An anechoic chamber or anechoid is a room designed to fully absorb the reflections produced by acoustic or electromagnetic waves on any of the surfaces that make it up (floor, ceiling and side walls). At the same time, the chamber is isolated from the outside from any source of noise or external sound influence. The combination of these two factors implies that the room emulates the acoustic conditions that would occur in a free field, independent of any type of effect or influence of the room resulting from said reflections.

The frequency range of the anechoic chamber is typically from approximately 200 Hz to 20 kHz, with absorption greater than 95%. It should be noted that there are difficulties at lower frequencies due to the response of the absorbing materials and the dimensions of the chamber.

Sound is a wave that transmits mechanical energy through a material medium such as a gas, liquid, or solid object. In this way, when an acoustic wave hits a surface, the wave is reflected "Reflection (physics)") and/or absorbed "Absorption (sound)") by said surface.

In nature, this phenomenon occurs in any environment except in a vacuum "Vacuum (physics)"), where sound cannot be transmitted. In any medium through which sound propagates, reflection and absorption take place. As a result of reflection, various phenomena such as reverberation and echo occur.

The anechoic room is designed to reduce, as far as possible, the reflection of sound: the anechoic chambers are isolated from the outside and consist of walls covered with pyramid-shaped wedges "Pyramid (geometry)") with the base "Base (geometry)") resting on the wall (among other arrangements), built with materials that absorb sound and increase the dispersion or diffusion of the little sound that is not absorbed. Some examples of these materials are fiberglass or certain foams.

In addition to acoustic chambers, radiofrequency anechoic chambers are enclosures with metal shielding on their walls, emulating a Faraday cage. The chambers are lined with material designed to absorb electromagnetic waves (different from the material used to absorb acoustic waves), in order to isolate the chamber from any type of external influence and simulate free space conditions inside. These cameras have multiple applications in the field of telecommunications, such as the measurement of the specific absorption rate (SAR, Specific absorption rate) of mobile terminals, or the design and characterization of radiating elements such as antennas and dipoles.