ScratchInput and Stane in the previous post are for gesture recognition. The acoustic sensing technology can be applied to implement touch screens or surfaces. Tai-chi (Tangible Acoustic Interfaces for Computer Human Interaction) is a EU-funded research project that changes ordinary objects to touch sensitive interactive surfaces. Just see a video clip below. In the demo video, the system detects touch (scratching) position using four microphones.
And this video shows a demo of touch gesture recognition.
Commercially, Elo TouchSystems has introduced a touch screen using Acoustic Pulse Recognition (APR), whose sensing technology is similar with that of Tai-Chi.
See also: Tai-Chi – Future Interactive Interface