In the Denge region, in Kent, England, the coastal landscape is filled with strange concrete constructions that look both decadent and futuristic. The explanation is simple. The place harboured, at the beginning of last century, a RAF base, where the first experiments of aerial detection were made, and the strange constructions were actually collossal acoustic mirrors, thatnow abandoned.
It all started around 1915, when the Royal Air Force decided to create a defense system that would detect the aerial aproximation to the English territory. For that, a huge network of listening sites were placed along the littoral border with the English Channel that had the mission of listening to possible sounds that would come from airplane engines. Each post was concieved as a concrete mega-structure, with reflectors with a parabolic shape turned towards the sea, which would redirect the sound to the microphones located in the focal point. It was basicly the same priciple that would go into the creation of today's parabolic antennas.
The system is impressive and reveals a great visionary ability, mostly if we take into account that aeronautics, acoustics and the use of concrete were still emerging at this point. And don't think it wasn't effective - because it was.
Throughout a decade, several of these posts were built along the English coast until the system was improved, in 1930. A new type of reflector was invented, more comprehensive and precise: a hemispheric wall, 60 metres wide and 10 metres tall. The detection ability of this new infrastructure was amazing. A person located at the focal point would be able to listen to the sound of an airplane engine 10 Km away and, with the help of microphones and ampliphiers, the range would go up to 32 Km!
The original plan was to build acoustic mirrors of this kind for every 40 km along the coastal line, with smaller parabolic reflectors in between. However, in the mid-1930s, the invention of radar and the rising speed of airplanes rendered this system obsolete. The military soon abandoned this idea, but left the posts built intact. The Denge complex is one of the best kept ones today and includes three reflectors, two parabolics, with a 6-metre and 9-metre radius each, and a huge 60-metre wall.