y0
your best bet is to make an inner wall (2 layers of wall)
and leave at least 5 cm between the walls the more air you leave the less sound will leak

best to use is plaster walls

that will make your room smaller
but quieter

my own solution was to move my studio to a neighbourless place

Vibez!

Ok, let’s 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?

Peace,
Zooot