The Sound mirrors at RAF Denge are located between Greatstone-on-Sea and Lydd airfield, Kent, on the edge of a disused gravel pit. They were built in the late 1920s and early 1930s as part of an early warning system for detecting hostile incoming aircraft by focusing sound waves onto a central microphone.
The Denge complex is the best-preserved of several along the south coast. It comprises three mirrors:
1) The 200 foot mirror – a near vertical, curved wall, 60 m (200 ft) in length.
2) The 30 foot mirror – a circular dish, 9 m (30 ft) across, supported on concrete buttresses, which retains a microphone pole at its centre.
3) The 20 foot mirror is similar to the 30 foot mirror, but with a smaller dish 6 m (20 ft) across.
The mirrors were capable of detecting the slow-moving aircraft of the period before they came into range, but were rendered obsolete by the invention of radar. They were abandoned and left to decay, though they remain reasonably well-preserved and are now scheduled under the Ancient Monuments and Archaeological Areas Act 1979.