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anechoic chamber
An anechoic chamber ("an-echoic" meaning non-reflective, non-echoing or echo-free) is a room designed to completely absorb reflections of either sound or electromagnetic waves.
They are also insulated from exterior sources of noise. The combination of both aspects means they simulate a quiet open-space of infinite dimension, which is useful when exterior influences would otherwise give false results.
Anechoic chambers, a term coined by American acoustics expert Leo Beranek, were originally used in the context of acoustics (sound waves) to minimize the reflections of a room. More recently, rooms designed to reduce reflection and external noise in radio frequencies have been used to test antennas, radars, or electromagnetic interference.
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 to be tested and the frequency range of the signals used, although scale models can sometimes be used by testing at shorter wavelengths (higher frequencies).
==Acoustic anechoic chambers==
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 is 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, humans do not like such quietness and are disoriented.〔]
The mechanism by which anechoic chambers minimize the reflection of sound waves impinging onto their walls is as follows. In the adjacent figure an incident sound wave I is about to impinge onto a wall of an anechoic chamber. This wall is composed of a series of wedges W with height H. After the impingement, the incident wave I is reflected as a series of waves R which in turn "bounce up-and-down" in the gap of air A (encircled with dotted lines) between the wedges W. Such bouncing may produce (at least temporarily) a standing wave pattern in A. During this process, the acoustic energy of the waves R gets dissipated via the air's molecular viscosity, in particular near the corner C. In addition, with the use of foam materials to fabricate the wedges, another dissipation mechanism happens during the wave/wall interactions. As a result, the component of the reflected waves R along the direction of I that escapes the gaps A (and goes back to the source of sound), denoted R', is notably reduced. Even though this explanation is two-dimensional, it is representative and applicable to the actual three-dimensional wedge structures used in anechoic chambers.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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