A device that converts one form of energy into another. Many types of transducers are reversible, for example converting electrical energy into acoustical energy and vice versa. The electrodynamic geophone is a reversible transducer that converts mechanical motion to electrical voltage, or passing a current through the coil causes the coil to move with respect to the case. Other reversible transducers are electrostatic, variable reluctance, magnetostrictive, piezoelectric, etc. Piezoelectric transducers of barium or lead zirconate or titanate are used in many hydrophones and sonar transducers.