During the 1930’s, the United Kingdom wanted to reinforce its defense system against the growing antagonism that would end up in the Second World War. That is how they invented the RADAR (RAdio Detection And Ranging) but before such achievements, they experimented an architectural system on the Kent coast that would allow an early warning of potential enemy planes and bring an idea of their directions. Those monumental sound mirrors were reflecting sound into a microphone that was able to determine the direction of the enemy aircraft by determining which area has reflected sound the most.
In order to know more about them -which were eventually gave up before the beginning of the Blitz- you can read the article Listening for the Enemy written by Solveig Grothe for German magazine Der Spiegle.
Thank you Carla.