Acoustic cleaners

Horn (acoustic)

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.

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.

Applications

Acoustic horns are used in:

  • horn-loaded loudspeakers
  • musical horns
  • signalling horns such as those used on cars, trucks, trains, boats, and bicycles
  • Megaphones, often used by lifeguards at public pools. Alternatively, a pylon can be (and often is) used in this way
  • ear 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)

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 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.

Signalling and vehicle horns

Classic bicycle horns usually consist of a single horn operating at a single resonance frequency, with a reed made of steel located in the throat of the horn, and supplied with air by a rubber squeeze bulb. Other variations include battery operated klaxon horns, and small air horns powered by a small can of compressed gas.

Automobile horns are usually electric klaxons, driven by a flat circular steel diaphragm that has an electromagnet acting upon it and is attached to a contactor that repeatedly interrupts the current to the electromagnet. This arrangement works like a buzzer or electric bell and is commonly known as "Sounding ones horn". There is usually a screw to adjust the distance/tension of the electrical contacts for best operation. A spiral exponential horn shape (sometimes called the "snail") is cast into the body of the horn to project the sound effectively. Sound levels are approximately 107-109 decibels, and current draw 5-6 amperes. Horns can be used singly, but are often arranged in pairs to produce a chord consisting of two notes, sounded together; although this only increases the sound output by 3 decibels, the use of two differing frequencies with their beat frequencies and missing fundamental is more perceptible than the use of two horns of identical frequency, particularly in an environment with a high ambient noise level. Typical frequencies of a pair of horns of this design are 500 and 405-420 Hz (approximately B4 and G#4).

Some cars, and many motor scooters or motorcycles, now use a cheaper and smaller alternative design, which, despite retaining the name "horn", abandons the actual horn ducting and instead relies on a larger flat diaphragm to reach the required sound level. Sound levels are approximately 109-112 decibels, and current draw 2.5-5 amperes. Again, these horns can be either single, or arranged in pairs; typical frequencies for a pair are 420-440 and 340-370 Hz (approximately G#4-A4 and F4-F#4) for this design.

Truck horns may be electromagnetic klaxons of similar design, but often are purely acoustic, driven by air from an air compressor which diesel trucks have already on board to operate the air brakes. Such air horns are often used as trim items, with chromed straight horns mounted on top of the cab. This design may also be installed on customized automobiles, using a small electrical compressor. Usually two are used, sometimes more. The frequencies vary in order to produce a variety of different chords, but in general are lower than those of automobile horns; for instance 125 through 180 Hz (approximately B2-F#3). Sound levels are approximately 117-118 decibels.

Train horns can be grouped from one to five horns, to form a chord that has the notes sounded together; these are operated by compressed air from the air brake system.

Ships signal to each other and to the shore with horns (sometimes referred to as whistles) that are driven with compressed air or from steam tapped from the power plant. Low frequencies are used because they travel further than high frequencies; ships horns have been heard as far as ten miles away. Traditionally, the lower the frequency, the larger the ship. The RMS Queen Mary, an ocean liner launched in 1934, had three horns based on 55 Hz, a frequency chosen because it was low enough that the very loud sound of it would not be painful to the passengers. Modern International Maritime Organization regulations specify ships' horn frequencies to be in the range 70-200 Hz for vessels that are over 200 meters in length.

Foghorns use low frequency tones to warn ships away from unseen coastlines. The large horn mouth is aimed out to sea.

Civil defense sirens and emergency service sirens often employ one or more acoustic horns to focus or distribute the sound.

Portable aerosol-driven air horns are used for small craft water safety as well as for sports events and recreational activities.

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. For instance, in a grain silo, such a horn may be mounted inside the silo and sounded as the silo is emptied to shake loose stuck granules. Typically, these use any fundamental frequency from around 120-250Hz, and are about 120dB SPL, and are powered by compressed air. They are sometimes called acoustic cleaners or acoustic horns in the materials handling industry.

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 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 harmonics 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.

References

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