Sound Mirrors 25 years old

Dean filmed huge concrete, curved structures built in Dungeness in the UK, which date back to the 1920s. They were constructed as a pre-radar sound tracking system, where the curved concrete structures gathered sound which were listened to by people with a sort of stethoscope, the aim of which was to give warning of hostile aircraft attacks. Since it was impossible to distinguish between hostile sounds and other sounds, the experiment was abandoned and in any case the entire installation became obsolete because of the development of radar.