An acoustic mirror is a passive device used to reflect and focus (concentrate) . Parabolic acoustic mirrors are widely used in parabolic microphones to pick up sound from great distances, employed in surveillance and reporting of outdoor sporting events. Pairs of large parabolic acoustic mirrors which function as "whisper galleries" are displayed in science museums to demonstrate sound focusing.
Interwar period, before the invention of radar, parabolic sound mirrors were used experimentally as early-warning devices by military air defence forces to detect incoming enemy aircraft by listening for the sound of their engines.
During World War II on the coast of southern England, a network of large concrete acoustic mirrors was in the process of being built when the project was cancelled owing to the development of the Chain Home radar system. Some of these mirrors are still standing today.[ The concrete blocks that once protected Britain BBC News website 7 January 2019]
Acoustic aircraft detection
The Dungeness mirrors, known colloquially as the "listening ears", consist of three large
concrete reflectors built in the 1920s–1930s. Their experimental nature can be discerned by the different shapes of each of the three reflectors: one is a long curved wall about high by long, while the other two are dish-shaped constructions approximately in diameter.
placed at the foci of the reflectors enabled a listener to detect the sound of aircraft far out over the
English Channel. The reflectors are not parabolic, but are actually spherical mirrors.
Spherical mirrors can be used for direction finding by moving the sensor rather than the mirror; another unusual example was the Arecibo Observatory.
[ The Arecibo Observatory – Past, Present and Future ]
Modern uses
Parabolic acoustic mirrors called "whisper dishes" are used as participatory exhibits in science museums to demonstrate focusing of sound. Examples are located at Bristol's We The Curious, Ontario Science Centre, Albuquerque's ¡Explora!, Baltimore's Maryland Science Center, Oklahoma City's Science Museum Oklahoma, San Francisco's
Exploratorium,
[ Exploratorium Listening Vessels exhibit ] the Science Museum of Minnesota, the Museum of Science and Industry in Chicago, Pacific Science Center in Seattle, Jodrell Bank Observatory, St. Louis Science Center, Parkes Observatory in Australia and on the north campus lawn of North Carolina State University.
Small portable parabolic microphones are used to record wildlife sounds such as bird song, in televised sports events to pick up the conversations of players, such as in the huddle during American Football games, or to record the sounds of the sport, and in audio surveillance to record speech without the knowledge of the speaker.
Locations
Acoustic aircraft detection mirrors are known to have been built at:
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RAF Denge, Kent
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Abbot's Cliff, Kent (at OS grid reference TR27083867)
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Boulby, Yorkshire
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Dover, Kent, at Fan Bay (OS grid reference TR352428)
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Hartlepool, County Durham, in the Clavering area
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Hythe, Kent
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Malta – five sound mirrors were planned for Malta, serialled alphabetically, but only the Magħtab wall is known to have been built:
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A. Il-Widna, "the ear": Magħtab
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B. Zonkor
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C. Ta Karach
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D. Ta Zura
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E. Tal Merhla
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Joss Gap, Kent
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Kilnsea, Yorkshire
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Leros, Greece
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Redcar, Yorkshire
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Seaham, County Durham
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Selsey, Sussex – converted into a residence
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Sunderland, at Namey Hill (OS grid reference NZ38945960
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Warden Point, Isle of Sheppey, Kent – the Warden Point mirror, sited on a cliff-top, fell onto the beach below ca 1978-9
Modern acoustic mirrors built for entertainment
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Pennypot, Royal Military Canal
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Wat Tyler country park, near Pitsea, Essex – modern sculpture in the form of functional sonic mirrors
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The Brickyard (NC State) – North Carolina State University campus
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Discovery Green in downtown Houston, which has a sculpture made of limestone called the Listening Vessels
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Very Large Array – Socorro County, New Mexico Visitor Center has a 'whispering gallery' pair of dishes
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Planetanya,
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Tūhura Otago Museum, Dunedin, New Zealand – A pair of 'whisper dishes' is situated outside of the Museum's entrance
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Luxembourg Science Center, Luxembourg – 2 parabolic antennas are situated in front of the Museum
See also
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Acoustic location
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Sound ranging, for the artillery use
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Parabolic microphone
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Whispering gallery
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Whispering-gallery wave
Further reading
External links
