What Are Anechoic Chambers?

Anechoic chambers are specially constructed rooms designed to absorb reflections of sound or electromagnetic waves, creating a free-space measurement environment within an enclosed structure. The name derives from the Greek for "without echo": the walls, floor, and ceiling are lined with absorptive materials that attenuate incident waves before they can reflect back into the measurement volume. Two distinct categories exist: acoustic anechoic chambers, which use foam wedges or fiberglass panels to absorb sound energy, and RF (radio frequency) or microwave anechoic chambers, which use pyramidal absorber tiles made from carbon-loaded polyurethane foam or ferrite-based materials to absorb electromagnetic energy. The IEEE uses anechoic chambers as reference measurement facilities for antenna characterization, electromagnetic compatibility (EMC) testing, and radiated emissions and immunity measurements governed by standards such as CISPR 16 and MIL-STD-461.

The defining performance parameter of an anechoic chamber is the quietzone quality: the region within the chamber where the field from a test antenna or device under test (DUT) behaves as it would in free space, with reflections suppressed to a specified level below the direct signal. For EMC testing, CISPR standards require that ambient electromagnetic levels in the chamber, measured with the DUT de-energized, be at least 6 dB below the applicable emission limits.

Acoustic Measurements

Acoustic anechoic chambers support measurements of sound power level, directivity patterns, and noise emission from loudspeakers, microphones, machinery, and consumer products. The absorptive boundary treatment eliminates reverberant energy, so microphone measurements in the quiet zone correspond to the free-field response of the source. A fully anechoic room absorbs energy above a lower cutoff frequency determined by the wedge depth; a semi-anechoic room (with a reflective floor simulating ground plane) is used for machinery noise testing per ISO standards. NIST maintains laboratory test method documentation for RF and acoustic measurements that addresses measurement uncertainty in absorber-lined test environments.

Antenna Measurements

Antenna anechoic chambers allow gain, radiation pattern, and polarization measurements without ground reflections or building interference that would corrupt outdoor range results. Far-field antenna ranges require the DUT to be placed in the far field of a source antenna, a condition that demands a long physical separation at low frequencies; compact antenna test ranges (CATRs) use a collimating reflector or lens to create a plane-wave quiet zone in a shorter distance, making antenna measurement of large apertures or phased arrays practical at indoor scales. IEEE publications on multipurpose wideband anechoic chamber design address the absorber selection, structural shielding, and quiet-zone validation procedures required to support antenna gain and pattern measurements from hundreds of megahertz through millimeter-wave frequencies.

Electromagnetic Compatibility and Immunity Testing

EMC anechoic chambers are the primary facility for radiated emissions testing (measuring how much electromagnetic energy a product radiates) and radiated immunity testing (measuring how a product responds to externally applied fields). Fully anechoic chambers with absorber on all six surfaces provide the best suppression of multipath reflections, and they are increasingly preferred over the semi-anechoic chambers specified in earlier EMC standards. Validation of chambers for radiated emissions compliance uses the normalized site attenuation (NSA) method, in which the measured transmission between a reference antenna pair is compared to the theoretical free-space value. IEEE Conference publications on validation of anechoic chambers for radiated emission tests describe the NSA measurement procedure and the acceptance criteria that determine whether a chamber qualifies for regulatory testing. TEM cells and reverberation chambers serve as complementary or alternative test environments for immunity testing at lower cost, but anechoic chambers remain the standard reference facility for emission compliance.

Applications

Anechoic chambers have applications in a wide range of disciplines, including:

• Radiated emissions testing for regulatory compliance of consumer electronics, industrial equipment, and medical devices
• Antenna characterization for wireless communications, radar, and satellite systems
• Acoustic noise measurement of audio equipment, household appliances, and vehicle components
• Electromagnetic immunity testing of avionics, automotive electronics, and military equipment
• Research and calibration work involving precise field measurements for sensor and transducer development