On 4 Aug 1997 22:11:51 -0700, audioman at HCTC.NET (Jeffrey Sirianni)
>A patient from our office asked me the other day about microphones used in
>hearing aids. As he has problems with detection of speech in noise, he
>inquired about Audio Zoom from Phonak, which he will be trying. He asked
>about parabolic microphones, used for surveillance and such, and wanted to
>know why the this technology could not be incorporated into a hearing aid.
>I know this is a silly question, but could someone with technical
>background in microphones explain how a parabolic microphone works and what
>are the essential components.
>>A parabolic mic requires a parabolic reflector and microphone
positioned at the focus of the reflector much the same as a satellite
The parabolic shape works by reflecting sound toward the mic no matter
where the wave hits the surface of the reflector, assuming the system
is aimed at the source of sound. This in effect gathers energy from
the wavefront and concentrates it at the microphone.
Two reasons I can think of why they are not used on hearing aids are
size and directionality.
The lowest frequency a parabolic reflector can reflect is a function
of the diameter of the reflector as compared to the wavelength of that
frequency. It is a good idea to design the diameter to be several
wavelengths of the lowest frequency desired. The wavelength of a 500
Hz tone is a little over 2 feet. This demonstrates the cosmetic
problems associated with the use of parabolic mics in hearing aids.
Then we have the problem of directionality. Parabolic mics are
extremely directional. They could be unfocused somewhat to reduce
directionality but then the user would still have the problem of
lugging around a couple of 5 foot dishes, synchronizing them and
aiming them at desired sound sources.