When amplified sounds leak out of the ear and re-enter the hearing aid microphone, they are re-amplified back into the ear at a louder sound level. The re-amplified sounds then leak out of the ear again and re-enter the microphone, causing it to be re-amplified back into the ear at an even louder sound level. If this cycle, known as a ‘feedback loop’, continues the hearing aid begins to ‘whistle’.
Digital hearing aids are able to break the feedback loop by replicating the leaked amplified sounds in the opposite ‘phase, thus cancelling or ‘nulling’ them.
If this method is not effective enough, some digital hearing aids can also perform a feedback calibration to determine at which frequency and amplification level a sound needs to be to create a feedback loop, before placing a cap so that the sound can never be amplified above this level.
Other digital hearing aids use internal memory to keep a record of all sounds that they have processed and amplified into the ear. This allows amplified sounds that leak out of the ear and re-enter the hearing aid microphone to be detected and discarded so they cannot create a feedback loop.
Some digital hearing aids also over the feedback loop by changing the frequency of sounds ever so slightly (frequency shifting) before amplifying them so that the same sound can never re-amplified.