An anechoic chamber 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 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. 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 between the wedges W. Such
bouncing may produce 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, 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 chambers=
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 over absorbent tiles found in full anechoic chambers. This
floor is damped and floating on absorbent buffers to isolate it from
outside vibration or electromagnetic signals. A recording studio may utilize
a semi-anechoic chamber to record music free of outside noise and unwanted
reflections / reverberation. Radio-frequency anechoic chambers
The internal appearance of the radio frequency 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
instead of acoustically absorbent material. The RF anechoic chamber is
typically used to house the equipment for performing measurements of antenna
radiation patterns, electromagnetic compatibility and radar cross section
measurements. Testing can be conducted on full-scale objects, including
aircraft, or on scale models where the wavelength of the measuring radiation is
scaled in direct proportion to the target size. Coincidentally, many RF
anechoic chambers which use pyramidal RAM also exhibit some of the properties
of an acoustic anechoic chamber, such as attenuation of sound and shielding from
outside noise. Performance expectations 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.
Antenna performance is simply affected by the increasing number of antenna,
components co-located with the antenna, enclosures, electronic sub-assemblies,
and PCBs. = Radiation absorbent material=
The RAM is designed and shaped to absorb incident RF 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 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. 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 be neither a good
electrical conductor nor a good electrical insulator as neither type
actually absorbs any power. Typically pyramidal RAM will comprise a rubberized
foam material impregnated with controlled mixtures of carbon and iron.
The length from base to tip of the pyramid structure is chosen based on the
lowest expected frequency and the amount of absorption required. For low
frequency damping, this distance is often 24 inches, while high frequency
panels are as short as 3–4 inches. Panels of RAM are installed with the
tips pointing inward to the chamber. Pyramidal RAM attenuates signal by two
effects: scattering and absorption. Scattering can occur both coherently,
when reflected waves are in-phase but directed away from the receiver, or
incoherently where waves are picked up by the receiver but are out of phase and
thus have lower signal strength. This incoherent scattering also occurs within
the foam structure, with the suspended carbon particles promoting destructive
interference. Internal scattering can result in as much as 10 dB of
attenuation. Meanwhile, the pyramid shapes are cut at angles that maximize
the number of bounces a wave makes within the structure. With each bounce,
the wave loses energy to the foam material and thus exits with lower
signal strength. 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.
There is also a hybrid type, a ferrite in pyramidal shape. Containing the
advantages of both technologies, the frequency range can be maximized while
the pyramid remains small. = Effectiveness over frequency=
Waves of higher frequencies have shorter wavelengths and are higher in energy,
while waves of lower frequencies have longer wavelengths and are lower in
energy, according to the relationship where lambda represents wavelength, v is
phase velocity of wave, and 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 and the pyramid height
is approximately equal to , where is the free space wavelength. Accordingly,
increasing the pyramid height of the RAM for the same 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. = 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 minimize reflections from the inner surfaces also require the
properties of a screened room to attenuate unwanted signals penetrating
inwards and causing interference to the equipment under test and prevent leakage
from tests penetrating outside. = Chamber size and commissioning=
The actual test setups usually require extra room—more 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 to be observed when
measuring antenna radiation patterns. Allowing for this and the extra space
that may be required for the pyramidal RAM means that a substantial capital
investment is required into even a modestly dimensioned chamber. For most
companies, such an investment in a large RF anechoic chamber is not justifiable
unless it is likely to be used continuously or perhaps rented out.
Sometimes, for radar cross-section measurements, it is possible to scale
down the objects under test and reduce the chamber size, provided that the
wavelength of the test frequency is scaled down in direct proportion.
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. = Operational use=
Test and supporting equipment configurations to be used within
anechoic chambers must expose as few metallic surfaces as possible, as these
risk causing unwanted reflections. Often this is achieved by using non-conductive
plastic or wooden structures for supporting the equipment under test.
Where metallic surfaces are unavoidable, they may be covered with pieces of RAM
after setting up to minimize such reflection as far as possible.
A careful assessment is required of whether to place the test equipment on
the interior or exterior of the chamber. 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, 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.
= Health and safety risks associated with RF anechoic chamber=
The following health and safety risks are associated with RF anechoic
chambers: RF radiation hazard
Fire hazard Trapped personnel
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. See also
Soundproofing Electromagnetic reverberation chamber
Reverberation room Sensory deprivation
GTEM cell References
^ a b Morton, Ella. "How Long Could You Endure the World's Quietest Place?".
Slate. The Slate Group. Retrieved 5 May 2014.
^ Novet, Jordan. "Look inside Microsoft’s anechoic chamber, officially
the quietest place on Earth". VentureBeat. Retrieved 2015-10-01.
^ "Interview with Dr. Leo Beranek". American Institute of Physics. Retrieved
2014-12-08. ^ Randall, R. H.. An Introduction to
Acoustics. Dover Publications. ^ E Knott, J Shaeffer, M Tulley, Radar
Cross Section. pp 528–531. ISBN 0-89006-618-3
^ Fully compact anechoic chamber using the pyramidal ferrite absorber for
immunity test External links
Pictures and description of an acoustic anechoic chamber
Anechoic Chambers, Past and Present How RF Anechoic Chambers Work
Video tour of an EMC/RF Test facility. Including the largest anechoic test
chamber in the southern hemisphere Some examples
Antenna Testing For An Anechoic Chamber Millimeter Wave Inc's Radio/MM Wave
anechoic chamber Bell Labs' Murray Hill anechoic chamber
"Acoustics Anechoic Chamber". The UK's National Measurement Laboratory.
National Physical Laboratory. Archived from the original on 29 September 2007.
Retrieved 22 February 2011. Anechoic chambers at Apple Inc. campus
used to test their mobile device products, via WaybackMachine
Photos from building an anechoic chamber in CTU, Prague
Sound examples The sound of clothes inside an anechoic
chamber Hallucinations in anechoic chambers: the
science behind the claim Listen to a subdued balloon burst in an
anechoic chamber