Wandering along the beaches of the county of Kent in southern England, the attentive tourist may notice concrete parabolas several meters in diameter. You might walk past what looks like the creation of an eccentric artist or an alien speedster, but make no mistake: it’s actually a military device.
Built around the Channel between the wars, the “acoustic mirrors” are quite simply artificial ears. Their structure adopts a concave shape used to amplify the sound produced by distant devices and to concentrate it towards a microphone, placed in its center. (It’s the same principle as when you wrap your hand around your ear and think you’re listening to the sea.) The goal of the maneuver: to spot enemy aircraft before it’s too late to alert the anti-aircraft units, and thus guard against lightning bombings that cost lives.
The lessons of the Great War have been learned. Certainly, in 1914, aviation did not yet have the level of sophistication that it would reach in 1940: the aircraft were mainly used for reconnaissance and the first combat planes carried pilots armed with machine guns, grenades or grappling hooks! On the ground, French units use giant “acoustic horns” to detect them. But their reliability often leaves something to be desired…
However, the improvement of bombers and fighter planes, as well as their exponential speed, was enough to prove that the next war would no longer be fought on the ground. “Gone are the days when armies on land or navies at sea could be the arbiters of a nation’s destiny in time of war,” comments Brigadier General Billy Mitchell, father of the US Air Force, in 1918.
This is how, in the 1920s and until the beginning of the 1930s, we saw “human radars” harnessing themselves to acoustic mirrors, headphones over their ears, in order to detect suspicious noises picked up by the device. In the best case scenario, if the intuition of an “ear” proved correct, then the Zeppelin bomber should be located about thirty kilometers from the coast in good weather.
Military command then had fifteen minutes in order to prepare anti-aircraft defenses. Selected for their hearing and concentration, trained to the different noises produced by aircraft engines, human radars make a thunderous entry onto the runways of the Royal Air Force or the US Air Force… But they all take off suddenly in 1938: radar had just been invented.
Cracked by time and covered in tags, the “giant ears” of the Channel, already a hundred years old, continue to listen to the sound of the waves. They will still inspire the creation of acoustic satellites intended to detect extraterrestrial noises. Human radars are therefore still active… listening to the little green men!
Source: www.slate.fr
