Absorption (acoustics)

Deformation causes mechanical losses via conversion of part of the sound energy into heat, resulting in acoustic attenuation, mostly due to the wall's viscosity.

They must be isolated from outside influences (e.g., planes, trains, automobiles, snowmobiles, elevators, pumps, ...; indeed any source of sound which may interfere with measurements inside the chamber) and they must be physically large.

The first, environmental isolation, requires in most cases specially constructed, nearly always massive, and likewise thick, walls, floors, and ceilings.

The National Research Council in Canada has a modern anechoic chamber, and has posted a video on the Web, noting these as well as other constructional details.

Doors must be specially made, sealing for them must be acoustically complete (no leaks around the edges), ventilation (if any) carefully managed, and lighting chosen to be silent.

An anechoic chamber must therefore be large to accommodate those absorbers and isolation schemes, but still allow for space for experimental apparatus and units under test.

Furthermore, some materials behave in a non-Newtonian way, which causes their viscosity to change with the rate of shear strain experienced during compression and rarefaction; again, this varies with frequency.

Gasses and liquids generally exhibit less hysteresis than solid materials (e.g., sound waves cause adiabatic compression and rarefaction) and behave in a, mostly, Newtonian way.