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An acoustic mirror is a passive device used to reflect and perhaps to focus (concentrate) sound waves. They were used during World War I to detect troop movements and artillery, and were a major area of study as an anti-aircraft early-warning device prior to the introduction of radar. A wide network of large acoustic mirrors was in the process of being set up in southern England when the project was suddenly canceled as the Chain Home system evolved.
Acoustic mirrors contrast with the much shorter-range acoustic location systems consisting of megaphone-like devices. These are used to locate the object with some degree of accuracy; acoustic mirrors provide limited location information by offering longer range detection.
Acoustic aircraft detection
Prior to World War II and the invention of radar, acoustic mirrors were built as early warning devices around the coasts of Great Britain, with the aim of detecting incoming enemy aircraft by the sound of their engines. The most famous of these devices still stand at Denge on the Dungeness peninsula and at Hythe in Kent. Other examples exist in other parts of Britain (including Sunderland, Redcar, Boulby, Kilnsea) and Selsey Bill, and Baħar iċ-Ċagħaq in Malta. The Maltese sound mirror is known locally as "the ear" (il-Widna) and appears to be the only sound mirror built outside Great Britain.
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 5 m high by 70 m long, while the other two are dish-shaped constructions approximately 4–5 m in diameter. Microphones 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 may be used for direction finding by moving the sensor rather than the mirror; another unusual example is the Arecibo Observatory; see also
Acoustic mirrors had a limited effectiveness, and the increasing speed of aircraft in the 1930s meant that they would already be too close to deal with by the time they had been detected. The development of radar put an end to further experimentation with the technique. Nevertheless, there were long-lasting benefits. The acoustic mirror programme, led by Dr William Sansome Tucker, had given Britain the methodology to use interconnected stations to pin point the position of an enemy in the sky. The system they developed for linking the stations and plotting aircraft movements was given to the early radar team and contributed to their success in World War II; although the British radar was less sophisticated than the German system, the British system was used more successfully.
Parabolic acoustic mirrors called "whisper dishes" are used as participatory exhibits in science museums to demonstrate focusing of sound. Examples are located at Ontario Science Centre, Baltimore's Maryland Science Center, Oklahoma City's Science Museum Oklahoma, San Francisco's Exploratorium, the Science Museum of Minnesota and Parkes Observatory in Australia. A pair of dishes, typically 6 to 10 feet in diameter, are installed facing each other, separated by hundreds of feet. A person standing at the focus of one can hear another person speaking in a whisper at the focus of the other, despite the wide separation between them.
Parabolic microphones depend on a parabolic dish to reflect sound coming from a specific direction into the microphone placed at the focus. Because of their small, portable size, they can easily be used in the same manner as acoustic mirrors for detection and direction finding of distant noise sources. They are also frequently used in televised sports events to pick up on the conversations of players, such as in the huddle during American Football games, or to record the sounds of the sport.
Acoustic mirrors are known to have been built at:
- Denge, Kent
- Abbot’s Cliff, Kent (at OS grid reference TR27083867)
- Boulby, Yorkshire
- Dover, Kent, at Fan Bay (OS grid reference TR352428)
- Hartlepool, Co. Durham, in the Clavering area
- Hythe, Kent
- Malta – five sound mirrors were planned for Malta, serialled alphabetically, but only the Magħtab wall is known to have been built:
- A. Magħtab (colloquially Il Widna - The Ear)
- B. Zonkor
- C. Ta Karach
- D. Ta Zura
- E. Tal Merhla
- Joss Gap, Kent
- Kilnsea, Yorkshire
- The Brickyard (NC State) – North Carolina State University campus
- Redcar, Yorkshire
- Romney Marsh, Kent – a series of horizontal discs
- Seaham, Co. Durham
- Selsey, Sussex – converted into a residence
- Sunderland, at Namey Hill (OS grid reference NZ38945960)
- Warden Point, Isle of Sheppey, Kent – the Warden Point mirror, sited on a cliff-top, fell onto the beach below ca 1978-9
- Pennypot, Royal Military Canal – a civilian installation for entertainment
- Wat Tyler country park, nr. Pitsea, Essex – modern sculpture in the form of functional sonic mirrors
- "SOUND MIRROR". http://jonathanseary.blogspot.com/. JONATHAN SEARY DEVELOPMENT. Retrieved 1 March 2013.
- DrPhysics.com. "Sound Mirrors". Retrieved 2010-06-08.
- The Arecibo Observatory - Past, Present and Future
- Exploratorium Listening Vessels exhibit
- Sunderland City Council. "World War 1 early warning acoustic mirror on Namey Hill". Retrieved 2011-01-14.
- "Acoustic mirror Summary:". http://www.subjectsummary.com. Retrieved 1 March 2013.
- Richard Newton Scarth, Echoes from the Sky: A Story of Acoustic Defence (Hythe Civic Society, 1999) (ISBN 1-900101-30-0) (As of January 2007[update], this book is out of print)
|Wikimedia Commons has media related to Acoustic mirrors.|
- Acoustic mirrors in Britain
- Military acoustic locators
- White Cliffs Underground further details of variety of East Kent defences
- Visiting information for UK (and Maltese) sound mirrors