Touch-sensitive Surfaces
From: NASA Tech Briefs Insider - 12/05/2006

European researchers have developed a series of acoustic sensors that turn
any surface into a touch-sensitive computer interface. The Tangible Acoustic
Interfaces for Computer-Human Interaction project, or "Tai-Chi", uses tiny
piezoelectric sensors to sense surface vibrations.  The sensors are connected
to a desktop computer loaded with software developed by the Tai-Chi team and
the system can track up to two objects at once. Tai-Chi could allow users to
type using a "keyboard" drawn on a desktop. 

The goal of the Tai-Chi project is to develop acoustics-based remote sensing
technology, which can then be adapted to any physical object to create
tangible interfaces. The methods for contact point localization developed in
Tai-Chi utilize the location-signature embedded in the acoustic wave patterns
caused by contact, as well as triangulation and acoustic holography. 

Read the entire article at:
Tangible Acoustic Interfaces for Computer-Human Interaction
http://www.taichi.cf.ac.uk/node/1

Links:
Video
http://www.youtube.com/watch?v=UdetI7glJP4

(Video) Tangible Acoustic Interfaces
http://www.techeblog.com/index.php/tech-gadget/video-tangible-acoustic-interfaces

Sound Rose: Tangible-Acoustic Interface 
http://www.techeblog.com/index.php/tech-gadget/sound-rose-tangible-acoustic-interface

Tangible Acoustic Interfaces for Computer-Human Interaction (Tai-Chi)
http://www.mec.cf.ac.uk/research/pubs/taichi.html

Acoustic sensors make surfaces interactive
http://www.newscientisttech.com/article/dn10673-acoustic-sensors-make-surfaces-interactive.html