The Denge sound mirrors near Lydd on Sea (Kent) were built in the late 1920's as a way to detect as yet 'out-of-sight' aircraft approaching from across the English Channel. The large concave concrete surface of each mirror was designed to collect sound at the center of the dish where a microphone would then amplify it to a listening operator. There are three acoustic mirrors here, the smallest 20' dish, a larger 30' dish and the vast 200' long concave wall. The mirrors worked well and were a precursor technology to radar which emerged in 1932, as soon as radar arrived though the benefits greatly outweighed that of the mirrors and they quickly became obsolete.
The mirrors are an important piece of wartime history and exist on an island surrounded by a flooded gravel pit and nature reserve that is run by the RSPB. Access to the mirrors on certain days can be arranged through the RSPB.
The fantastically desolate and otherworldly shingle beach of Dungeness with its ruined fishing boats and huts is just 2 miles down the road and so a trip here with your camera is most definitely worth the effort.