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An anechoic chamber is a shielded room designed to attenuate sound or electromagnetic energy. Anechoic chambers were originally used in the context of absorbing acoustic (sound) echoes caused by internal reflections of a room, but more recently anechoic chambers have also been used to provide a shielded environment for radio frequency (RF) and microwaves.
An RF anechoic chamber is designed to suppress the electromagnetic wave energy of echoes: reflected electromagnetic waves, from the internal surfaces. Both types of chamber are constructed with echo suppression features and with effective isolation from the acoustic or RF noise present in the external environment. In a well-designed acoustic or RF anechoic chamber, the equipment under test receives acoustic, mechanical or RF signals from a signal source, a perfect chamber will not internally reflect these transmitted waves. This ensures the integrity of the subject being tested is not influenced by external or internal reflected noise. Furthermore, the shielding of the chamber limits interference from equipment located outside of the chamber.
Anechoic chambers range from small compartments to ones as large as aircraft hangars. The size of an anechoic chamber depends on the size of the objects to be tested and the frequency range of the radio or microwave signals used. Radio frequency interference (RFI) is the unwanted reception of radio signals: sources include lightning, electrical equipment, fluorescent lighting, cell phones, and transmissions from television and radio stations. RFI testing helps determine which frequencies affect particular electronic systems and provide clues to mitigating the risks to communication devices or developing measures to counter the interference.
http://en.wikipedia.org/wiki/Anechoic_chamber