Acoustic Mirrors, Precursor of Radars…
Mirrors are fascinating. During my school days, I was greatly enamoured by the physics of mirrors. Mirrors seemed to have their own parallel universe. And when we used to put a ball between two parallel mirrors and observed infinite reflections, it was Hogwarts all over, never mind the physics.
However, what we are discussing today are not traditional mirrors, but acoustic sound mirrors. They were used as enemy aircraft detection devices before the advent of radars. Invented by Major William Sansom Tucker, these were spherical acoustic mirrors put up in coasts across Great Britain in the early 20th century, notably in Kent, Sunderland, Redcar and Boulby. These mirrors consisted of reflectors and a microphone at the foci of the reflectors which enabled a listener to listen to the sound of incoming enemy aircrafts. They had the ability to reflect and focus sound waves from great distances.
As fighter aircrafts with more speeds came up during 1930s, use of these mirrors for their detection became impractical as very little time was left after detection to take necessary preventive action. Invention of radar put an end to more research and deployment of such mirrors.
There are still a lot of such mirrors strewn over the English countryside. Will be worth a sight, a little known but revolutionary piece of relatively recent history!