A loudspeaker is a device for converting variations of electric energy into corresponding variations of acoustic energy or sound.
Most speakers have three main parts — a permanent magnet, a coil of wire surrounding the magnet called a voice coil, and a cone-shaped piece of paper or plastic called a diaphragm. When electric signals from an amplifier pass through the voice coil varying magnetic forces are produced in the coil. These magnetic forces interact and drive the coil in a rapid forward and backward motion. The diaphragm which is attached to the voice coil vibrates with it, and in turn sets into vibration the air in contact. Since the surface area of the diaphragm is quite large it sets up vibrations in a large mass of air and the sound produced is quite loud.
