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anechoic chamber[¦an·ə¦kō·ik ′chām·bər]
a specially equipped room for acoustic measurements under conditions approaching those in free open space (a free sound field). The walls, ceiling, and floor of an anechoic chamber are covered with sound-absorbing materials, providing an almost total absence of reflected sound waves. In modern anechoic chambers the sound-suppressing finish consists of wedges of light porous material (glass fibers) with their bases toward the walls. In large anechoic chambers the absorption of sound energy may be as high as 99 percent in the frequency range from 50 to 70 hertz (Hz) to the highest audible frequencies. For anechoic chambers with dimensions of 4–5 m, the lower limit for operating frequencies is usually about 100–120 Hz. The absence of noticeable reflections achieved in an anechoic chamber minimizes interferences and standing waves, thus providing a close approximation to the ideal form of a sound wave (a pure traveling plane or spherical wave).
Anechoic chambers are used for the calibration of sound-measuring microphones in a free field; in testing loudspeaker outputs and response patterns; in studying the noise generated by machines, transformers, and other objects; in determining audibility thresholds; and in other subjective measurements of loudness. In all such research it is important not only to achieve a close approximation of a pure traveling wave but also to provide highly efficient soundproofing and vibration-free construction, that is, to protect the anechoic chamber against external noise and vibrations. The chamber dimensions must be such that the sound receiver and source are located far enough apart so that the receiver can be placed in the zone of virtually plane waves (for frequencies below 100 Hz this distance should be greater than 1 m).
REFERENCESFurduev, V. V. Akusticheskie osnovy veshchaniia. Moscow, 1960.
Molodaia, N. T., L. Z. Papernov, and V. I. Shorov. “Zaglushennaia zvukomernaia kamera.” Elektrosviaz, 1969, no. 8.
I. G. RUSAKOV