Acoustic mirror

From Wikipedia, the free encyclopedia

"Sound mirrors" redirects here. For the album by Coldcut, see Sound Mirrors.

An acoustic mirror is a device used to focus and amplify sound waves.

1 Overview
2 See also
3 References
4 External links

[edit] Overview

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 and at Hythe in Kent. Other examples exist in other parts of Britain (including Sunderland, Redcar, Boulby and Kilnsea) and in Malta.

The Dungeness mirrors, known colloquially as the "listening ears", consist of three large concrete reflectors. 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 20 miles on a good day), they could also detect direction. 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, while 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. However, there were some additional long lasting benefits to come out of the program.

The acoustic mirror program gave Britain the methodology by which to use interconnected stations to pin point the position of an enemy in the sky. The system developed by the acoustic mirror team (lead by Dr Tucker) for linking the ranging stations together and plotting aircraft movements by using more than one station was given to the early radar team and led to their success in WW2. The British radar was less sophisticated than the German system, yet the British one was used more successfully. That is because of all the work done by the totally unacknowledged acoustic mirror team.

Acoustic mirrors are used today to amplify sounds of players and coaches for broadcast at live athletic competitions, where use of conventional microphones would be too intrusive. They are also used as novelty items — "whisper dishes" — in science museums to allow patrons to whisper across long distances, for example at Ontario Science Centre.