per wiki, an acoustic anechoic chamber is
a room designed to absorb as much sound as possible. The walls consist of a number of baffles with highly absorptive material arranged in such a way that the fraction of sound they do reflect is directed towards another baffle instead of back into the room. This makes the chamber almost devoid of echos which is useful for measuring the sound pressure level of a source and for various other experiments and measurements.
Anechoic chambers are expensive for several reasons and are therefore not common.
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.
What is the miminal volumn an acoustic anechoic chamber need to be? How to compute this quantity? Is there an existing formula?