Loudspeaker
Horn loudspeakers

Horn loudspeakers are the oldest form of loudspeaker system. The use of horns as voice-amplifying megaphones dates at least to the 17th century, and horns were used in mechanical gramophones as early as 1877. Horn loudspeakers use a shaped waveguide in front of or behind the driver to increase the directivity of the loudspeaker and to transform a small diameter, high-pressure condition at the driver cone surface to a large diameter, low-pressure condition at the mouth of the horn. This improves the acoustic—electro/mechanical impedance match between the driver and ambient air, increasing efficiency, and focusing the sound over a narrower area.

The size of the throat, mouth, the length of the horn, as well as the area expansion rate along it must be carefully chosen to match the driver to properly provide this transforming function over a range of frequencies. The length and cross-sectional mouth area required to create a bass or sub-bass horn dictates a horn many feet long. Folded horns can reduce the total size, but compel designers to make compromises and accept increased cost and construction complications. Some horn designs not only fold the low-frequency horn but use the walls in a room corner as an extension of the horn mouth. In the late 1940s, horns whose mouths took up much of a room wall were not unknown among hi-fi fans. Room-sized installations became much less acceptable when two or more were required.

A horn-loaded speaker can have a sensitivity as high as 110 dB at 2.83 volts (1 watt at 8 ohms) at 1 meter. This is a hundredfold increase in output compared to a speaker rated at 90 dB sensitivity and is invaluable in applications where high sound levels are required or amplifier power is limited.
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