During the 1930s, and leading up to World War II, the United Kingdom built a network of giant "acoustic mirrors" across its southern and eastern coast. The concrete structures, which differed in height and length, were an experimental early warning system. Built in the shape of spherical mirrors, they could reflecting and focus sound waves. Using them, military air defense forces could detect incoming enemy aircraft by listening for the sound of their engines.
The experimental nature of acoustic mirrors 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. Using microphones placed at the foci of the reflectors enabled a listener to detect the sound of aircraft far out over the English Channel. However, the increasing speed of aircraft during the 1930s, meant that they were detected when they were already too close (the system's range was about 25 miles) to deal with them.
With the development of the Chain Home radar system at the beginning of World War II, the acoustic mirrors project was cancelled as it was now obsolete. Many of the acoustic mirrors built, stand till this day in coastal areas like at Denge on the Dungeness peninsula and at Hythe, Kent. Other examples exist in other parts of Britain (including Sunderland, Redcar, Boulby, Kilnsea). The only acoustic mirror constructed outside the UK, was built in Baħar iċ-Ċagħaq in Malta.
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