Internal noise in hearing aid microphones: Its causes and consequences

It is always possible to make the microphone and circuit noise in a hearing aid audible. One can carefully cover the sound port of the microphone, turn up the amplifier gain to a very high level, and listen to the output. (If you try this, be very careful to keep the port well covered to avoid feedback oscillation while listening!) The sound that is heard originates either in the microphone or the hearing aid circuitry. However, in normal operation, with the frequency-dependent gain of the aid set according to a reasonable fitting rationale, the microphone noise should be masked by environmental noise in all but the quietest environments. Continual reductions in the noise level of the circuits within hearing aid microphones have brought us to a point Where the electrical circuit noise is not the dominant source of microphone noise. The noise that you may hear from the microphone actually originates in random motion of the air molecules within the small passageways of the microphone.