Uncle Monster
Mon, 26
Dec 2016 23:44:34 GMT in alt.home.repair, wrote:

Back in the 1970's, ultrasonic motion detectors for security
systems were quite common in businesses. The last of the old
Polaroid instant cameras used an ultrasonic transducer to measure
distance to the subject of a picture and focus the camera. There
were some ultrasonic motion detectors at one time that detected a
shopper in the isle at a retailer so an electronic advertising
display could start playing music and the voice of a pitchman. I
haven't seen one for a while but I could hear the harmonics and
clicks.

It's unmistakable if you've heard it before.



--
Sarcasm, because beating the living **** out of deserving people is
illegal.