During the first World War, the Royal Air Force built gigantic acoustic mirrors, known colloquially as 'listening ears', that served as an early warning system for incoming air invasion. The parabolic shape of the mirrors collected and magnified sound waves in the air over the English Channel and directed them at a microphone positioned just in front of the parabola. The mirrors effectively gave Britain a fifteen-minute advance warning of an impending attack.
Acoustic mirrors were built at several places around Great Britain, but the ones at Denge are the best preserved. There are three acoustic mirrors in Denge - a 200 foot near vertical, curved wall, a 30 foot circular dish shaped mirror and another 20 foot mirror.
Acoustic mirrors did work, and could effectively be used to detect slow moving enemy aircraft before they came into sight. However, their use became limited as aircraft became faster. Finally in 1932, the invention of radar rendered the acoustic mirrors obsolete.
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