The AM Forum

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 mirrors were built in the 1920s as an experimental early warning system for incoming aircraft. Several were built along the south and east coasts, but the complex at Denge is the best preserved.There are three acoustic mirrors in the complex, each consisting of a single concrete hemispherical reflector.

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 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 in Malta.

http://www.artificialowl.net/2009/03/abandoned-acoustic-mirrors-denge-uk.html