When one mentions the term Doppler Effect, then automatically what comes into mind is Sound waves interference. Doppler's effect explains why a higher frequency and a higher-pitched siren results if the ambulance or police car speeds towards you and as the opposite result as it speeds away.
This effect has been explored to come up with gesture control based on the changes in soundwaves.
Sidhant Gupta, a graduate student at the University of Washing ton stumbled on the idea while working on a different project. Quite interesting how sound waves can interpret movement made by hands but with the speaker playing an ultrasonic tone, one that is high-pitched enough to be inaudible to the user. The microphone then detects changes in the ultrasound and interprets those changes as movement.
The clip below show how this was made possible.
http://www.youtube.com/v/wK_u8-UQmOs
source:popularmechanics (http://www.popularmechanics.com/technology/gadgets/home-theater/next-up-in-kinect-style-motion-sensing-ultrasound-9118795)