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I once took an ultrasonic receiver and built a frequency copnverter to bring the ultrasonics down to audible frequencies. It was interesting to hear what items were loudest. Dropped silverware could be pretty good, as well as keys.
Some time later I had a Heathkit with ultrasonic. We came home to find the dog watching TV. At first I thought his chain collar jingling had done it, but couldn't duplicate it. Much later, we found he was hitting the big 2"-square on-off buttons accicentally with his tail!
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