Daniel Rapp has written a blog post about using the doppler effect to sense motion. In his experiments he’s used hand gestures in free space to generate the scroll, tap, and double-tap gestures on his laptop. He’s produced a library which allows a computer to transmit a tone from its speaker while at the same time receiving through its microphone. As the hand moves in specific directions near the microphone it creates subtle nuances in the sound being received. These nuances are then translated into specific commands.
If your computer were to produce a tone that’s audible, you’d end up with a musical instrument called the Theremin. If your computer were to produce an inaudible tone you might feel like you’re in the movie Minority Report.
I’m currently experimenting with something somewhat similar to this. It’s called passive radar. I live in an area with a lot of electromagnetic interference — often called noise. If I set my radio receiver’s sensitivity to high I hear a lot of noise. I can actually print this noise onto a jpeg image. If I print an image as large objects move through free space and interrupt the ambient noise, I can see them. It’s called passive radar because I didn’t need to generate sound to see this. Instead, already present ambient noise was the baseline. Active radar generates sound first, then listens. Doppler (above) is active radar.