Anechoic

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 often isolated from waves entering from their surroundings. This combination means that a person or detector exclusively hears direct sounds (no reverberant sounds), in effect simulating being inside an infinitely large room.

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 RF 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.

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, some humans may not like such quietness and can become disoriented.

The mechanism by which anechoic chambers minimize the reflection of sound waves impinging onto their walls is as follows: In the included 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 (bounded by 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.

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