“Black background” — What really contributes to this phenomenon?

there are 3 sources of noise:

Mechanical noise coming from unwanted vibrations and resonance...

Electrical noise floor level of the HOUSE not only from the room and gear...

Acoustical treatment and controls to help listening what has been gained by controls of the mechanical and electrical dimensions.. But the room non isolated will be also a source of noise...

redwooodaudio B and K calibrated mic into Studio Six on an ipad. The trusty and hyper durable Radio Shack analog meter is quite good for low frequency work. Yes dedicated line is wonderful 

true fast is all the drivers start in the right direction , see the impulse response test for loudspeakers. The flat earth random discovery society is anti meter and anti laser. They are no substitute for listening but they are valuable tools. We don’t attain black backgrounds in the recording studio by letting environmental noise in. 

RT 60

or wander randomly.....