page_title: Acoustic mirror
section_title: Modern uses
hierarchical_section_title: Acoustic mirror / Modern uses
caption_attribution_description: English: Whispering dish, Jodrell Bank Observatory.
context_page_description: 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.
context_section_description: 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, 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.  
  A pair of dishes, typically 2 to 3 m (7 to 10 ft) in diameter, is installed facing each other, separated by around a hundred metres.  A person standing at the focus of one can hear another person speaking in a whisper at the focus of the other, despite the wide separation between them.
  Parabolic microphones depend on a parabolic dish to reflect sound coming from a specific direction into the microphone placed at the focus.  They are extremely directional: sensitive to sounds coming from a specific direction.  However, they generally have poor bass response because a dish small enough to be portable cannot focus long wavelengths.    
  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.
caption: A parabolic whispering dish that's part of an installation at Jodrell Bank Observatory.
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