If memory serves me correct, playing around with an ultrasonic
receiver years ago I noticed that car keys gave off quite a bit of
'noise' (ringing and jangling) when shaken
It took me years to figure out that some keys, in a certain order on a
ring, would trigger some EAS (electronic article surveillance) systems,
as well as trigger some demagnetizers at checkouts. A real PITA.
Kurt
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