I'm planning to sound-isolate an already-converted garage built on concrete slab. I want to modify the walls to have mass-air-mass with two layers of drywall inside and out.

I'm on the 7th and top floor in an office building and the professional advice is I need a box in a box with around 100mm concrete for the suspended floor with a 100mm air gap. Proper engineered and calculated rubber blocks and the correct type and amount of insulation in the gap.