feedback as an auditory representation of the infinity mirror trick. It's caused by its own reflection.
This type of feedback is a natural acoustical phenomenon, therefore, it's hard to avoid completely. There is nothing that can stop all feedback in all circumstances. Many hearing aid manufacturers have specific feedback reduction features in their instruments to pick out feedback and reduce the gain of it when it's present. This year, Oticon released their Opn S platform, powered by the Velox S chip, which has a new means of feedback reduction.
These graphs are called spectrum. The colors represent how much of each frequency there is through time. Dark red would show the highest density of sound and dark blue would show the lowest density. In the dark gray circle is a period of feedback. As most modern hearing aids, there is a longer solution to keeping the feedback from continuing indefinitely. This period of feedback lasts about a third of a second, which isn't too bad, but certainly audible.
The second chart shows the new OpenSound Optimizer from Oticon at work. The hearing aid sensed that initial increase of energy, as show by the small red line at the start of that circle, but once it noticed the feedback, it began to introduce a feedback breaker, which is marked by the small green lines. The breaker turns off that specific frequency for a millisecond, and then turns it back on. These breakers are so short that they aren't audible to the majority of the population, but keep that frequency from gaining enough power to feedback until the longer term feedback reducer kicks in. This reduces the feedback from a third of a second, to less than a tenth of a second.
There is another type of feedback that hearing aid uses can experience. Does your hearing aid have a telecoil? This is a specialized copper coil that connect the hearing aids to non-Bluetooth phones, such as a landline or office phone. Telecoils would use a magnetic pulse to draw in the incoming signal of the phone. They also still use telecoils in many churches, meeting rooms, airports and even the NYC subway!
Having a telecoil has one downside which is potential magnetic feedback. This is still feedback, but sounds more like a grinding or humming than a squealing. Places with large flourscent lights are most likely to trigger magnetic feedback, such as a home improvement or club store. Unfortunately, there is no real cure to this type of feedback, as the industry is moving towards Bluetooth compatibility instead of telecoils, but older hearing aids may still have telecoils. In all actuality, magnetic feedback is a good thing, because if you walk into Costco and your hearing aids begin to grind and hum, that means the telecoils are working well, which is what you want! The real cure is making your spouse do the club shopping!!!