What is the miminal volumn an acoustic anechoic chamber need to be? How to compute this quantity? 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?
 A: I haven't seen any such specification for anechoic room/chamber design. I have experience (not designing but working in) small anechoic chambers of some cubic metres (I don't know exactly the number but if I had to guess this would be around 50).
Obviously, since this specific room was not designed to accommodate the needs to measure big setups, big objects, or on "big" distances (the term big here is somewhat relative to the application/measurement), it was not anechoic down to low frequencies (the lower limit was meant to be around 200 Hz to 250 Hz if I recall correctly). Still, for the frequency range it was designed to be anechoic it was rather good, especially at the centre of the room.
Most probably, the bigger the room the better will be for the sound attenuation (you get some free attenuation from the mean free path of the reflected sound), but still, this could increase the frequency span by not much (low frequencies are mainly the problem, and they are not very much absorbed by air).
Since the anechoic design is based on absorption (most often, if not always, found on the walls) I don't think there is any "specification" (if someone finds out that this works well for them, then they shouldn't wait for specs to come out to use it) of minimum of needed volume. The existence of modes in the low-frequency regime for small rooms will most probably complicate things but still, to achieve adequate absorption on low frequencies someone would need a considerable amount of absorptive surface, which in turn dictates the need for more space. More space means a bigger room, which means that the "wave-phenomena" (modal) frequency region moves lower.
