Acoustic mirror

:"Sound mirrors" redirects here. For the album by Coldcut, see Sound Mirrors."An acoustic mirror is a passive device used to reflect and perhaps to focus (concentrate) sound waves.

Overview

Prior to World War II and the invention of radar, acoustic mirrors were built as early warning devices around the coasts of Great Britain, with the aim of detecting airborne invasions. The most famous of these devices still stand at Denge on the Dungeness peninsula and at Hythe in Kent. Other examples exist in other parts of Britain (including Sunderland, Redcar, Boulby, Kilnsea) and Selsey Bill, and in Malta.

The Dungeness mirrors, known colloquially as the "listening ears", consist of three large concrete reflectors built in the 1920s–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 several kilometres out in the English Channel. The reflectors are not parabolic as sometimes imagined, but are in fact hemispherical mirrors. This design element is their genius, because in addition to being able to detect range (over 30 km, or 20 statute miles, on a good day), they could also detect direction.

Acoustic mirrors 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. Nevertheless, there were long-lasting benefits. The acoustic mirror programme, led by Dr William Sansome Tucker, had given Britain the methodology to use interconnected stations to pin point the position of an enemy in the sky. The system they developed for linking the ranging stations and plotting aircraft movements was given to the early radar team and contributed to their success in WW2; although the British radar was less sophisticated than the German system, the British system was used more successfully.

Acoustic lenses similar to acoustic mirrors are used today as novelty items — "whisper dishes" — in science museums to allow patrons to whisper across long distances, for example at Ontario Science Centre and San Francisco's Exploratorium [ [http://www.exploratorium.edu/cmp/exhibits/l/listening_vessels.html Exploratorium "Listening Vessels" exhibit] ] . Such whisper dishes must be parabolic for greatest effect.

Parabolic microphones appear to use acoustic mirror properties but instead depend on a parabolic dish to reflect sound coming from a specific direction into the microphone placed at the foci. Because of their small, portable size, they can easily be used in the same manner as acoustic mirrors for detection and direction finding of distant noise sources.

See also

*Japanese War Tuba
*Sound ranging

References

*Richard Newton Scarth, "Echoes from the Sky: A Story of Acoustic Defence" (Hythe Civic Society, 1999) (ISBN 1-900101-30-0)This book is now out of print (January 2007) - Hythe Civic Society

External links

* [http://www.ajg41.clara.co.uk/mirrors/ Acoustic mirrors in Britain]
* [http://www.youtube.com/watch?v=qsR3qyJDk0c Blackout] Sound Mirror film by art group Disinformation, 1997
* [http://www.discogs.com/release/117617 Antiphony] Sound Mirror imagery from the Disinformation 2xCD "Antiphony", 1997
* [http://www.dself.dsl.pipex.com/MUSEUM/COMMS/ear/ear.htm Military acoustic locators]