If your grandparents had hearing aids, you may know what feedback is. It is the annoying high pitch sound that comes out of a hearing aid.
Basically, it is the sound signal that leaks out of the hearing aid, cycles around and is picked up again by the microphone from the hearing aid. It is the same acoustic principal as hearing that loud noise when someone on a stage has the microphone too close to the speaker. In earlier days, feedback in hearing aids was a major issue that hearing aid technology could not fix or control very well. The features within the hearing aid technology was not advanced enough to tell the hearing aid to eliminate feedback when it heard it. The styles of hearing aids that were available at the time were also not elaborate enough to cope with all degrees and shapes of hearing losses.
The good news, the styles of hearing aids that are now available and the advancements in technology are good enough that that you should not experience feedback. This said. Feedback is still an acoustic phenomenon that you can experience with hearing aids. If you do, it typically means that your hearing aids are not properly fit for you on an acoustic level, and or not programmed correctly.
As an example, I sometimes have patients who prefer one hearing aid style over another, regardless of their hearing loss. In some cases, I have to warn them that with certain styles that may not be as appropriate for their hearing loss, they could run into feedback problems. The typical configuration of hearing aid that may cause feedback would be a Receiver-in-canal or Slim-tube Behind-The-Ear hearing aid, that is fit with a dome in the ear, for a hearing loss that is more severe, and that requires too much volume to handle this configuration.
Another cause of feedback that I have seen is when someone has been wearing their hearing aids for over five years, and it no longer fits properly in their ear canal, therefore the sound leaks out of the ear.
Essentially, if you are experiencing feedback, you should contact your Audiologist to make sure they can troubleshoot the issue and figure out how to help you best.