A device, often fitted into the bridges of acoustic guitars, where the mechanical vibrations in the bridge cause microscopic distortions to the shape of a piezo-electrical crystal, generating a small voltage in the process. The other common type of pick-up is the electro-magnetic pick-up used in electric guitars. EM pick-ups use a coil of wire which senses any changes in the magnetic field created by a small permanent magnet. As the guitar string above the coil vibrates, it disturbs the magnetic field, and the coil generates a small electrical current, which is passed onto an amplifier and loudspeaker. This is the pick-up part of the DI.