Piezo-electric Transducers:

Piezo-electric transducers are ceramic devices that pickup and amplify mechanical vibrations within the material they are directly in contact with. Typically they are attached to either the body or the soundboard of an instrument. A special long thin transducer has been developed that can be placing directly under the bridge saddle of an instrument.

Some form of local pre-amplification close to the transducer is usually required due to the high-impedance of piezo-electric devices (i.e. they are vulnerable to pickup electrical interference).