earphones are the most common transducer.
can use doppler frequencies from headphone w a seeperate mic to detect motion. This may be ultrasonic? Need to use bandpass filter to just get relevant frequencies. Also, there needs to stuff in the audio range, so filter that w low pass to make sure it stays where it should.
demo video shows this works when a guy waves his hands around.
now we see a chart of error between movements and detections. Moving away has more error.
the earbuds are meant to be held and not worn, so it actually doesn't need to handle normal range of sounds too.
to make system wireless, use a portable music player.
people over 18 cannot hear the ultrasonic frequencies. There are hardware limitations with nyquist. Or ability to play 18kHz