World War One was the first major conflict that saw the use of airplanes. During the early years of the fighting, aircraft were mostly used for surveillance and observation, but as the war progressed, airplanes took on an offensive role. By the end of the war, airplanes had become so deadly that it became necessary for the Allies to develop new tactics for air defense, if the next war was to be won.

The most important ingredient of an effective air defense system is an early warning system that could detect and track enemy aircraft before they arrive. But in the days before radar, the only way to detect enemy aircraft was to listen to the sound of approaching airplanes.

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The Royal Air Force established a series of listening posts around the coasts of Great Britain. They consisted of huge spherical, concrete reflectors called “sound mirrors” that were able to amplify the noise of airplane engines as they approached over the English Channel, by reflecting sound waves off their curved surfaces and concentrating them to a focal point, like rays of light reflecting off a curved mirror. Devices like this are often found in science museums today as “whispering galleries”.

Several microphones were placed in front of the reflector and depending on which microphone received the strongest signal, the direction of approach of the plane could be determined.

By the end of the 1920s, the first sound mirror went up at Hythe, in Kent, on the south coast of England. The location was chosen because it fell under the flight path of commercial aircraft heading towards France, thereby providing sufficient test subjects. At first, there were five concave mirrors made of steel and concrete, each six to nine meters high. These mirrors could efficiently amplify sound waves having wavelength up to three feet. But the sound waves the military was interested in had wavelengths of 15 to 18 feet.

In 1930, the sixth and final mirror was erected. It was a huge curved wall, 60 meters long and 8 meters high. With a listening range of about 20 miles, it was this mega-mirror that finally delivered the required results.

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At once construction of sound mirrors started along the coast from Norfolk down to Dorset, to make it impossible for enemy planes to approach the UK from the English Channel undetected. In addition to the mirrors, a huge team of more than 500 people were trained for rapid and accurate communications required between the listening post, telephone operators and officers in the control room and at headquarters.

But airplanes were becoming faster, which meant that by the time they were detected they would already be too close to deal with. Soon doubts about the sound mirror project began to arise. Then in 1935, the project came to an abrupt end. The alternative to the sound mirrors — Radio Detection and Ranging, or radar for short— had been perfected.

The concave sound mirrors, that never saw action, still stands around the English coast. The most famous of these mirrors are at Denge, near Dungeness, in Kent, England. The site has three mirrors —a 60-meter-long curved wall, a 9-meter circular dish and a 6-meter dish.

Sound mirrors stand in at least 19 different places, including Malta which is the only place outside Great Britain to have these structures.

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Photo credit: Greg/Flickr

Photo credit: Paul/Flickr

Photo credit: Paul/Flickr

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Sources: Wikipedia / www.andrewgrantham.co.uk / www.spiegel.de