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The Sound Mirrors, also known as Acoustic Mirrors, Concrete Dishes or Listening Ears, are large concrete structures designed as an early warning system for Britain to detect enemy aircraft. There were three designs of mirrors and all three can be seen in Greatstone, Kent.
The three concrete "listening ears" at Greatstone range in size from 20 to 200 feet in size. Built between 1928-30, the sound mirrors were part of Britain's national defence strategy. They were designed to pick up the sound of approaching enemy aircraft. Sound waves were caught in the belly of the mirror and relayed back through microphones and a stethoscope to an operator who raised the alarm. Anti-aircraft defences were then deployed. The mirrors effectively gave Britain a fifteen-minute warning of an impending attack.
1928 The 20 feet diameter mirror was the first to be built and was precast as one huge slab of curved concrete.
1930 Lessons were quickly learnt and a 30 feet diameter mirror, set at a different angle and providing greater accuracy, was built alongside.
1930 The third mirror was 200 feet in length and 26 feet high, and was built alongside the other two smaller sound mirrors.
Microphones were attached to the curved surfaces and in favourable conditions could pick up the sounds of aircraft up to 24 miles away.
The mirrors did work, and could effectively be used to detect slow moving enemy aircraft before they came into sight. They worked by concentrating sound waves towards a central point, where the microphone would have been located. (Note the microphone in the pictures of the 30 foot mirror). However, their use was limited as aircraft became faster. Operators also found it difficult to distinguish between aircraft and seagoing vessels.
They quickly became obsolete due to the invention of radar in 1935, being finally abandoned in 1939.