An acoustic horn or waveguide is a tapered sound guide designed to provide an acoustic impedance match 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. Acoustic horns are found in nature in the form of the burrows constructed by male mole crickets to amplify their song. The earliest appearance of the horn in connection to sound in The Times was published in 1786: Applications Acoustic horns are used in: horn loudspeakers brass and woodwind musical instruments vehicle horns such as those used on cars, trucks, trains, boats, and bicycles megaphones, often used by lifeguards at public swimming pools. foghorns, used to warn ships ear horns, used by people who are hard of hearing (the human ear itself having the form of a horn) pickup horns, used e.g. on acoustic phonograph players Horn loudspeakers Loudspeakers are often built into horn-shaped enclosures or use horns. Most often the higher-frequency elements (tweeters and midranges) use horns, sometimes with acoustic diffraction lenses to spread the sound waves in a horizontal pattern at ear-level and limit the vertical pattern. 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 the area of the horn mouth, with little sound output below. Bass sounds are usually produced by conventional speaker cones, since a circular horn mouth sufficient to reproduce 20 Hz would have a diameter of about , except when a building, ground surface, or room itself is considered an extension of the horn. Use of the surroundings as part of the horn Large bass speakers often take advantage of the surroundings as part of the horn. For example, they can be put in the corners of a room, so the walls act as part of the horn. 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 very 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