Acoustic Mirrors – Before the Invention of the Radar
November 23, 2012
First images: Denge, a site of the British Royal Air Force near the Channel, where was built experimental acoustic mirrors in the 1920s and 1930s. Their experimental nature can be discerned by the different shapes of each of the three reflectors: one is a long, curved wall about 5 m high by 70 m long, while the other two are dish-shaped constructions approximately 4–5 m in diameter. Microphones placed at the foci of the reflectors enabled a listener to detect the sound of aircraft far out over the English Channel. The reflectors are not parabolic, but are actually spherical mirrors. Spherical mirrors may be used for direction finding by moving the sensor rather than the mirror; another unusual example is the Arecibo Observatory.
An acoustic mirror is a passive device used to reflect and to focus (concentrate) sound waves. it had a limited effectiveness, and the increasing speed of aircraft in the 1930s meant that they would already be too close to deal with by the time they had been detected. The development of radar put an end to further experimentation with the technique.
These listening devices were created by the Dutch during World War II to detect incoming air attacks in advance.