English: Diagram showing how a reflex horn loudspeaker works. Shows a section through the axis of the speaker. The reflex speaker, also called a trumpet speaker, megaphone, bullhorn, or loudhailer, is widely used in public address systems. The function of a horn in a speaker is to couple the diaphragm more efficiently to the air, so more of the vibrational energy is radiated as sound. Horn speakers can have ten times the efficiency of cone speakers, so they are used in high power applications such as P.A. systems. However, horns that can reproduce the human voice adequately must be several feet long, so to make the speaker more compact "folded horns" are often used, in which the sound path is bent into a zigzag shape by reflections. Developed in the 1940s, a reflex speaker is a type of folded horn in which the sound path is a series of concentric ducts.
The sound is produced by a specialized diaphragm speaker called a compression driver (d) which is designed to work loaded by the high sound pressures in the horn. This consists of a stiff metal diaphragm (a) driven by a voice coil (b) mounted between the poles of a cylindrical magnet (c) The sound follows a zigzag path through a series of exponentially widening ducts through the central projection of the horn (e) and out through the concentric outer horn (f).