On the southeast coast of England are a set of massive concrete acoustic mirrors — or “listening ears — that were built after World War I to monitor the skies for the telltale sounds of an airborne invasion. Constructed between 1927 and 1930, the Denge Sound Mirrors were part of Britain’s national defense strategy. Their shape collected and magnified sound waves in the air over the English Channel and directed them at a microphone positioned just in front of the structure. Anti-aircraft defenses were then deployed. The mirrors effectively gave Britain a 15-minute warning of an impending attack. The site features three different reflectors, including a 200-foot-long curved wall, a 30-foot-tall curved dish, and a 20-foot-tall shallow dish. Once the receivers were adjusted for the best reception, the results would be compared with those from one of the other “ears" and used to calculate an aircraft’s height, speed and flight path. They worked best at a range of between 8 and 24 miles, but by the time the wall was ready in 1930, technology was already on its way towards radar. Today the site is open to the public from August through December, with guided tours provided by the Royal Society for the Protection of Birds.
