Talk:Anechoic chamber
From Wikipedia, the free encyclopedia
 |
This article is within the scope of WikiProject Physics, which collaborates on articles related to physics. | |
| B | This article has been rated as B-Class on the assessment scale. | |
| ??? | This article has not yet received an importance rating within physics. | |
|
This article has been rated but has no comments. If appropriate, please review the article and leave comments here to identify the strengths and weaknesses of the article and what work it will need. |
||
As an aviation enthusiast I've heard of anechoic chambers being used for radar signature measurement, where the echos absorbed are radar echos, not sound echos. Anechoic chambers for EM radiation should be mentioned in addition to ones meant for audio purposes.
I'll try to do a significant revision and addition to incorporate this. mnemonic 10:49, 2004 Jun 21 (UTC)
Should the words "dampening" and "dampened" be used? Doesn't that mean the panels are making something wet? ;)
Perhaps "damping" and "damped" should be used.
More here:
http://en.wikipedia.org/wiki/Dampening
Contents |
[edit] Acoustic and RF (electromagnetic) anechoic chambers
I have added several paragraphs about RF anechoic chambers as opposed to acoustic ones and I have not as yet deleted anything. I think it improves the article but it could do with a tidy up, or perhaps it might be worth splitting it into the two different types. If nobody else does I'll get back to it in a few weeks.
I think some clarification is needed about the acoustic and RF wavelengths. It is true that a typical audio frequency of 500Hz acoustic wavelength (0.7 m) is very different from 500 Hz electromagnetic (600 km) but I don't think many RF tests are done as low as 500 Hz. But 0.7 m RF wavelength I make about 454 MHz. Many tests are done around these UHF frequencies. ChrisAngove 17:43, 10 November 2006 (UTC)
[edit] Substantial Changes
After a common introduction I have tried to separate out the anechoic and RF versions. I have not intentionally deleted any of the previous factual information. This is the original:
An anechoic chamber is a room that is isolated from external sound or electromagnetic radiation sources, sometimes using sound proofing, and prevents the reflection of wave phenomena (reverberation). In rooms such as these, the only sounds which exist are emitted directly from their source, and not reflected from another part of the chamber. Anechoic rooms have the characteristic of being muted, muffled, and silent. Anechoic chambers are widely used for measuring the acoustic properties of acoustic instruments, measuring the transfer functions of electro-acoustic devices, testing microphones and performing psychoacoustics experiments (such as measuring the quality of audio codecs or measuring head-related transfer functions).
Anechoic chambers, modeled after the world's first wedge-based anechoic chamber at Murray Hill, Bell Labs, typically use fiberglass wedges (Anechoic tile) on all walls of the chamber to absorb incoming sound waves. The wedge shape acts as a waveguide to focus incoming sound into the fiberglass wedge, where the acoustic energy is converted to heat. The alternating pattern is used to achieve a more uniform angular absorption. Frequencies below 200 Hz are not as effectively absorbed by the wedges.
Anechoic chambers in which the bottom is also composed of wedges have the floor formed by a wire mesh suspended above the bottom by wires. Other anechoic chambers only use wedges for five of the six sides of the room. To prevent external sounds from entering anechoic chambers, most are encased in a meter or more of cement and may be surrounded by additional insulating materials.
John Cage, a 20th century composer, cited his experience in 1951 in Harvard University's anechoic chamber — a room in which he expected to hear nothing, but heard instead what was believed to be the sound of his own bloodflow and nervous system — as the inspiration for his famous "silent" composition, 4' 33".
Anechoic chambers for electromagnetic radiation absorption are used in the aerospace industry for radar cross section measurement, among other areas. Anechoic-chamber technology is also used in soundproofing rooms for indoor shooting ranges, and for hearing aid test chambers.
In electromagnetics, anechoic chambers are equipped with absorber material[1] to damp the reflection of electromagnetic waves and are used to measure the properties of antennas or of electronic devices which emit, or are susceptible to interference from, electromagnetic (radio/microwave) energy. The absorber material is typically pyramidal in shape and made of a carbon-impregnated foam that acts as a resistance, dissipating any energy that strikes it. These Radio Frequency/Microwave anechoic chambers can range in size from a small room to a large airplane hangar, depending on the size of the object to be measured and the frequency range of the radio or microwave signals employed. Testing can be conducted on full-scale objects, including aircraft, or on scale models where the frequency of the measuring radiation is scaled up as the size of the object is scaled down. Most radio-frequency/microwave anechoic chambers are located in a screen room, which is a shielded facility that prevents the leakage of radio-frequency/microwave energy in or out of the chamber, thus ensuring the accuracy of measurements and preventing interference with outside systems. Absorber material can be quite flammable and is often protected with an automatic fire-extinguishing system for safety.
Incidentally, electromagnetic anechoic chambers exhibit noticeable degrees of acoustic noise damping in practice, despite great differences in wavelengths between microwave signals and audible (voice) signals.
[edit] The Radio Frequency Anechoic Chamber
The internal appearance of the radio frequency (RF) anechoic chamber is often 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.
[edit] Radiation Absorbent Material
RAM is designed to absorb incident RF radiation in the form of electronmagnetic plane waves, also known as non-ionising radiation, as effectively as possible from as many incident directions as possible. The more effectively this is achieved, the less will be the level of RF radiation reflected. Many measurements in the specialist areas of electromagnetic compatibility (EMC) and antenna radiation patterns require that spurious signals from sources, including reflections, are negligible to avoid the risk of causing measurement errors and ambiguities. Performing these in an RF anechoic chamber will heavily suppress this source of spurious signals.
One of the most effective types of RAM comprises regular arrays of pyramid shaped pieces, each of which is constructed from a suitably lossy material. To work effectively, all internal surfaces of the anechoic chamber must be entirely covered with RAM, though sections of RAM may be temporarily removed to install equipment provided they are replaced before performing any tests. To be sufficiently lossy, RAM can neither be a good electrical conductor nor a good electrical insulator as neither type actually absorbs any power. It has to be an intermediate grade of material which absorbs incident power in a controlled way as the incident wave penetrates. Typically pyramidal RAM will comprise a rubberised foam impregnated with controlled mixtures of carbon and iron, depending on the frequency of operation.
An alternative type of RAM comprises flat plates, often of ferrite materials, in the form of flat tiles fixed to the interior surfaces of the chamber. This type is only effective over a narrower frequency band than pyramidal RAM and is designed to be fixed to good conductive surfaces, in order to permit a controlled degree of penetration and absorption of the incident wave. It is generally cheaper, easier to fit and more durable than the pyramidal RAM but is less effective at lower frequencies. Its performance might however be quite adequate if tests are limited to the higher RF or microwave frequencies.
[edit] Effectiveness over Frequency
The overall effectiveness of an RF anechoic chamber is normally determined by its lowest test frequency of operation, at which measured reflections from the internal surfaces will be the most significant compared to higher frequencies. Pyramidal RAM is at its most effective (absorptive) when the incident wave is at normal incidence to the internal surface. The frequency at which it is most effective is when the pyramid height is approximately equal to λ / 4, where λ is the free space wavelength. Accordingly, increasing the pyramid height of the RAM for the same (square) base size improves the effectiveness of the chamber at low frequencies but this results both in increased cost and in a reduced unobstructed working volumefor a given chamber size.
[edit] Installation into a Screened Room
An RF anechoic chamber is usually built into a screened room, designed using the Faraday cage principle. This is because most of the RF tests that require an anechoic chamber to minimise reflections from the inner surfaces also require the properties of a screened room to:
attenuate unwanted signals penetrating inwards and
prevent leakage from high power tests penetrating outside.
[edit] Chamber Size and Commissioning
RF anechoic chambers are typically used to perform electromagnetic compatability (EMC) tests and antenna radiation patterns measurements. The actual test setups installed usually require extra space than that required to simply house the test equipment, the hardware under test and associated cables. For example, the far field criteria sets a minimum distance between the transmitting antenna and the receiving antenna required when measuring antenna radiation patterns. Allowing for this and the extra space occupied by the pyramidal RAM means that a substantial capital investment is required into even a modestly dimensioned chamber. For most companies a such an investment in a large RF anechoic chamber is not justifable unless it is likely to be used frequently. It may be rented out during quiet times.
RF anechoic chambers are normally designed to meet the electrical requirements of one or more accredited standards. Once built, tests are performed during commissioning to verify that the standard(s) are met and a certificate will be issued, effective for a limited period.
[edit] Operational Use
Test equipment configurations to be used within RF anechoic chambers must expose as few metallic (conductive) surfaces as possible, other than the actual equipment under test, as these risk causing unwanted reflections. Often this is achieved by using non-conductive plastic, composite or wooden structures for supporting the equipment under test. Where such surfaces are unavoidable, they may be covered with loose pieces of RAM to minimse such reflection as far as possible.
A careful assessment of whether to place the test equipment (as opposed to the equipment under test) on the interior or exterior is required. Normally test equipment may be located outside of the chamber provided it is not susceptible to interference from exterior fields which, otherwise, would not be present inside the chamber. This has the advantage of reducing reflection surfaces inside but it requires extra cables to carry the signals including good quality filtering. Unnecessary cables and/or poor filtering can collect interference present externally and conduct it to the inside. A good compromise is to install human interface equipment (such as PCs), electrically noisy and high power equipment on the outside and the sensitive equipment on the inside.
One useful application of fiber optic cables is to provide the communications links to carry signals within the chamber. This is because fiber optic cables are non-conductive and therefore cause negligible reflections.
It is normal to filter electrical power supplies for use within the anechoic chamber as unfiltered supplies present a risk of unwanted signals being conducted into and out of the chamber.
[edit] The Health and Safety Risks associated with the RF Anechoic Chamber
There are three significant health and safety risks associated with RF anechoic chambers:
RF radiation hazard
Fire hazard
Trapped personnel
Personnel are normally excluded from the RF chamber interior during measurements as this can not only cause unwanted reflections from the human body but also be a possible radiation hazard to the personnel concerned if tests are being performed at high RF powers. Such risks are from RF or non-ionising radiation, not to be confused with the higher energy ionising radiation.
As RAM is absorptive of RF radiation, incident radiation will generate heat within it. If this cannot be dissipated adequately there is a risk that the RAM temperature may rise to the point of combustion. This can be a risk if a transmitting antenna inadvertently gets too close to the RAM. Even for quite modest transmitting power levels, high gain antennas can concentrate the power sufficiently to cause high power flux near their apertures. Although recently manufactured RAM is normally treated with a fire retardant to reduce such risks, it is difficult to completely eliminate.
Safety regulations normally require the installation of a gaseous fire suppression system including [smoke detector|smoke detectors]]. Gaseous fire suppression avoids damage caused by the extuinghishing agent which would otherwise worsen damage caused by the fire itself. A common gaseous fire suppression agent is carbon dioxide. Normally the fire detection system is linked into the power supply to the chamber arranged to disconnect it if smoke or a fire activates the system.
[edit] External links
- Bell Labs' Murray Hill anechoic chamber
- UK National Physics Laboratory anechoic chamber
- Photos from building an anechoic chamber in CTU, Praguede:Reflexionsarmer Raum
es:Cámara anecoica nl:Dode kamer ja:無響室 fr:Chambre anéchoïque
[edit] Suggested Improvements
Picture of the interior of an RF anechoic chamber if possible showing something being tested. References to the particular international standards (a) that chambers are built to and (b) that are typically tested for in the chambers. A picture or two showing the shell construction ie. the screened room for the RF and the 'metre thick cement' or similar for the acoustic one. ChrisAngove 17:39, 18 November 2006 (UTC)
[edit] Acoustic Testing of Satellite Components
Does anybody know anything about the very high level acoustic noise testing that used to be done on components to go into space? I believe it may have been to similate launch conditions. Do they still do it or has it been superceeded? The levels would need a heavy duty chamber i think. ChrisAngove 17:39, 18 November 2006 (UTC)
