A speaker horn is a flared apperture used to convey sound from a transducer into the air. The expanding dimensions of the horn effectively act as an acoustic transformer to couple the high impedance environment of the air in front of the transducer's diaphgram with the low impedance of the atmosphere around the speaker. This arrangement allows for near-ideal energy transfer and so allows the source of sound to operate with a much higher power efficiency than a direct-radiating driver (typically around ten times better).
In modern applications, horns are normally used with compression drivers, where the diaphragm of the compression driver is larger than the throat apperture of the horn, and generates a very high sound pressure level. A 'phase plug' is often employed at the throat of the horn, to help guide the sound waves from the transducer's diaphgram into the horn, minimising local destructive phase cancellations and extending the operating bandwidth of the complete unit. The dimensions of the flared exit of the horn can be chosen to help control the angle of horizontal and vertical dispersion.
Horns are most commonly used with midrange and high-frequency drivers, but they can also be used in larger folded configurations for low frequency applications, as well as in portable megaphones and pole-mounted PA systems.
SOS Contributor Paul Ward describes the operation of a horn thus: "But how, I hear you ask, does a horn increase efficiency? It’s all about radiation impedance. When a naked driver diaphragm moves in a fluid (air, for instance), the power it transfers depends on the fluid’s density. A big difference between the density of the diaphragm and the density of the fluid is equivalent to an impedance mismatch in an electrical circuit. Imagine trying to power yourself along by doing a front crawl swimming stroke in air rather than in water. It doesn’t work in air because air is very much less dense than water. The higher density of water stops the molecules from moving sideways to get out of the way of your gracefully sweeping hands. Fish can move rapidly in the water with small fins, while birds need big wings to stay in the air, all for the same reason. Adding a horn to the front of a speaker works because, down at the business end, otherwise known as the throat, the horn essentially stops the air moving sideways and getting out of the way of the diaphragm. The length and flare of the horn then progressively blend the radiation impedance at the throat into that of free air.