An acoustic anechoic chamber is a room designed to be free of reverberation (hence non-echoing or echo-free). The walls, ceiling and floor are usually lined with a sound absorbent material to minimise reflections and insulate the room from exterior sources of noise. All sound energy will travel away from the source with almost none reflected back. Thus a listener within an anechoic chamber will only hear the direct sound, with no reverberation.
The anechoic chamber effectively simulates a quiet open-space of infinite dimension. Thus, they are used to conduct acoustics experiments in ‘free field’ conditions. They are often used to measure the radiation pattern of a microphone or of a noise source, or the transfer function of a loudspeaker.
An anechoic chamber is very quiet, with noise levels typically close to the threshold of hearing in the 10–20 dBA range (the quietest anechoic chamber has a decibel level of -9.4dBA, well below hearing). Without the usual sound cues, people find the experience of being in an anechoic chamber very disorienting and often lose their balance. They also sometimes detect sounds they would not normally perceive, such as the beating of their own heart.
One of the earliest anechoic chambers was designed and built by Leo Beranek and Harvey Sleeper in 1943. Their design is the one upon which most modern anechoic chambers is based. In a lecture titled ‘Indeterminacy,’ the avant-garde composer John Cage described his experience when he visited Beranek’s chamber.
“in that silent room, I heard two sounds, one high and one low. Afterward I asked the engineer in charge why, if the room was so silent, I had heard two sounds… He said, ‘The high one was your nervous system in operation. The low one was your blood in circulation.’”
After that visit, he composed his famous work entitled 4’33”, consisting solely of silence and intended to encourage the audience to focus on the ambient sounds in the listening environment.
In his 1961 book ‘Silence,’ Cage expanded on the implications of his experience in the anechoic chamber. “Try as we might to make silence, we cannot… Until I die there will be sounds. And they will continue following my death. One need not fear about the future of music.”