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Hearing aid hears theft detectors?

David Hotvet dahotvet at sonarhh.com
Tue Jul 8 19:48:29 EST 1997

"Sandra and Dan Hobson" <danhobson at netins.net> wrote:

>I have worn hearing aids over 30 years and also have had the unsettling
>experience of picking up Transmissions in airports and certain "secured"
>buildings ... more so when I have worn BTE's with a T-coil or a
>programmable.  One theory proposed was the oscillations of the security
>system is what my circuits were picking up. Sometimes fluorescent light can
>be an interference. Generally I haven't had too much trouble with the
>magnetic security systems though on occassion they do react.   On the whole
>I haven't worried about it, and take notes when my clients report it
>happening to them and where.
>Sandra Hobson 
When the telecoil is activated, magnetic fields can certainly be
picked up. When it is turned off, however, it probably is not magnetic
detection that you are hearing, rather it is probably the ultrasonic
alarms. Typically, they run at 40 kHz and can cause all sorts of
problems with the front end of hearing aids, particularly AGC hearing
aids. It may sound as if the hearing aid 'shuts off'. Actually the AGC
circuit is trying to reduce the level from the microphone- the stuff
you can't hear, but the mic does.

Hope this helps!

   Dave Hotvet                   
   IS & Design Specialist        Internet: dahotvet at sonarhh.com  
   Sonar Hearing Health          Phone:    612-733-6350 
   3130 Lexington Ave. S.        Fax:      612-736-7558   
   Eagan, MN. 55121
 "Without deviation from the norm, there can be no progress"

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