Horn (acoustic)
From Wikipedia, the free encyclopedia
A horn is a tapered, sound guide designed to provide an acoustic impedance match between a sound source and free air. This has the effect of maximising the efficiency with which sound waves from the particular source are transferred to the air. Conversely, a horn can be used at the receiving end to optimise the transfer of sound from the air to a receiver.
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[edit] Kinds of acoustic horns
- horn-loaded loudspeakers
- musical instrument horns
- signalling horns such as those used on cars, trucks, trains, boats, bicycles, etcetra
- 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 (and the human ear is itself constructed in the form of a horn)
- pickup horns, used for acoustic pickup (e.g. on acoustic phonograph players)
[edit] Horn-loaded loudspeakers
Loudspeakers are often built into horn-shaped enclosures or use horns. Most often the higher-frequency elements (tweeters; midrange) use horns, sometimes with acoustic diffraction lenses to spread the sound waves in a horizontal pattern at ear-level. An audio driver (e.g., a speaker cone or dome) is mounted at the small, inner end. Horn speakers are very efficient, but have a sharp cutoff frequency, depending on their size, with little sound output below. Bass sounds are usually produced by conventional speaker cones, since a (straight or folded) horn sufficient to reproduce the lowest audible frequency (20 Hz) can be about 12 ft. (4 meters) long, except when a building, ground surface, or room itself is considered as part of the horn.
[edit] Use of the surroundings as part of the horn
Large butterfly bass speakers often take advantage of the surroundings as part of the horn. For example, they are often put in corners of a room, so the sound folds out onto the walls. Even outdoors, the ground can form part of the horn surface, and thus a partial horn can help provide a good impedance match to ground, or one or more walls, even at low frequencies.
[edit] 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 by 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 can be grouped from one to five bells, to form a chord that has the notes sounded together.
[edit] Horn-loaded musical instruments
Many wind instruments have some kind of bell shaped output to provide a good impedance match with freespace air. They are curved and folded to get the required length into a manageable size, and most have valves to vary this length to play different notes.
[edit] Functional (mathematical shape) of horns
Ideally, horns have exponentially tapered sides, but a linear taper is often 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 the material. 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 and are used by cheerleaders and lifeguards at public pools. They are easy to make from coated cardboard or plastic, with a handle on one side. A person can yell into the cut-off small end and sound comes out the big end, with a resulting improvement in impedance match, and directionality.
[edit] Acoustic horns in popular culture
The notion of "blowing a horn" is often used as an idiomatic expression for self-promotion.
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.
[edit] See also
- Horn (disambiguation page)
- Horn (instrument)
- Horn speaker
- Klaxon
