John Cage and the anechoic chamber

 

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.”

Breaking the sound barrier

Consider a source moving at the speed of sound (Mach 1). The sounds it produces will travel at the same speed as the source, so that in front of the source, each new wavefront is compressed to occur at the same point. A listener placed in front of the source will not detect anything until the source arrives. All the wavefronts add together, creating a wall of pressure. This shock wave will not be perceived as a pitch but as a ‘thump’ as the pressure front passes the listener.

Pilots who have flown at Mach 1 have described a noticeable “barrier” that must be penetrated before achieving supersonic speeds. Traveling within the pressure front results in a bouncy, turbulent flight.

Now consider a sound source moving at supersonic speed, i.e., faster than the speed of sound. In this case, the source will be in advance of the wavefront. So a stationary listener will hear the sound after the source has passed by. The shock wave forms a Mach cone, which is a conical pressure front with the plane at the tip. This cone creates the sonic boom shock wave as a supersonic aircraft passes by. This shock wave travels at the speed of sound, and since it is the combination of all the wavefronts, the listener will hear a quite intense sound. However, supersonic aircraft actually produce two sonic booms in quick succession. One boom comes from the aircraft’s nose and the other one from its tail, resulting in a double thump.

The speed of sound varies with temperature and humidity, but not directly with pressure. In air at sea level, it is about 343 m/s. But in water, the speed of sound is far quicker (about 1,484 m/s), since molecules in water are more compressed than in air and sound is produced by the vibrations of the substance. So the sound barrier can be broken at different speeds depending on air conditions, but is far more difficult to break underwater.