A loudspeaker is a device that transforms a varying electric signal into sound that can be heard at a considerable distance. Loudspeakers are used in radio and television receivers, phonographs, high fidelity systems, and public address systems. There are many different kinds of loudspeakers, the most common of which is the dynamic, or moving-coil, loudspeaker.
The dynamic loudspeaker has a cone, usually made of paper, that is attached to a coil of fine wire called the voice coil. The voice coil is located between the poles of a large magnet, which may be a permanent magnet or an electromagnet. When the varying electric current from an amplifier is passed through the voice coil, it sets up a varying magnetic field. This field interacts with the unchanging magnetic field of the large magnet, causing the voice coil and the cone to move rapidly back and forth. The moving cone strikes the air molecules next to it and produces sound waves.
Less common than the dynamic loudspeaker is the electrostatic loudspeaker. It consists essentially of two electrodes, one a fixed metallic plate and the other a flexible plastic diaphragm covered with a very thin me tallic film. The two electrodes are separated by a very small air space, and they are maintained at a very high voltage difference by an unvarying current. When the varying current from an amplifier is superimposed on this current, variations in voltage difference between the two electrodes result. The variations produce corresponding variations in the force of attraction between the two electrodes, and the diaphragm vibrates rapidly back and forth, producing sound waves.