English: Animation showing the operation of a reflex loudspeaker. Drawing shows a cross section through the speaker, the construction is cylindrically symmetrical. The reflex or reentrant loudspeaker, also called a megaphone, bullhorn, or loudhailer, is a type of folded horn speaker widely used in public address systems. It consists of a specialized diaphragm loudspeaker called a compression driver (a) attached to a metal or plastic horn (d). The driver has a small metal diaphragm (blue) driven to vibrate by the audio signal current flowing through a coil of wire (red) between the poles of a cylindrical magnet (green). The horn serves to couple the diaphragm more efficiently to the air, allowing it to radiate more of its energy as sound than a conventional horn speaker, making it louder. The horn sound path must be several feet long, so to make the speaker more compact the sound path is "folded" into a zigzag shape by several concentric exponentially-widening ducts. The sound waves pass forward through an inner tube (b) in the central projection in the horn, back through a larger concentric tube (c), then out through the surrounding horn (d).

Reentrant loudspeakers can be 10 times as efficient as cone loudspeakers, and so their output can be 10 dB louder when operating from the same amplifier. However their frequency response is very uneven and limited, with a bandwidth of 300-400 Hz to 5 kHz, so they have a distinctive "tinny" sound and are only used for the human voice.

In this animation the rate of vibration of the diaphragm and the speed of the sound waves are shown greatly reduced.