Motion sensing using the doppler effect

'' In order to measure the doppler effect for motion detection on a conventional computer, what you can do is send out a sinusoid at some known (inaudible) frequency, say, 20 kHz. If something is moving in the room, then, after the sinusoid has bounced around on the walls and into the microphone, the sound will shift in frequency. This can be measured by looking at the frequency spectrum in the nearby region of the 20 kHz tone. 

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The most obvious application for this is motion sensing. '' [source]