An acoustic horn (also known as a waveguide) is a tapered sound guide designed to provide an acoustic impedance match between a sound source and free air. This configuration maximizes the efficiency with which sound waves from a 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.
Principle of Operation
The horn functions as an acoustic transformer. Its gradually expanding cross-sectional area allows sound waves to transition from the high-pressure, low-velocity environment at the throat (narrow end) to the low-pressure, high-velocity environment of open air at the mouth (wide end). This impedance matching reduces reflection of sound energy back toward the source, thereby increasing radiation efficiency.
Applications
Acoustic horns are employed in a wide range of devices:
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Horn loudspeakers: Loudspeakers built into horn-shaped enclosures, most often used for higher-frequency elements (tweeters and midranges). Horn loudspeakers are highly efficient but have a sharp cutoff frequency determined by the area of the horn mouth. Bass reproduction via horn loading is impractical for very low frequencies, as a circular horn mouth sufficient to reproduce 20 Hz would require a diameter of approximately 5.5 meters (18 feet).
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Musical instruments: Brass and woodwind instruments incorporate flaring bell shapes. These modify standing wave patterns, raising the frequencies of low-pitched resonances and contributing to the characteristic "brassy" timbre. The bell also enhances radiation efficiency in the frequency range to which the human ear is most sensitive.
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Vehicle horns: Used on cars, trucks, trains, boats, and bicycles.
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Megaphones: Handheld conical devices used to project the voice, commonly employed by lifeguards and public address operators.
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Foghorns: Large horns used to produce audible warning signals for maritime navigation.
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Ear horns: Passive acoustic devices historically used by people with hearing impairment to collect and concentrate sound into the ear canal.
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Pickup horns: Used on acoustic phonograph players to mechanically amplify sound from the stylus.
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Material handling "sound horns": In agriculture and dry material handling, compressed-air-powered acoustic horns (typically 120–250 Hz, approximately 120 dB SPL) are used inside silos to dislodge stuck granular materials. These are sometimes called acoustic cleaners.
Use of Surroundings as Part of the Horn
Large bass speakers often take advantage of room boundaries as extensions of the horn. Placing a speaker in a corner allows walls to act as part of the horn surface, providing a better impedance match at low frequencies. Outdoors, the ground can similarly serve as part of the horn surface.
In Nature
Acoustic horns occur naturally in the form of burrows constructed by male mole crickets (Gryllotalpa vineae). These burrows are shaped as double exponential horns with a bulb, carefully smoothed to contain no irregularities larger than 1 millimeter. The cricket's song, produced at 3.5 milliwatts of mechanical power, delivers a peak of 92 decibels at a distance of one meter and can be heard up to 600 meters away.
