A horn is a tapered sound guide designed to provide an acoustic

Acoustics

Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics...

 impedance match

Impedance matching

In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....

 between a sound source and free air. This has the effect of maximizing 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 optimize the transfer of sound from the air to a receiver.

Applications

Acoustic horns are used in:

• horn-loaded loudspeakers
• musical horns
  Horn (instrument)
  The horn is a brass instrument consisting of about of tubing wrapped into a coil with a flared bell. A musician who plays the horn is called a horn player ....
  
  
• vehicle horn
  Vehicle horn
  A vehicle horn is a sound-making device used to warn others of the approach of the vehicle or of its presence. Automobiles, trucks, ships, and trains are all required by law to have horns...
  
  s such as those used on cars, trucks, trains
  Train horn
  Train horns are audible warning devices found on most diesel and electric locomotives. Their primary purpose is to alert persons and animals to the presence of a train, especially when approaching a grade crossing. They are also used for acknowledging signals given by railroad employees Train horns...
  
  , boats, and bicycles
• Megaphone
  Megaphone
  A megaphone, speaking-trumpet, bullhorn, blowhorn, or loud hailer is a portable, usually hand-held, cone-shaped horn used to amplify a person’s voice or other sounds towards a targeted direction. This is accomplished by channelling the sound through the megaphone, which also serves to match the...
  
  s, often used by lifeguards at public swimming pools.
• ear horns
  Hearing aid
  A hearing aid is an electroacoustic device which typically fits in or behind the wearer's ear, and is designed to amplify and modulate sound for the wearer. Earlier devices, known as "ear trumpets" or "ear horns", were passive funnel-like amplification cones designed to gather sound energy and...
  
  , used by people who are hard of hearing (the human ear is itself constructed in the form of a horn)
• pickup horns, used for acoustic pickup (e.g. on acoustic phonograph
  Phonograph
  The phonograph record player, or gramophone is a device introduced in 1877 that has had continued common use for reproducing sound recordings, although when first developed, the phonograph was used to both record and reproduce sounds...
  
   players)

Horn-loaded loudspeakers

Loudspeakers are often built into horn-shaped enclosures or use horns. Most often the higher-frequency elements (tweeter

Tweeter

A tweeter is a loudspeaker designed to produce high audio frequencies, typically from around 2,000 Hz to 20,000 Hz . Some tweeters can manage response up to 65 kHz...

s; 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 loudspeakers 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 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.

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.

Material handling 'sound horns'

In agriculture, and dry material handling generally, sound horns are often used to start material flow or to force release of impacted materials. In a grain silo, such a horn may be mounted inside the silo and sounded as the silo is emptied to loosen stuck granules. Typically, these use any fundamental frequency from around 120-250Hz, and are about 120dB SPL

Sound pressure

Sound pressure or acoustic pressure is the local pressure deviation from the ambient atmospheric pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water...

, and are powered by compressed air. They are sometimes called acoustic cleaners or acoustic horns.

Horn-loaded musical instruments

Many wind instruments have some kind of flaring bell shape. These are generally not exponential in configuration, and are in fact used to modify the standing wave

Standing wave

In physics, a standing wave – also known as a stationary wave – is a wave that remains in a constant position.This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling...

 patterns of the instrument, and thereby the musical notes which can be produced.

"The flared section of the bore in many instruments are almost conical. First let's look at what this does to the spacing of the frequencies. In the page about pipes and harmonics, we saw that closed conical pipes have resonances whose frequencies are both higher and more closely spaced than those of a closed cylindrical pipe. So one can think of introducing a conical or flared section of the pipe as raising the frequencies of the standing waves, and raising the frequencies of the low pitched resonances most of all. The bell also contributes to this effect: in the rapidly flaring bell, the long waves (with the low pitches) are least able to follow the curve of the bell and so are effectively reflected earlier than are the shorter waves. (This is because their wavelengths are very much longer than the radius of curvature of the bell.) One might say therefore that the long waves 'see' an effectively shorter pipe."

This has the effect of providing both the "brassy" sound of horn instruments versus woodwinds or even metal instruments which lack a flare, and also of increasing the perceived loudness of the instrument, as harmonic

Harmonic

A harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency, i.e. if the fundamental frequency is f, the harmonics have frequencies 2f, 3f, 4f, . . . etc. The harmonics have the property that they are all periodic at the fundamental...

s in the range to which the ear is most sensitive are now delivered more efficiently. However, this enhanced radiation in the higher frequencies means by definition less energy imparted to the standing waves, and thus less stable and well-defined notes in the higher registers, making the instrument more difficult to play.