There are few places on earth where you can experience complete darkness. These places include the depths of the ocean, caves, and mines. Even in these places, however, there is sound. Complete silence does not happen naturally. In order to have complete silence, you need a special type of room called an anechoic chamber.
The word anechoic (meaning “no echo”) was derived from Greek by acoustics expert Leo Beranek. Special measures must be considered when designing an echo-free room. As much as possible, an anechoic chamber room should be decoupled from the building in which it is built. HVAC systems to supply and return air from the room must be silences. Often, this is achieved by adding silencers to the system and by increasing the length of duct between fans and vents.
All potentially reflective surfaces—floors, ceilings, walls, doors—are covered with acoustic wedges. These wedges, which give anechoic chambers a distinctively science fiction look—absorb and dissipate sound energy.
Full anechoic chambers protect wedges on the floor with a suspended mesh or wire net floor. Semi-anechoic chambers (or hemi-anechoic chambers as some call them) leave the floor free of acoustic wedges. Experiments and tests done in semi-anechoic chambers take reflections from the floor into account.
Anechoic chambers are used in research to create a free field environment. Free field means that sound only travels away from the source and none of it is reflected back. Free field experiments are useful to examine the directivity of sound sources, as well as other acoustic characteristics like sound power and acoustic intensity.
Since anechoic chambers are so expensive to build, they are generally only found at top research and testing facilities. Many of these facilities offer anechoic chamber tours. Some people find the quiet of an anechoic chamber unnerving, many people find it relaxing and peaceful. Many people report hearing the sound of their heartbeat, blood rushing in their veins, breathing, and stomach grumbles clearly in an anechoic chamber. Contrary to popular belief, staying in an anechoic chamber will not cause you to go crazy, although it may cause sensitivity to sound for a brief time after leaving the chamber.
If you don’t have access to one of these quietest places on earth, this video does a great job of giving an idea of what it’s like inside an anechoic chamber.
360-degree Video of an Anechoic Chamber