Anechoic chamber

An anechoic chamber (an-echoic meaning "non-reflective, non-echoing, 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.

360 anechoic chamber salford university uk
360 image of an acoustic anechoic chamber
360 image of an electromagnetic anechoic chamber
360 image of an electromagnetic anechoic chamber

Acoustic anechoic chambers

Anechoic chamber dissipation
Minimization of the reflection of sound waves by an anechoic chamber's walls.
Consumer Reports - product testing - headphones in anechoic chamber
Testing headphones in the Consumer Reports anechoic chamber

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.[1] In 2015, an anechoic chamber on the campus of Microsoft broke the world record with a measurement of −20.6 dBA.[2] 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 quietness and can become disoriented.[1]

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.[3] 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.[4]

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. Recording studios are often semi-anechoic.

Radio-frequency anechoic chambers

Radio-frequency-anechoic-chamber-HDR-0a
An RF anechoic chamber.
Large Drive-In EMC Test Chamber
A large drive-in EMC RF anechoic test chamber. Note the orange caution cones for size reference
40th Flight Test Squadron F-16 Fighting Falcon sits in the anechoic chamber
An F-16 Fighting Falcon in the anechoic test chamber at Eglin Air Force Base.

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, radars, and is typically used to house the antennas for performing measurements of antenna radiation patterns, 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

Anechoic chamber wall
Close-up of a pyramidal RAM

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 (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.

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

At lower radiated frequencies, far-field measurement can require a large and expensive chamber. Sometimes, for example for radar cross-section measurements, it is possible to scale down the object under test and reduce the chamber size, provided that the wavelength of the test frequency is scaled down in direct proportion by testing at a higher frequency.

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. The chamber will need to be periodically retested.

Operational use

Test and supporting equipment configurations to be used within anechoic chambers must expose as few metallic (conductive) 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 may be required as to whether the test equipment (as opposed to the equipment under test) should be placed inside or outside the chamber. Typically most of it is located in a separate screened room attached to the main test chamber, in order to shield it from both external interference and from the radiation within the chamber. Mains power and test signal cabling into the test chamber require high quality filtering.

Fiber optic cables are sometimes used for the signal cabling, as they are immune to ordinary RFI and also cause little reflection inside the chamber.

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.

See also

References

  1. ^ a b Morton, Ella (5 May 2014). "How Long Could You Endure the World's Quietest Place?". Slate. Retrieved 5 May 2014.
  2. ^ Novet, Jordan (1 October 2015). "Look Inside Microsoft's Anechoic Chamber, Officially the Quietest Place on Earth". VentureBeat. Retrieved 1 October 2015.
  3. ^ Beranek, Leo (10 August 2009). Written at Boston. "Oral History Interview with Leo Beranek" (Interview). Interviewed by Richard Lyon. College Park, MD: Niels Bohr Library & Archives, American Institute of Physics. Retrieved 8 December 2014.
  4. ^ Randall, R. H. (2005). An Introduction to Acoustics. Dover Publications.

External links

Absorption (acoustics)

Acoustic absorption refers to the process by which a material, structure, or object takes in sound energy when sound waves are encountered, as opposed to reflecting the energy. Part of the absorbed energy is transformed into heat and part is transmitted through the absorbing body. The energy transformed into heat is said to have been 'lost'.

When sound from a loudspeaker collides with the walls of a room part of the sound's energy is reflected, part is transmitted, and part is absorbed into the walls. Just as the acoustic energy was transmitted through the air as pressure differentials (or deformations), the acoustic energy travels through the material which makes up the wall in the same manner. Deformation causes mechanical losses via conversion of part of the sound energy into heat, resulting in acoustic attenuation, mostly due to the wall's viscosity. Similar attenuation mechanisms apply for the air and any other medium through which sound travels.

The fraction of sound absorbed is governed by the acoustic impedances of both media and is a function of frequency and the incident angle. Size and shape can influence the sound wave's behavior if they interact with its wavelength, giving rise to wave phenomena such as standing waves and diffraction.

Acoustic absorption is of particular interest in soundproofing. Soundproofing aims to absorb as much sound energy (often in particular frequencies) as possible converting it into heat or transmitting it away from a certain location.

In general, soft, pliable, or porous materials (like cloths) serve as good acoustic insulators - absorbing most sound, whereas dense, hard, impenetrable materials (such as metals) reflect most.

How well a room absorbs sound is quantified by the effective absorption area of the walls, also named total absorption area. This is calculated using its dimensions and the absorption coefficients of the walls. The total absorption is expressed in Sabins and is useful in, for instance, determining the reverberation time of auditoria. Absorption coefficients can be measured using a reverberation room, which is the opposite of an anechoic chamber (see below).

Auralization

Not to be confused with audiation.Auralization is a procedure designed to model and simulate the experience of acoustic phenomena rendered as a soundfield in a virtualized space. This is useful in configuring the soundscape of architectural structures, concert venues, public-spaces and in making coherent sound environments within virtual immersion systems.

Benefield Anechoic Facility

Benefield Anechoic Facility (BAF) is an anechoic chamber located at the southwest side of the Edwards Air Force Base main base. It is currently the world's largest anechoic chamber. The BAF supports installed systems testing for avionics test programs requiring a large, shielded chamber with radio frequency (RF) absorption capability that simulates free space.

The facility is named after Rockwell test pilot and flight commander Tommie Douglas "Doug" Benefield who was killed in a crash 22 miles northeast of Edwards Air Force Base in the desert east of Boron on August 29, 1984 during a USAF B-1 Lancer flight test.

Biconical antenna

In radio systems, a biconical antenna is a broad-bandwidth antenna made of two roughly conical conductive objects, nearly touching at their points.Biconical antennas are broadband dipole antennas, typically exhibiting a bandwidth of three octaves or more. A common subtype is the bowtie antenna, essentially a two-dimensional version of the biconial design which is often used for short-range UHF television reception. These are also sometimes referred to as butterfly antennas.

Eckel Industries

Eckel Industries is an acoustics noise control company founded 1952 in Cambridge, Massachusetts. The company engineers and constructs anechoic (echo-free) sound chambers.

GTEM cell

A GTEM or gigahertz transverse electromagnetic cell is a type of electromagnetic compatibility (EMC) test chamber used for radiated EMC testing.

Loudspeaker acoustics

Loudspeaker acoustics is a subfield of acoustical engineering concerned with the reproduction of sound and the parameters involved in doing so in actual equipment.

Engineers measure the performance of drivers and complete speaker systems to characterize their behavior, often in an anechoic chamber, outdoors, or using time windowed measurement systems -- all to avoid including room effects (e.g., reverberation) in the measurements.

Designers use models (from electrical filter theory) to predict the performance of drive units in different enclosures, now almost always based on the work of A N Thiele and Richard Small.

Important driver characteristics are:

Frequency response

Off-axis response (dispersion pattern, lobing)

Sensitivity (dB SPL for 1 watt input)

Maximum power handling

Non-linear distortion

Colouration (i.e., more or less, delayed resonance).It is the performance of a loudspeaker/listening room combination that really matters, as the two interact in multiple ways. There are two approaches to high-quality reproduction. One ensures the listening room be reasonably 'alive' with reverberant sound at all frequencies, in which case the speakers should ideally have equal dispersion at all frequencies in order to equally excite the reverberant fields created by reflections off room surfaces. The other attempts to arrange the listening room to be 'dead' acoustically, leaving indirect sound to the dispersion of the speakers need only be sufficient to cover the listening positions.

A dead or inert acoustic may be best, especially if properly filled with 'surround' reproduction, so that the reverberant field of the original space is reproduced realistically. This is currently quite hard to achieve, and so ideal loudspeaker systems for stereo reproduction would have a uniform dispersion at all frequencies. Listening to sound in an anechoic "dead" room is quite different from listening in a conventional room, and, while revealing about loudspeaker behaviour it has an unnatural sonic character that some listeners find uncomfortable. Conventional stereo reproduction is more natural if the listening environment has some acoustically reflective surfaces.It is in large part the directional properties of speaker systems, which vary with frequency that make them sound different, even when they measure similarly well on-axis. Acoustical engineering in this instance is concerned with adapting these variations to each other.

Loudspeaker measurement

Loudspeaker measurement is the practice of determining the behavior of loudspeakers by measuring various aspects of performance. This measurement is especially important because loudspeakers, being transducers, have a higher level of distortion than other audio system components used in playback or sound reinforcement.

Lucent

Lucent Technologies, Inc., was an American multinational telecommunications equipment company headquartered in Murray Hill, New Jersey, in the United States. It was established on September 30, 1996, through the divestiture of the former AT&T Technologies business unit of AT&T Corporation, which included Western Electric and Bell Labs.Lucent was merged with Alcatel SA of France in a merger of equals on December 1, 2006, forming Alcatel-Lucent. Alcatel-Lucent was absorbed by Nokia in January 2016.

MIL-STD-461

MIL-STD-461

is a United States Military Standard

that describes how to test equipment for electromagnetic compatibility.

Various revisions of MIL-STD-461 have been released.

Many military contracts require compliance to MIL-STD-461E.

The latest revision (as of 2015) is known as "MIL-STD-461G".While MIL-STD-461 compliance is technically not required outside the US military, many civilian organizations also use this document.Electromagnetic compatibility test labs typically set up their anechoic chamber to comply with MIL-STD-461.

Test labs attempt to comply with this standard for two reasons:

Even if no potential customer requires MIL-STD-461 compliance, if a device complies with (or is very close to complying to) the (relatively strict) MIL-STD-461, then it is certain to comply with the (relatively looser) FCC Part 15 and EMC standards of other countries, and it is simpler to run one test than to run a separate test for each one.

Even if only a few of the potential customers require MIL-STD-461, it's simpler to design a single commercial off-the-shelf product that complies with the most strict standard—MIL-STD-461—rather than trying to track several versions of a product that each comply with separate standard.In 1999, MIL-STD-462 was combined with MIL-STD-461D into MIL-STD-461E.

Muffler

A muffler (silencer in British English) is a device for reducing the noise emitted by the exhaust of an internal combustion engine.

Naval Submarine Medical Research Laboratory

The Naval Submarine Medical Research Laboratory (NSMRL) is located on the New London Submarine Base in Groton, Connecticut. The laboratory's mission is to protect the health and enhance the performance of United States sailors through focused submarine, diving, and surface research solutions. It is a subordinate command of the Naval Medical Research Center.

Pennsylvania State University Applied Research Laboratory

The Pennsylvania State University Applied Research Laboratory (short: Penn State ARL or simply ARL), is a specialized research unit dedicated to interdisciplinary scientific research at the Penn State, University Park campus. The ARL is a DoD designated U.S. Navy University Affiliated Research Center. It is the university's largest research unit with over 1,000 faculty and staff. The Laboratory ranks 2nd in DoD and 10th in NASA funding to universities.ARL maintains a long-term relationship with the Naval Sea Systems Command and the Office of Naval Research.

Radiation-absorbent material

Radiation-absorbent material, usually known as RAM, is a material which has been specially 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.

Reference antenna

A reference antenna is an antenna with known performance. It is normally used to calibrate other systems.

Reference antennas are built with particular care taken to make them simple, robust and repeatable. In a common usage scenario a reference antenna would be used as a transfer standard. First the reference antenna's performance is measured using high accuracy measurement facility. This test may be done using an electromagnetic anechoic chamber or another type of antenna test range (see Antenna measurements). The antenna is then measured using a second antenna test facility. The results from the two are compared, the comparison can reveal the accuracy of the second test facility. It can also be used to calibrate the second facility.

Sometimes rather than measure the performance of the reference antenna theoretical methods are used. The antenna may be simulated using electromagnetic simulation, or its properties derived from formulae based on electromagnetic theory. These methods are only useful if the materials and dimensions of the antenna can be characterised very well, and the mathematics of the simulation or formulae used is known to be accurate.

Normally the parameter of interest is antenna gain. In this case the reference antenna is built to have a high degree of repeatability in its radiation pattern and boresight gain. A common practice is to measure the boresight gain of a reference antenna across its operational frequency band. Other parameters are sometimes of interest though, such as antenna efficiency.

Common reference antennas are horns, dipoles, monopoles and biconicals. These types are chosen because they are mechanically simple and quite electrically simple. Mechanical simplicity makes building repeatable antennas easier. Electrical simplicity makes design easier and allows use of design formulae that are known to be accurate.

The International Telecommunication Union maintains a database of reference antenna radiation patterns. These radiation patterns are theoretical equivalents to physical reference antennas.

Sound pressure

Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average or equilibrium) atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydrophone. The SI unit of sound pressure is the pascal (Pa).

Soundproofing

Soundproofing is any means of reducing the sound pressure with respect to a specified sound source and receptor. There are several basic approaches to reducing sound: increasing the distance between source and receiver, using noise barriers to reflect or absorb the energy of the sound waves, using damping structures such as sound baffles, or using active antinoise sound generators.

Two distinct soundproofing problems may need to be considered when designing acoustic treatments - to improve the sound within a room (see reverberation), and reduce sound leakage to/from adjacent rooms or outdoors (see sound transmission class and sound reduction index). Acoustic quieting and noise control can be used to limit unwanted noise. Soundproofing can suppress unwanted indirect sound waves such as reflections that cause echoes and resonances that cause reverberation. Soundproofing can reduce the transmission of unwanted direct sound waves from the source to an involuntary listener through the use of distance and intervening objects in the sound path.

Tension grid

A tension grid is a type of non-standard largely-transparent catwalk. Tension grids are composed of tightly woven wire rope steel cables that create a taut floor strong enough for technicians to walk on.

Vivaldi antenna

A Vivaldi antenna or Vivaldi aerial or tapered slot antenna is a co-planar broadband-antenna, which can be made from a solid piece of sheet metal, a printed circuit board, or from a dielectric plate metalized on one or both sides.

The feeding line excites an open space via a microstrip line or coaxial cable, and may be terminated with a sector-shaped area or a direct coaxial connection. From the open space area the energy reaches an exponentially tapered pattern via a symmetrical slot line.

Vivaldi antennas can be made for linear polarized waves or – using two devices arranged in orthogonal direction – for transmitting / receiving both polarization orientations.

If fed with 90-degree phase-shifted signals, orthogonal devices can transmit/receive circular-oriented electromagnetic waves.

Vivaldi antennas are useful for any frequency, as all antennas are scalable in size for use at any frequency. Printed circuit technology makes this type antenna cost effective at microwave frequencies exceeding 1 GHz.

Advantages of Vivaldi antennas are their broadband characteristics (suitable for ultra-wideband signals ), their easy manufacturing process using common methods for PCB production, and their easy impedance matching to the feeding line using microstrip line modeling methods .

The MWEE collection of EM simulation benchmarks includes a Vivaldi antenna.

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