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An anechoic chamber is a room that is isolated from external sound or electromagnetic radiation sources, sometimes using sound proofing, and prevents the reflection of wave phenomena (reverberation). In rooms such as these, the only sounds which exist are emitted directly from their source, and not reflected from another part of the chamber. Anechoic rooms have the characteristic of being muted, muffled, and silent. Anechoic chambers are widely used for measuring the acoustic properties of acoustic instruments, measuring the transfer functions of electro-acoustic devices, testing microphones and performing psychoacoustics experiments (such as measuring the quality of audio codecs or measuring head-related transfer functions). Anechoic chambers, modeled after the world's first wedge-based anechoic chamber at Murray Hill, Bell Labs, typically use fiberglass wedges (Anechoic tile) on all walls of the chamber to absorb incoming sound waves. The wedge shape acts as a waveguide to focus incoming sound into the fiberglass wedge, where the acoustic energy is converted to heat. The alternating pattern is used to achieve a more uniform angular absorption. Frequencies below 200 Hz are not as effectively absorbed by the wedges. Anechoic chambers in which the bottom is also composed of wedges have the floor formed by a wire mesh suspended above the bottom by wires. Other anechoic chambers only use wedges for five of the six sides of the room. To prevent external sounds from entering anechoic chambers, most are encased in a meter or more of cement and may be surrounded by additional insulating materials. John Cage, a 20th century composer, cited his experience in 1951 in Harvard University's anechoic chamber — a room in which he expected to hear nothing, but heard instead what was believed to be the sound of his own bloodflow and nervous system — as the inspiration for his famous "silent" composition, ''4' 33"''. Anechoic chambers for electromagnetic radiation absorption are used in the aerospace industry for radar cross section measurement, among other areas. Anechoic-chamber technology is also used in soundproofing rooms for indoor shooting ranges, and for hearing aid test chambers. In electromagnetics, anechoic chambers are equipped with absorber material* to damp the reflection of electromagnetic waves and are used to measure the properties of antennas or of electronic devices which emit, or are susceptible to interference from, electromagnetic (radio/microwave) energy. The absorber material is typically pyramidal in shape and made of a carbon-impregnated foam that acts as a resistance, dissipating any energy that strikes it. These Radio Frequency/Microwave anechoic chambers can range in size from a small room to a large airplane hangar, depending on the size of the object to be measured and the frequency range of the radio or microwave signals employed. Testing can be conducted on full-scale objects, including aircraft, or on scale models where the frequency of the measuring radiation is scaled up as the size of the object is scaled down. Most radio-frequency/microwave anechoic chambers are located in a screen room, which is a shielded facility that prevents the leakage of radio-frequency/microwave energy in or out of the chamber, thus ensuring the accuracy of measurements and preventing interference with outside systems. Absorber material can be quite flammable and is often protected with an automatic fire-extinguishing system for safety. Incidentally, electromagnetic anechoic chambers exhibit noticeable degrees of acoustic noise damping in practice, despite great differences in wavelengths between microwave signals and audible (voice) signals.
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