You're no doubt familiar with gesture-motion controls like Microsoft Kinect and the lot. These things employ censors to detect your motion and then execute the commands. Daniel Rapp, a Swedish student decided to use an approach first explored by Microsoft in 2012 called the Doppler effect.

The Doppler effect has been around for a long time. The technique applied uses just a microphone and speaker to measure when soundwave reflects off moving objects.

The shift is measured by using the microphone, which is then interpreted as a gesture by the computer. Check out Rapp's webpage here.