The largest of three experimental sound mirrors built in the 1930s on the coast at Denge, Kent, England, for air defence. These were used to detect incoming enemy aircraft by listening for the sound of their engines. This mirror formed a large horizontal arc about 5 m high and 70 m long. In use, a series of microphones were placed at the focus in front so that the direction of the sound detected could be determined. The wall has a circular rather than parabolic curve, so sounds coming from different directions come to a focus at different points. Standing at the focus today can be quite a surreal experience - the sound of passing trains on the nearby Romney, Hythe & Dymchurch Railway is massively amplified and the direction of the sound is mirrored to the opposite direction from which it comes! This was the last functional mirror built at this site and was made obsolete by the advent of radar during World War 2.
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