Anechoic chamber

A room specifically engineered to prevent the reflection or echoing of sound or electromagnetic waves is known as an anechoic chamber, a term derived from an-echoic, signifying 'non-reflective' or 'without echoes'. These chambers are frequently designed to be impervious to external energy ingress. This dual design principle ensures that any individual or sensor within the chamber perceives only direct sounds, effectively replicating the conditions of an open, free-field environment.

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.

The designation 'anechoic chamber', originally introduced by the American acoustics authority Leo Beranek, initially pertained solely to acoustic applications. However, its usage has since broadened to encompass radio frequency (RF) anechoic chambers, which are engineered to mitigate reflections and external interference generated by electromagnetic waves.

Anechoic chambers exhibit considerable variation in scale, from compact units comparable to domestic microwave ovens to expansive structures the size of aircraft hangars. The dimensions of a specific chamber are determined by the size of the items under examination and the spectrum of frequencies being investigated.

Acoustic Anechoic Chambers

The initial demand for what later became known as an anechoic chamber arose from the necessity to test loudspeakers producing sound levels of such intensity that outdoor evaluation in populated regions was impractical.

Within the field of acoustics, anechoic chambers are routinely employed for conducting experiments under conditions approximating a 'free field,' which implies the absence of reflected signals. In such an environment, virtually all sound energy propagates away from its source with minimal reflection. Typical experiments conducted within these chambers include the measurement of a loudspeaker's transfer function or the assessment of noise radiation directivity from industrial equipment. Generally, the internal environment of an anechoic chamber is exceptionally quiet, with characteristic noise levels ranging from 10 to 20 dBA. A notable achievement occurred in 2005, when an anechoic chamber recorded a noise level of −9.4 dBA. Subsequently, in 2015, a chamber situated at the Microsoft campus established a new world record with a measurement of −20.6 dBA. Given that the human auditory system typically perceives sounds above 0 dBA, an individual within such a chamber would experience an environment utterly devoid of audible sound. Anecdotal accounts suggest that some individuals find this profound silence unsettling and may experience disorientation.

The operational principle by which anechoic chambers attenuate the reflection of incident sound waves upon their surfaces is detailed below: As depicted in the accompanying illustration, an incident sound wave, denoted I, approaches a chamber wall, which is constructed from a series of wedges, W, each possessing a height, H. Upon impact, the incident wave I is reflected as a series of waves, R, which subsequently oscillate within the air gap, A (delineated by dotted lines), between the wedges W. This oscillatory motion can potentially generate a standing wave pattern within A, at least temporarily. During this process, the acoustic energy of waves R is dissipated through the molecular viscosity of the air, particularly concentrated near corner C. Furthermore, when foam materials are utilized in the fabrication of these wedges, an additional dissipation mechanism occurs during the interactions between the waves and the wall. Consequently, the component of the reflected waves R that propagates along the original direction of I and exits the gaps A (returning towards the sound source), designated R', is significantly diminished. While this explanation is presented in two dimensions, it accurately represents and applies to the actual three-dimensional wedge configurations employed in anechoic chambers.

Semi-Anechoic and Hemi-Anechoic Chambers

Full anechoic chambers are engineered to achieve comprehensive energy absorption across all directions. This necessitates covering every internal surface, including the floor, with precisely shaped acoustic wedges. A mesh grille is commonly installed above the floor to facilitate pedestrian access and equipment placement. This mesh flooring is typically situated at the same elevation as the surrounding building's floor, implying that the chamber itself extends below the main floor level. Furthermore, this mesh floor is damped and supported by absorbent buffers, ensuring its isolation from external vibrations or electromagnetic interference.

Conversely, semi-anechoic or hemi-anechoic chambers incorporate a solid floor, which serves as a robust working surface capable of supporting substantial items such as automobiles, domestic appliances, or industrial machinery—objects that a full anechoic chamber's mesh grille could not accommodate. It is noteworthy that recording studios frequently employ a semi-anechoic design.

The precise differentiation between "semi-anechoic" and "hemi-anechoic" remains ambiguous. In certain contexts, these terms are used interchangeably or only one is employed. Conversely, some definitions distinguish them based on the presence of an ideally reflective floor (establishing free-field conditions with a singular reflective surface) versus a simple, untreated flat floor. Furthermore, distinctions are sometimes drawn regarding their dimensions and operational capabilities, with one potentially representing an existing space retrofitted with acoustic treatment, and the other a purpose-built facility, typically larger and exhibiting superior anechoic characteristics.

Radio-frequency Anechoic Chambers

The interior design of a radio frequency (RF) anechoic chamber can resemble that of an acoustic anechoic chamber; however, its internal surfaces are lined with radiation absorbent material (RAM) rather than acoustically absorbent material. RF anechoic chambers are utilized for applications such as testing antennas and radars, commonly serving as enclosures for antennas during the measurement of radiation patterns and electromagnetic interference.

Achieving specific performance criteria, including gain, efficiency, and pattern characteristics, presents significant challenges in the design of both standalone and embedded antennas. Contemporary antenna designs are increasingly intricate, with individual devices integrating diverse technologies such as cellular communication, WiFi, Bluetooth, LTE, MIMO, RFID, and GPS.

Radiation-Absorbent Material

Radiation-absorbent material (RAM) is engineered and configured to maximize the absorption of incident RF radiation (also referred to as non-ionizing radiation) across a broad range of incident angles. Enhanced RAM effectiveness directly correlates with a reduction in reflected RF radiation levels. In numerous electromagnetic compatibility (EMC) and antenna radiation pattern measurements, it is imperative that spurious signals originating from the test environment, particularly reflections, are minimized to prevent measurement inaccuracies and ambiguities.

Frequency-Dependent Effectiveness

Waves characterized by higher frequencies possess shorter wavelengths and greater energy, whereas lower-frequency waves exhibit longer wavelengths and reduced energy, as defined by the relationship ${\displaystyle \lambda =v/f}$, where λ denotes wavelength, v represents the wave's phase velocity, and ${\displaystyle f}$ signifies frequency. To achieve effective shielding for a particular wavelength, the absorbing cone must be appropriately sized. The operational quality of an RF anechoic chamber is primarily determined by its lowest functional test frequency, as reflections from internal surfaces are most pronounced at these lower frequencies compared to higher ones. Pyramidal RAM demonstrates optimal absorption when the incident wave strikes the internal chamber surface at normal incidence and the pyramid's height approximates ${\displaystyle \lambda /4}$, where ${\displaystyle \lambda }$ refers to the free-space wavelength. Consequently, increasing the pyramidal RAM's height while maintaining the same (square) base dimensions enhances the chamber's low-frequency performance, but this comes at the expense of increased cost and a reduction in the available unobstructed working volume within a fixed-size chamber.

Integration within a Screened Room

An RF anechoic chamber is typically constructed within a screened room, which is engineered based on the Faraday cage principle. This design choice is necessitated by the fact that most RF tests demanding an anechoic environment for internal reflection minimization also require the shielding capabilities of a screened room. Such shielding attenuates extraneous signals from external sources that could interfere with the equipment under test and prevents test emissions from escaping the chamber.

Chamber Dimensions and Commissioning

For lower radiated frequencies, far-field measurements often necessitate a substantial and costly chamber. In certain scenarios, such as radar cross-section measurements, it is feasible to scale down the object under examination and consequently reduce the chamber's dimensions. This approach is viable if the test frequency's wavelength is proportionally decreased by conducting tests at a higher frequency.

Radio frequency (RF) anechoic chambers are typically engineered to satisfy the electrical specifications of one or more recognized standards. For instance, the aerospace sector might evaluate aircraft equipment in accordance with corporate or military specifications, such as MIL-STD 461E. Upon construction, acceptance tests are conducted during commissioning to confirm adherence to the specified standard(s). If compliance is verified, a corresponding certificate is issued. Subsequent periodic retesting of the chamber is also required.

Operational Use

Equipment configurations, including both test and supporting apparatus, deployed within anechoic chambers must minimize the exposure of metallic (conductive) surfaces, as these can induce undesirable reflections. This objective is frequently accomplished through the utilization of non-conductive plastic or wooden structures to support the equipment under test. In instances where metallic surfaces are indispensable, they can be subsequently covered with pieces of Radar Absorbent Material (RAM) following setup to mitigate reflections to the greatest extent possible.

A thorough evaluation is often necessary to determine the optimal placement of test equipment (distinct from the equipment under test) either inside or outside the anechoic chamber. Typically, the majority of this equipment is situated in a separate, shielded room adjacent to the primary test chamber. This arrangement serves to protect the equipment from both external interference and the radiation present within the chamber. Furthermore, mains power and test signal cabling entering the test chamber necessitate high-quality filtering.

Fiber optic cables are occasionally employed for signal transmission due to their immunity to typical radio frequency interference (RFI) and their minimal reflective properties within the chamber.

Health and Safety Risks Associated with RF Anechoic Chambers

The following health and safety hazards are linked to RF anechoic chambers:

• RF radiation hazards
• Fire hazards
• Personnel entrapment

Personnel are typically prohibited from entering the chamber during measurements. This restriction is imposed not only because the human body can introduce undesirable reflections but also due to potential radiation hazards to individuals if tests are conducted at high RF power levels. These risks stem from RF or non-ionizing radiation, distinct from higher-energy ionizing radiation.

Given that Radar Absorbent Material (RAM) exhibits high absorption of RF radiation, incident radiation will generate heat within the material. Inadequate heat dissipation poses a risk of localized hot spots developing, potentially elevating the RAM's temperature to its combustion point. This hazard can arise if a transmitting antenna inadvertently approaches the RAM too closely. Even at relatively modest transmitting power levels, high-gain antennas can sufficiently concentrate power to create a high power flux near their apertures. While recently manufactured RAM is typically treated with fire retardants to mitigate such risks, their complete elimination remains challenging.

• Soundproofing
• Buffer (disambiguation)
• Damping ratio
• Electromagnetic reverberation chamber
• Sensory deprivation
• References

References

• 360-degree video of an anechoic chamber
• Anechoic Chambers, Past and Present
• Video tour of an EMC/RF Test facility. Including the largest anechoic test chamber in the southern hemisphere
• Some Examples
• 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.
• "Acoustics Anechoic Chamber". The UK's National Measurement Laboratory. National Physical Laboratory. Archived from the original on 29 September 2007. Retrieved 22 February 2011.Source: TORIma Academy Archive