Acoustic Mirrors at Denge, a site of the British Royal Air Force near the Channel, where some experimental models were built in the 1920s and 1930s. One is a long, curved wall about 5 m high by 70 m long. The other two are spherical and approximately 4–5 m in diameter. Acoustic mirrors were used to reflect and to concentrate sound waves. Microphones placed at the foci of the reflectors enabled a listener to detect the sound of aircraft approaching. The microphones could be moved for direction finding.
These Acoustic mirrors were built to detect the sound of aircraft engines far out over the Channel but the increasing speed of aircraft in the 1930s meant that they had a limited effectiveness. Eventually, the development of radar technology put an end to its use.