Hearing aid hears theft detectors?

Jeff Joslin jjoslin at nortel.ca
Fri Jul 4 13:13:48 EST 1997


In article <Pine.GSO.3.96.970703150843.11468A-100000 at speedy>, Jim Lellman
<lellman at daugherty.com> wrote:

>Apparently more stores are using those little labels and the detectors
>at the doorway.  My son wears a hearing aid with a bone vibrator.
>He can "hear" the detectors at the doorway when he walks by them.
>Some are so loud that they cause him pain.  He has mystied more than
>one store manager when he told them that their antitheft detectors
>were not turned on.
>
>My question is, is this a common occurance?  I don't know if it's
>the hearing aid proper, or the bone transducer that's picking it up.

Picking up the "sound" of theft-detection systems occurs because
some hearing aids act as radio receivers, pick up the electro-magnetic
field created by the detectors.  It's the same as when hearing aids 
pick up interference with certain types of cellular phones (e.g.,
GSM, PCS).

The problem is due to improperly shielded input stages in the
hearing aid; when the hearing aid is exposed to an EM field, 
the input stages of the amplifier circuit can act as antenna
and demodulator, feeding an audio frequency into the main 
amplifier.  My high school library's theft detection system used
to create painfully-loud interference in my old Widex BTE; the
Unitrons that replaced it were no better.  My current Sensos
don't suffer from this problem to nearly the same degree, though
they're not completely immune.

You can try complaining to the hearing aid manufacturer, 
but it's a difficult problem to fix, and a fix probably can't 
easily be retrofitted to an existing design.  It would be nice
if manufacturers gave specs for E-M rejection so that buyers
could make a semi-informed decision.

Jeff

-- 
Jeff Joslin                              All opinions mine.



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