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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Anechoic chamber | 1/2 | https://en.wikipedia.org/wiki/Anechoic_chamber | reference | science, encyclopedia | 2026-05-05T09:03:19.715865+00:00 | kb-cron |
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 radio frequency (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.
== Acoustic anechoic chambers ==
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. 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.
=== Semi-anechoic and hemi-anechoic chambers === Full anechoic chambers aim to absorb energy in all directions. To do this, all surfaces, including the floor, need to be covered in correctly shaped wedges. A mesh grille is usually installed above the floor to provide a surface to walk on and place equipment. This mesh floor is typically placed at the same floor level as the rest of the building, meaning the chamber itself extends below floor level. This mesh floor is damped and floating on absorbent buffers to isolate it from outside vibration or electromagnetic signals. In contrast, semi-anechoic or hemi-anechoic chambers have a solid floor that acts as a work surface for supporting heavy items, such as cars, washing machines, or industrial machinery, which could not be supported by the mesh grille in a full anechoic chamber. Recording studios are often semi-anechoic. The distinction between "semi-anechoic" and "hemi-anechoic" is unclear. In some uses they are synonyms, or only one term is used. Other uses distinguish one as having an ideally reflective floor (creating free-field conditions with a single reflective surface) and the other as simply having a flat untreated floor. Still other uses distinguish them by size and performance, with one being likely an existing room retrofitted with acoustic treatment, and the other a purpose-built room which is likely larger and has better anechoic performance.
== Radio-frequency anechoic chambers ==
The internal appearance of the radio frequency (RF) anechoic chamber is sometimes similar to that of an acoustic anechoic chamber; however, the interior surfaces of the RF anechoic chamber are covered with radiation absorbent material (RAM) instead of acoustically absorbent material. Uses for RF anechoic chambers include testing antennas and radars, and they are typically used to house the antennas for performing measurements of antenna radiation patterns and electromagnetic interference. Performance expectations (gain, efficiency, pattern characteristics, etc.) constitute primary challenges in designing stand alone or embedded antennas. Designs are becoming ever more complex with a single device incorporating multiple technologies such as cellular, WiFi, Bluetooth, LTE, MIMO, RFID and GPS.
=== Radiation-absorbent material ===
RAM is designed and shaped to absorb incident RF radiation (also known as non-ionising radiation) as effectively as possible, from as many incident directions as possible. The more effective the RAM, the lower the resulting level of reflected RF radiation. Many measurements in electromagnetic compatibility (EMC) and antenna radiation patterns require that spurious signals arising from the test setup, including reflections, are negligible to avoid the risk of causing measurement errors and ambiguities.
=== Effectiveness over frequency ===