A horn is a tapered sound-guide designed to provide an acoustic impedance match between a sound-producing device and the characteristic impedance of free space.

Kinds of acoustic horns

• horn-loaded loudspeakers
• musical instrument horns
• signalling horns such as those used on cars, trucks, trains, boats, bicycles, and the like
• speaking horns, often used by lifeguards at public pools. Alternatively, a pylon can (and often is) used in this way.
• listening horns, often used by people who are hard of hearing
• pickup horns, used for acoustic pickup (e.g. on acoustic phonograph players and the like)

Horn-loaded loudspeakers

Loudspeakers are often built into horn-shaped enclosures or use horns. Most often the higher frequency loudspeakers (tweeters) use horns, and often these horns also include acoustic lenses to spread the sound waves in a horizontal direction in order to create a non-isotropic radiation pattern that concentrates the acoustic energy at what will be ear-level for most of the space being filled.

Duplicity of signalling horns

Bicycle horns usually consist of a single horn operating at a single resonant frequency, with a reed made of stainless steel, located in the throat of the horn, and supplied with air be a rubber squeeze bulb.

Automobile horns are generally arranged in pairs, to produce a major third chord consisting of two notes: F and A, sounded together. Automobile horns are usually driven by a flat circular stainless steel membrane that has an electromagnet acting upon it, and a contactor that repeatedly interrupts the current to the electromagnet. This arrangement works like a buzzer (similar to the way the old-style telephone bells worked). There is usually a screw to adjust the distance/tension of the electrical contacts for best operation.

Truck horns use the same notes, F and A, but are usually voiced an octave lower than automobile horns. Truck horns are usually supplied with air from an air compressor, because most big trucks have air compressors already on board to operate the brakes (big trucks often use air brakes).

Train horns are usually grouped in 3 or 4, to form a chord that has 3 or 4 notes sounded together.

Horn-based musical instruments

Horn-loaded musical instruments

Many musical instruments have some kind of bell shape output to provide a good impedance match with freespace air.

Horn as a figure of speech for certain musical instruments

The word "horn" by itself is often used to refer to certain musical instruments, most commonly the French horn (as in the words to the popular children's song: "The horn, the horn, it sounds so forlorn"), as well as the tuba.

Functional (mathematical shape) of horns

Ideally horns have an exponential taper, but often a linear taper is used for ease of implementation (i.e. for the large plywood shapes of speaker enclosures such as "butterfly bass bins"), where it is difficult to make an exponential taper out of certain materials such as plywood. Alternatively, sometimes wooden horns taper in only one dimension, since it is easy to build a speaker enclosure with a taper (usually at the sides) and straight top and bottom.

Conical horns are also quite common, because they are easy to make. Conical speaking horns are used by lifeguards at public pools. Such horns are simply a plastic cone with a handle on one side, in which a person can yell into the small end and sound comes out the big end, with a resulting improvement in impedance match as well as directionality.

Acoustic horns in popular culture

The notion of "blowing a horn" is often used as an idiomatic expression for self-promotion.

The popular children's song "I've been working on the railroad" makes reference to blowing of a horn:

 Ive been working on the railroad All the live long day Ive been working on the railroad Just to pass the time away Can't you hear the whistle blowing Rise up so early in the morn Cant you hear the whistle blowing Dinah, blow your horn 

 Dinah wont you blow Dinah wont you blow Dinah, wont you blow your horn Dinah, wont you blow, Dinah, wont you blow, Dinah, wont you blow your horn 

See also

• Horn (instrument), Horn speaker