A device that converts an electrical signal from an amplifier into sound. A loudspeaker driver is an electromechanical-acoustic device with two electrical input terminals, to which an electrical signal is applied, and a diaphragm which vibrates and radiates sound. An electromechanical motor mechanism exerts a force on the diaphragm to cause it to vibrate. By far the most common type of motor mechanism, the electromagnetic-mechanical transducer, employs a coil of wire immersed in a magnetic field; electric current flowing through the coil causes a force to be exerted on the coil which is mechanically coupled to the diaphragm. A less common type of motor mechanism is the electrostatic-mechanical transducer. It uses a capacitor to which an electric voltage is applied, causing a force to be exerted between the capacitor plates, which is mechanically coupled to the diaphragm. Two types of electrostatic transducers are used: the piezoelectric transducer and the condenser transducer. The piezoelectric transducer uses a piezoelectric crystal between the capacitor plates. The condenser transducer uses an air dielectric. One plate of the capacitor is a flexible membrane which serves as the diaphragm.