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Concept
Anechoic chamber
Summary
An anechoic chamber (an-echoic meaning "non-reflective" or "without echoes") is a room designed to stop reflections or echoes of either sound or electromagnetic waves. They are also often isolated from energy entering from their surroundings. This combination means that a person or detector exclusively hears direct sounds (no reflected sounds), in effect simulating being outside in a free field.
Anechoic chambers, a term coined by American acoustics expert Leo Beranek, were initially exclusively used to refer to acoustic anechoic chambers. Recently, the term has been extended to other radio frequency (RF) and sonar anechoic chambers, which eliminate reflection and external noise caused by electromagnetic waves.
Anechoic chambers range from small compartments the size of household microwave ovens to ones as large as aircraft hangars. The size of the chamber depends on the size of the objects and frequency ranges being tested.
The requirement for what was subsequently called an anechoic chamber originated to allow testing of loudspeakers that generated such intense sound levels that they could not be tested outdoors in inhabited areas.
Anechoic chambers are commonly used in acoustics to conduct experiments in nominally "free field" conditions, free field meaning that there are no reflected signals. All sound energy will be traveling away from the source with almost none reflected back. Common anechoic chamber experiments include measuring the transfer function of a loudspeaker or the directivity of noise radiation from industrial machinery. In general, the interior of an anechoic chamber can be very quiet, with typical noise levels in the 10–20 dBA range. In 2005, the best anechoic chamber measured at −9.4 dBA. In 2015, an anechoic chamber on the campus of Microsoft broke the world record with a measurement of −20.6 dBA. The human ear can typically detect sounds above 0 dBA, so a human in such a chamber would perceive the surroundings as devoid of sound. Anecdotally, some people may not like such silence and can become disoriented.
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