An anechoic chamber is a shielded room designed to attenuate waves. Anechoic chambers were originally used in the context of acoustic (sound) echoes caused by reflections from the internal surfaces of the room, but more recently anechoic chambers have been used to provide a shielded environment for radio frequency (RF) and microwaves. An RF anechoic chamber is designed to suppress the electromagnetic wave analogy of echoes: reflected electromagnetic waves, from the internal surfaces. Both types of chamber are constructed with echo suppression features and with effective isolation from the acoustic or RF noise present in the external environment. In a well-designed acoustic or RF anechoic chamber, the equipment under test receive acoustic, mechanical or RF signals from the signal source, not reflected from another part of the chamber. This ensures the integrity of the testing being conducted. Furthermore, the shielding of the chamber limits interference from equipment located outside of the chamber. Anechoic chambers are specialized chambers designed for electromagnetic or sound waves. They have specialized walls to absorb and reflect waves of electromagnetic radiation. Acoustic anechoic chambers are soundproof and used for testing applications. They are usually constructed with cement or brick walls to keep outside sound from entering the chamber. Inside, the chamber is lined with fiberglass wedges to absorb the sound waves. Anechoic chambers that are used to study or test electromagnetic interference (EMI) are lined with an absorbent material, such as carbon-impregnated foam shaped into pyramids. The pyramidal shape acts to resist and dissipate the electromagnetic waves. EMI testing is done in anechoic chambers to analyze the properties of antennas and other electronics that are susceptible to radio or microwave interference.
Anechoic chambers range from small compartments to chambers as large as aircraft hangars. The size of an anechoic chamber depends on the size of the objects to be tested and the frequency range of the radio or microwave signals used. Radio frequency interference (RFI) is the unwanted reception of radio signals and can be problematic to the other electronic equipment onboard aircraft and other vehicles. Radio frequency interference sources include lightning, electrical equipment, fluorescent lighting, cell phones, and transmitting equipment from radio stations. RFI testing helps determine which frequencies affect particular electronic systems and provide clues to mitigating the risks to communication devices or developing measures to counter the interference.
Full anechoic chambers aim to absorb energy in all directions. Semi-anechoic chambers have a solid floor that acts as a work surface for supporting heavy items, such as cars, washing machines, or industrial machinery, rather than the mesh floor grille found over absorbent tiles present in full anechoic chambers. This floor is damped and floating on absorbent buffers to isolate it from outside vibration or electromagnetic signals. Recording artists recording studio may utilize the semi-anechoic chamber to produce high-quality music free of outside noise and unwanted echoes.
One of the most effective types of RAM comprises 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. Sections of RAM may be temporarily removed to install equipment but they must be 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 power gradually in a controlled way as the incident wave penetrates it. Typically pyramidal RAM will comprise a rubberized foam material impregnated with controlled mixtures of carbon and iron.
An alternative type of RAM comprises flat plates of ferrite material, in the form of flat tiles fixed to all interior surfaces of the chamber. This type has a smaller effective frequency range than the pyramidal RAM and is designed to be fixed to good conductive surfaces. It is generally easier to fit and more durable than the pyramidal type RAM but is less effective at higher frequencies. Its performance might however be quite adequate if tests are limited to lower frequencies (ferrite plates are have a damping curve that makes them most effective between 30-1000MHz)
Waves of higher frequencies have smaller amplitudes and are higher in energy, while waves of lower frequencies have larger amplitudes and are lower in energy, according to the relationship where lambda represents wavelength, v is phase velocity of wave, and f is frequency. To shield for a specific wavelength, the cone must be of appropriate size to absorb that wavelength. The performance quality of an RF anechoic chamber is 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 absorptive when the incident wave is at normal incidence to the internal chamber surface when the pyramid height is approximately equal to , where 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 results in increased cost and a reduced unobstructed working volume that is available inside a chamber of defined size.
RF anechoic chambers are normally designed to meet the electrical requirements of one or more accredited standards. For example, the aircraft industry may test equipment for aircraft according to company specifications or military specifications such as MIL-STD 461E. Once built, acceptance tests are performed during commissioning to verify that the standard(s) are in fact met. Provided they are, a certificate will be issued to that effect, valid for a limited period.
A careful assessment of whether to place the test equipment (as opposed to the equipment under test) on the interior or exterior of the chamber is required. Normally this 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 and particularly good filtering. Unnecessary cables and/or poor filtering can collect interference on the outside and conduct them to the inside. A good compromise may be to install human interface equipment (such as PCs), electrically noisy and high power equipment on the outside and sensitive equipment on the inside.
One useful application of fiber optic cables is to provide the communications links to carry signals within the chamber. Fiber optic cables are non-conductive and of small cross-section and therefore cause negligible reflections in most applications.
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 along the power cables.
Personnel are not normally permitted inside the chamber during a measurement as this not only can cause unwanted reflections from the human body but may also be a radiation hazard to the personnel concerned if tests are being performed at high RF powers. Such risks are from RF or non-ionizing radiation and not from the higher energy ionizing radiation.
As RAM is highly absorptive of RF radiation, incident radiation will generate heat within the RAM. If this cannot be dissipated adequately there is a risk that hot spots may develop and 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, they are difficult to completely eliminate.
Safety regulations normally require the installation of a gaseous fire suppression system including smoke detectors. Gaseous fire suppression avoids damage caused by the extinguishing 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, so that the fire detection system can disconnect the power supply if smoke or a fire is detected.