Sound mirror

An acoustic mirror is a passive device used to reflect and perhaps to focus (concentrate) sound waves. 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.

Between the World Wars, 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 2 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. Many of these mirrors are still standing today.

Before 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 (16 ft) high by 70 m (230 ft) long, while the other two are dish-shaped constructions approximately 4–5 m (13–16 ft) 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 can be used for direction finding by moving the sensor rather than the mirror; another unusual example is the Arecibo Observatory; see also

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