December 10, 2013
by fosco lucarelli
A sound mirror (or acoustic mirror) is a device employed to reflect and concentrate sound waves. An experimentation on large scale acoustic mirrors has been run after WWI in Great Britain to detect incoming enemy aircrafts taking off on the other side of the English Channel. By enhancing the sound of enemy aircraft’s engines, the mirrors would have enabled the British defense to detect incoming airplanes fifteen minutes before they were visible.
Built between 1927-30, the sound mirrors were part of Britain’s national defense strategy, but they became rapidly obsolete due to the rapid increment in speed of the aircrafts and the invention of radar. The most famous examples of acoustic mirrors are preserved in Denge on the Dungeness peninsula in England. Three different kinds existed: two made of dishes of different proportions and one, a 70 m long curved wall. Headphones were placed at the foci of the reflectors enabling a listener to detect the sound of an aircraft.