Hearing aid
Hearing aids use adaptive feedback cancellation to improve the amount of gain. When hearing aids are misplaced or turned to loud volumes they can have large feedback. Noise picked up through the hearing aid's microphone creates this feedback, which is then amplified creating a ringing noise. Adaptive feedback cancellation cancels the noise with an adaptive filter. The product of the adaptive filter cancels the feedback from the microphone creating clear sound from the hearing aid.