Answer: Acoustic mirrors
If you walk along the coast of Britain, you will find weird concrete structures that iron the coast, as the closures of an earlier civilization. These curved concrete walls and concrete half-fields are the acoustic mirrors-the remains of Britain's ambitious flight detection system of the First World War.
Before the advent of radar-based aerial port detection, the only way to detect an aircraft was to either see it (at which point it was almost on top of you) or hear it (which, due to the limitations of human hearing, was almost as bad as waiting for to see it). The acoustic reflects hyper-elevated human hearing, however, by focusing distant sound waves on a single point where a microphone was mounted. Soldiers could listen to incoming aircraft and let the alarm out interceptors.
The concrete acoustic mirrors worked well enough for what they were (and the age they were constructed for the first time), but the increasing speed of aircraft was made them obsolete in the mid-1