Poor internal wall

[Salpion]

Hello everyone!


I am in the process of building a recording studio above my home residence, which is also under construction. I think I have made a big mistake and I need a kind of undo-advice from you. The thing is I invested more into sound insulation on the external walls of both control and live room (to avoid complaints from my really crabbed neighbour! ) instead of the internal wall between the two rooms.


External walls: regular brick (4 inches / 10 cm) + 2 layers of polysterine (4 inches / 10 cm) + regular brick (4 inches / 10 cm) + rockwool (2 inches / 5 cm) + 1 layer of half-inch plasterboard.


Internal wall: regular brick (10 inches / 25 cm) and that's it!!!


Since the internal wall is probably quite poor for insulation, I am thinking of adding to the existent 10-inch brick wall a 4-inch brick wall (with 4-inch air space in between) + 1 or 2-inch rockwool + 1 layer of half-inch plasterboard. That's a total of 19.5 or 20.5 inches (49.5 or 52 cm).


My questions are:


1. Taking for granded that 52 cm is the maximum sum of thickness I can get (or even less if possible) for my internal wall, is there a better combination of the materials above or different calculations of their thickness?


2. What is the ideal thickness of air space between the two brick walls in my case?


3. Is there any sound insulating material to use (instead of having an air space) that would have given me a better result?


4. What kind of rockwool should I look for to get effective sound insulating and absorbing conditions? Any helpful links online? How much thickness should I go for?


5. Is there any other material I could use to replace rockwool for more effective sound insulation or acoustic absorption?


Thanks a lot for taking the time to read about my concerns!