Bruno Zamborlin collaborated with Norbert Schnell to use a contact microphone connected to a system that processes sound in real time to turn any rigid surface into a touch interface. There's no way to explain it adequately in words, so just watch the video:

Through gesture recognition techniques we detect different kind of fingers-touch and associate them with different sounds. In the video we used two different audio synthesis techniques:
- physic modelling, which consists in generating the sound by simulating physical laws;
- concatenative synthesis (audio mosaicing), in which the sound of the contact microphone is associated with its closest frame present in a sound database.

To put it another way, the system is transforming the vibrations transmitted from touch through a rigid body into waveforms that a computer can, in real time, recognize and either transmute into audible sound or use as a triggering mechanism for other sounds.