A compression driver (cylindrical box at rear) on a midrange horn speaker used in a home audio system
A compression driver (A) in a horn loudspeaker consists of a metal diaphragm (blue) vibrated by the audio signal current in a coil of wire (red) between the poles of a cylindrical magnet (green). The sound waves pass out through an acoustic horn (B).
A compression driver is a small specialized diaphragm loudspeaker which generates the sound in a horn loudspeaker. It is attached to an acoustic horn, a widening duct which serves to radiate the sound efficiently into the air. It works in a "compression" mode; the area of the loudspeaker diaphragm is significantly larger than the throat aperture of the horn so that it provides high sound pressures. Horn-loaded compression drivers can achieve very high efficiencies, around 10 times the efficiency of direct-radiating cone loudspeakers. They are used as midrange and tweeter drivers in high power sound reinforcement loudspeakers, and in reflex or folded-horn loudspeakers in megaphones and public address systems.