Sound Mirrors Used During Wartime

World War II gave us many advances in technology, but one of the lesser known things that is directly related to echolocation is the “Sound Mirror” At first they seem baffling and quite strange to uninitiated onlookers, but when you understand what they were used for and how they work it's really quite simple yet quite remarkable.

Rising a good twenty feet off the ground and composed of concrete somewhere near the Greatstone Lakes, these structures that highly resemble some kind of ancient wonder built by a prehistoric kingdom are collectively known as Acoustic Mirrors, Concrete Dishes, Listening Ears and, of course, Sound Mirrors. And no, contrary to whatever impressions come to mind they were most assuredly not objects of worship, they were actually built by the British government in conjunction with the military so as to provide an early warning system against enemy air-strikes. Riddled with microphones and other listening equipment, the sound mirrors provided people with a 15 minute warning ahead of time in the event of an enemy attack. Simple but very effective, this allowed Britain and its citizens time enough to evacuate while anti-aircraft weapons are prepared for the eventual arrival of the enemy.

The interesting part here is to note that these are objects designed for sound to bounce off of them.  There aren't too many objects like that.  They acted to focus a small amount of sound energy (over the area of the face of the 30' dish) into a focal point, just like a radio telescope of satellite TV dish.  Somewhere in the center of the dish is where all of the sound would be focused.  If you were to put your head in that central focal point, which is where the microphones would have been placed, you would be able to hear all of the sounds coming into the dish.

An interesting concept when it comes to picturing how echolocation works!