Denge, a former Royal Air Force site in Kent, England, is best known for the early experimental acoustic mirrors that were originally built in the 1920s. Used as an early warning system, they could effectively detect slow moving enemy aircrafts before they came into sight. They worked by concentrating sound waves towards a central point, where a microphone would have been located. Their use was limited as aircrafts became faster and eventually became obsolete with the invention of radar in 1932.
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