Ultrasonic motion detectors

Paul H. Stypulkowski phstyp at SONARHH.COM
Wed Sep 10 09:40:34 EST 1997


We have investigated this problem for several individuals
that have run into this situation. The output of these
motion detectors is very high, often in the range of
95-100 dB SPL. Since the frequency is usually around 25 kHz they
are inaudible (at least to humans). The microphones of
most hearing aids are, however, sensitive at these frequencies.
The hearing aids see this signal as a very high level input
causing one of several reactions (depending upon the
circuit design of the aid): 1) the front end preamplifier
will overload; and/or 2) the aid will go heavily into compression,
and reduce the gain, as it is designed to do, in response
to high level inputs.

For the user, the aid appears to turn off, have static, or 
become intermittent. The intermittency comes from the fact that
the wavelength is very short at these frequencies and 
if the user moves around the room there can be large
fluctuations in level over fairly short distances. 

Hope this helps. If you have a patient with this problem,
which is usually associated with motion detectors designed
to turn off office lighting, have he/she request that
a switch be put on the system for the office (cite ADA
if necessary).  


Paul Stypulkowski
SONAR Hearing Health



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