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goroth wrote:I'm gonna wade in here with knowledge that is potentially made up, but with regard to the gap the air is not doing anything. What you are aiming at is mechanical separation of the room within the room. As a medium air does just fine transmitting sound.

Yup mechanical separation is the main thing, the absence of transmitting material is the key, but I wondered the size of this empty space (air) could somehow influence the effectiveness of everything.

i am in the home stretch of building a similar space in my basement. I'm doing the room-within-a-room method - i came off the existing ceiling joists with isolation clips & hat channel, then did the ceiling & walls with 2 layers of drywall & green glue in between. I'm currently about to start sanding the drywall mud, so i can prime/paint and start hanging some acoustical foam treatment on the walls & bass traps in the corners. I don't expect it to be soundPROOF, but i'm hoping for a bit more isolation than i had previously (which was basically none - i live in a 120+ year old house with a brick exterior & wooden floors, so sound made in the basement shoots up through the rest of the house like a cannon.)
I guess my advice would be to do as much as you can afford to do (- i was lucky in that my wife got $500 worth of gift cards from the hardware store with credit card points, so most of my materials were essentially free)