Ok, lets give this 1 more go.
Acoustical treatment, like foam, diffusers, bass traps, membranes etc. r there in order to regulate the way sound decays FREQUENCY vs. TIME wise.
Isolating however, means locking the sound energy inside a room. This means that ALL energy must die (absorbed by structure) by structure only.
That means that the room should b made air tight (otherwise energy will b conducted through air to the outside) and structure should b shock insulated from outer walls, floor and ceiling.
This indeed means building a room inside a room, floating the inner structure from floor, walls and ceiling by air, rubber, springs, the works.
LF energy being the major component of the energy in total, is the hardest to control.
Floating the vibrating parts of floor, absorbing LF energy by means of bass traps and membranes may REDUCE LF leakage but not eliminate it.
BTW, Reuven, in order to avoid dealing with the same issues over and over, y not make a FAQ, containing the Frequently Asked Questions (elementary, my dear Watson
) and addressing newbies there?