An anechoic chamber is a room with special walls that absorb as much sound as possible. Anechoic means "without echoes". Sometimes the entire room even rests on shock absorbers, negating any vibration from the rest of the building or the outside.
The material covering the walls of an anechoic chamber uses wedge-shaped panels to dissipate as much audio energy as possible before reflecting it away. Their special shape reflects energy into the apex of the wedge, dissipating it as vibrations in the material rather than the air. Anechoic chambers are frequently used for testing microphones, measuring the precise acoustic properties of various instruments, determining exactly how much energy is transferred in electro-acoustic devices, and performing delicate psychoacoustic experiments.
The world's first wedge-based anechoic chamber was built in 1940 on Murray Hill, at Bell Labs in New Jersey. It is encased in more than a meter of concrete to shield it from external noise. Its creators have boasted that the chamber absorbs over 99.995% of the incident acoustic energy above 200 Hz. The wedge-shaped panels are poor at absorbing lower frequencies, but these frequencies carry little energy and are inaudible to human ears. At one point, the Murray Hill chamber received the Guinness Book of World Records' award for being the world's quietest room.
John Cage, a famous experimental composer, was inspired when he entered Harvard's anechoic chamber in the 1940s and heard the sound of his own blood circulating. He ended up composing a three-minute piece that consisted of nothing but silence, to allow audiences to reflect on the reality that no person has yet been able to escape noise entirely -- except presumably the deaf.
Special anechoic chambers are also constructed to test a variety of electromagnetic devices. Different shaped wedges allow the reflection of different frequencies, such as radio.
Research in anechoic chambers regarding the specific ways in which the human head reflects sound energy has lead to the development of speakers that project virtual sound around the listener. These speakers exploit the way we hear sound to make us think it is coming from one direction when it is truly coming from another. One day it could be possible to simulate entire orchestras with merely a couple of speakers.