Anechoic chambers, also called dead rooms or field-free rooms, are rooms
designed to absorb acoustic echoes, or sound, in order to minimize
internal and external sound reflections as well as providing a shielded
environment for radio frequency, or RF, and microwaves. Offering a sound
energy absorption level of 99% to 100%, or a reflected sound pressure
level of 10% or less, anechoic chambers simulate echo-free conditions in
which the decibel levels given off and affected by industrial products
can be measured.
Anechoic chambers are mainly used in order to create quiet environments for worker safety, audio mixing and research as well as testing the electromagnetic interference, or EMI, of products such as computers, microphones, lightning, fluorescent lighting, cell phones, loudspeakers and electrical components. Ranging from small compartments to chambers as large as aircraft hangers, an anechoic chamber’s size depends upon the product being tested as well as the frequency range of the microwave or radio signals used. Industries that benefit from anechoic chamber testing include electronics, aerospace, automotive, medical, telecommunications, music, industrial, military and audiology. Sound enclosures such as anechoic chambers help eradicate noise pollution, or echoes which distort and dilute the main audio, from the work environment.
There are two types of anechoic chambers: full anechoic chambers are the most common type in which every surface is covered with sound absorbing material while hemi anechoic chambers have sound absorbent materials on the walls and ceiling only in order to test heavy-duty equipment requiring solid floors. Both types of anechoic chambers are constructed using cement or brick walls in order to keep external sound from entering and are typically asymmetrical in design in order to reduce stationary waves. The higher the ratio of sound barrier surface area to open space, the greater the sound absorption; for this reason, anechoic chamber manufacturers line surfaces with corrugated foam wedges, which absorb far more sound than flat foam or other acoustic panels. The corrugated foam wedges can be injected with materials such as carbon or ferrite in order to provide increasing resistive loss. All materials used must have good non-conductive properties in addition to a relative permittivity, or amount of energy a material absorbs when subject to an electric field, that is near unity in order to minimize sound reflection at the interface. The materials are commonly shaped as inward-facing pyramids, which reduce the amount of sound reflected back into the interior of the chamber. Product testing rooms and acoustical research centers requiring absolutely minimal noise pollution build anechoic chambers that are lined with a combination of acoustic foam panels, acoustic drywall and acoustical ceilings.