Before Radar, they used these giant concrete “Sound Mirrors” to detect incoming enemy aircraft

 
 
 
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Scottish physicist Sir Robert Watson-Watt invented the first practical radar system back in  1935. The system used pulsed radio waves, and it could detect airplanes up to 100 miles away.

A few years later, a chain of radar stations was established throughout England, which helped defend the British and played a crucial role in the Allied victory in World War II. It was a huge discovery and the inventor Sir Robert Watson-Watt was knighted for it in 1942.

But what about the period before the radar was invented? What was the forerunner of radar and how the British defended themselves from the German Zeppelins during World War I?

Acoustic mirrors at Denge. Photo Credit
4.5 meter high (14ft 9in) WW1 concrete acoustic mirror near Kilnsea Grange, East Yorkshire, UK. Photo Credit

Dr. William Sansome Tucker developed early warning systems known as ‘acoustic mirrors’ around 1915, and up until 1935, Britain built a series of concrete acoustic mirrors around its coasts. The acoustic mirror was the forerunner of radar, and it was invented to help detect zeppelins and other enemy aircraft by the sound of their engines.

The British used these devices and with their help, they managed to detect many enemy raids. The acoustic mirrors could detect an incoming aircraft up to 15 miles away, which gave English artillery just enough time to prepare for the attack of the German bombers.

Kent acoustic mirror. Photo Credit
The farmland has been ploughed around the mirror. The rusty pole in front of the mirror had a microphone attached to it when listening for aircraft in the First World War. Photo Credit
Round acoustic mirror in Denge, Kent, England, 19 July 2009. Photo Credit

Many of these strange looking devices remain on Britain’s coastlines, more or less intact. They can be seen in many places throughout the country, including Hartlepool, Seaham, Redcar, Sunderland, Dover, Romney Marsh and Selsea. However, some of the finest examples of acoustic mirrors can be seen on the Dungeness peninsula and at Hythe in Kent.

On the former Royal Air Force site near Dungeness, in Kent, England, there are three acoustic mirrors built between the 1920s–1930s; one of them is a 70m (230 ft) curved wall, around 5m high, while the other two are dish-shaped around 5m in diameter.

Sound Mirrors at Denge. Photo Credit
WW1 Acoustic Mirror Kilnsea. Photo Credit

Hot-wire microphones designed personally by Dr. William Sansome Tucker that were placed at the focus of each structure made it possible for the listeners to detect the sound of oncoming enemy aircraft and gave a fifteen-minute warning to the English artillery to prepare for the attack. Some of the most sophisticated acoustic mirrors could detect the sounds of oncoming enemy aircraft up to 25 miles (40km) away.

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As aircraft performance increased, its sound could no longer be heard or located 15 minutes before it reached its target, and that is why acoustic mirrors were no longer useful and were replaced by the newly developed radar technology.