Acoustic mirror
From Biocrawler, the free encyclopedia.
An acoustic mirror is a device used to focus and amplify sound waves.
Prior to World War II and the invention of radar, acoustic mirrors were built as early warning devices around the coasts of Britain, with the aim of detecting airborne invasions. The most famous of these devices still stand at Denge on the Dungeness peninsula in Kent. Other examples exist in other parts of Britain (including Sunderland, Marske, Boulby and Kilnsea) and in Malta.
The Dungeness mirrors, known colloquially as the "listening ears", consist of three large concrete parabolic reflectors. Microphones were placed at the foci of the parabolas that enabled a listener to detect the sound of aircraft several kilometres out in the English Channel. The listening ears were built in the 1920s–1930s, and 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, the other two are dish-shaped constructions approximately 4-5 m in diameter.
Acoustic mirrors had a limited effectiveness, and the increasing speed of aircraft in the 1930s meant that by the time they had been detected, they would already be too close to deal with. The development of radar finally put an end to further experimentation with the technique.
They are today used to amplify sounds of players and coaches for broadcast at live athletic competitions, where use of conventional microphones would be too intrusive. They are used as novelty items — "whisper dishes" — in science museums to allow patrons to whisper across long distances [1] (http://www.ontariosciencecentre.ca/plan/services/assets/accessguide.en.pdf).
External links
Dungeness acoustic mirrors (http://www.ajg41.clara.co.uk/mirrors/dungeness.html)
