Originally Posted by Presbyter

It's a hearing aid with two microphones. The main one in the front and another one in the back. The one in the back are faced behind. If the microphone in the back picks up sound BEFORE it hits the front microphone, then the hearing aid will know that the sound is coming front behind. The hearing aid will try to reduce background sounds like noise. If the sound reaches the front before it reaches the back, then the aid will know it's coming from the front and will not filter it.