Trip Start Nov 14, 2007
29Trip End May 18, 2011
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A forerunner of Radar, acoustic mirrors were built on the south and northeast coasts of England between about 1916 and the 1930s. The ‘listening ears’ were intended to provide early warning of incoming enemy airplanes and airships about to attack coastal towns. With the development of faster aircraft the sound mirrors became less useful, as an aircraft would be within sight by the time it had been located, and radar finally rendered the mirrors obsolete
Denge is a former Royal Air Force site near Dungeness, in Kent, England. It is best known for the early experimental acoustic mirrors which remain there. The acoustic mirrors, known colloquially as 'listening ears', at Denge are located between Greatstone-on-Sea and Lydd. Several were built along the south and east coasts, but the complex at Denge is the best preserved. The mirrors were built in the 1920s as an experimental early warning system for incoming aircraft, developed by Dr William Sansome Tucker.
Acoustic mirrors did work, and could effectively be used to detect slow moving enemy aircraft before they came into sight. They worked by concentrating sound waves towards a central point, where a microphone would have been located. However, their use was limited as aircraft became faster. Operators also found it difficult to distinguish between aircraft and seagoing vessels. In any case, they quickly became obsolete due to the invention of radar in 1932. The experiment was abandoned, and the mirrors left to decay. The gravel extraction works caused some undermining of at least one of the structures.There are three acoustic mirrors in the complex, each consisting of a single concrete hemispherical reflector.
- The 200 foot mirror is a near vertical, curved wall, 200
feet (60m) long. It is one of only two similar acoustic mirrors in the
world, the other being in Maghtab, Malta.
- The 30 foot mirror is a circular dish, similar to a deeply
curved satellite dish, 9m (30 ft) across, supported on concrete
buttresses. This mirror still retains the metal microphone pole at its
- The 20 foot mirror is similar to the 30 foot mirror, with a
smaller, shallower dish 6m (20 ft) across. The design is close to that
of an acoustic mirror in Kilnsea, East Riding of Yorkshire.