greenlounge wrote: I will measure the current isolation between the main room and another room next to it, with the same internal wall construction
You could do that, but if there's a door between these rooms or into a hall and back to the other room you will get a result lower than expected. You'd really be measuring the isolation of an unsealed (maybe hollow) door.
It may be worth a measure, but I wouldn't bank on it being reliable.
Dan
Stay up at night reading books on acoustics and studio design, learn Sketchup, bang your head against a wall, redesign your studio 15 times, curse the gods of HVAC silencers and door seals .... or hire a studio designer.