Acoustic mirror


Image:WW1AcousticMirrorKilnseaJan2007.jpg|right|thumb|300px| WW1 concrete acoustic mirror near Kilnsea Grange, East Yorkshire, UK. The pipe which held the "collector head" can be seen in front of the structure.
An acoustic mirror is a passive device used to reflect and focus sound waves. Parabolic acoustic mirrors are widely used in parabolic microphones to pick up sound from great distances, employed in surveillance and reporting of outdoor sporting events. Pairs of large parabolic acoustic mirrors which function as "whisper galleries" are displayed in science museums to demonstrate sound focusing.
Between the World Wars, before the invention of radar, parabolic sound mirrors were used experimentally as early-warning devices by military air defence forces to detect incoming enemy aircraft by listening for the sound of their engines.
During World War II on the coast of southern England, a network of large concrete acoustic mirrors was in the process of being built when the project was cancelled owing to the development of the Chain Home radar system. Some of these mirrors are still standing today.

Acoustic aircraft detection

Image:ListeningEarsMar2005.jpg|thumb|right|Acoustic mirrors at RAF Denge
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 high by long, while the other two are dish-shaped constructions approximately 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 can be used for direction finding by moving the sensor rather than the mirror; another unusual example was the Arecibo Observatory.

Modern uses

Image:Whispering dish, Jodrell Bank.jpg |thumb|right|A parabolic whispering dish that is part of an installation at Jodrell Bank Observatory
Image:Echobos Muiderberg Akoestische spiegels.jpg|thumb|A pair of smaller acoustic mirrors in a park in Muiderberg
Parabolic acoustic mirrors called "whisper dishes" are used as participatory exhibits in science museums to demonstrate focusing of sound. Examples are located at Bristol's We The Curious, Ontario Science Centre, Albuquerque's ¡Explora!, Baltimore's Maryland Science Center, Oklahoma City's Science Museum Oklahoma, San Francisco's Exploratorium, the Science Museum of Minnesota, the Museum of Science and Industry in Chicago, Pacific Science Center in Seattle, Jodrell Bank Observatory, St. Louis Science Center, Parkes Observatory in Australia and on the north campus lawn of North Carolina State University.
Small portable parabolic microphones are used to record wildlife sounds such as bird song, in televised sports events to pick up the conversations of players, such as in the huddle during American Football games, or to record the sounds of the sport, and in audio surveillance to record speech without the knowledge of the speaker.

Locations

Acoustic aircraft detection mirrors are known to have been built at:
Modern acoustic mirrors built for entertainment