At the most fundamental level, a horn speaker uses a "horn" to get more sound (volume) from the driving loudspeaker. The horn is not amplifying anything in the sense of adding energy. It gets more sound from the driver by improving the coupling between the driver (typically made of paper or, more recently, more exotic materials such as titanium) and the air (which has a very low density).

In this sense some people have described a horn as an "acoustic transformer". Stated another way, it converts large pressure variations in a small amount of air into a low pressure variation (the human ear is very sensitive indeed to pressure variations - even quite loud sounds are actually very small pressure variations!) in a large amount of air.

The most well known early horn speakers are those on 78 RPM phonographs. In this case the record moved a heavy metal needle that excited vibrations in a metal disk typically a couple of inches in diameter. If played without the horn, this sounds very quiet. The horn improves the loading and thus gets a better "coupling" of energy from the metal diaphram into the air, and the pressure variations then get smaller as the volume expands and the sound travels up the horn.

A modern electric horn speaker works the same way, but just replaces the mechanically excited diaphram with a dynamic loadspeaker (or sometimes piezo speaker).

As usual, once a principle of operation has been defined, the technology can be adapted and improved almost without limit.

The horn should not just be a cone of fixed length, since this would resonate at the natural frequency of its length. Typically modern horns have some form of exponential flare, notably the tractrix taper.

Some claim that the small-amplitude, high-pressure environment makes it possible to build better drivers than "normal" dynamic speakers, which are hampered by the need to make the rotor as lightweight as possible. In particular, the driver can be very small, even for bass frequencies. This makes it possible to produce wide range drivers and, in some cases, avoid crossovers entirely.

Horn speakers can provide very high efficiencies, making them especially desirable for very low-powered amplifiers, such as single-ended triode amplifiers. See also Voight, Lowther, Foster, Bruce Edgar.

Horn speaker efficiency is also applied to provide very high sound pressure levels needed for sound reinforcement and public address applications, although in these applications, fidelity is usually severely compromised and large multispeaker combinations or arrays are used (see also bass bins).

See also: JB Lansing , Altec Lansing, Voice of the Theatre