Sound isolation ("soundproofing") is an entirely different thing from the internal acoustics of the room. In my experience building a room in our basement, they can work in opposite directions. The less leaky the room (to sound), the stronger will be bass nodes and peaks, and the greater the need for acoustic treatment.
When using two layers of sheetrock, I suggest using different thicknesses. Or you can use a commercial product like QuietRock, which has two different thicknesses with a resilient layer (like Green Glue) bonding them.
I worked with an acoustician who pointed out that any holes allow for sound transfer. So if that’s important in your room (it was here, to keep out HVAC noise), all electrical boxes need acoustical putty pads, edges of sheetrock should be sealed with acoustical caulk, and so on. You can find more info about construction practices on the QuietRock Web site. I also got a few things (door seals, mostly) and advice from
The Soundproofing Company.
I used QuietRock rather than resilient channel because I couldn’t afford to lose the few inches of height that the latter takes. The finished room is just 7 ft high.
The hardest thing to reduce proved to be structure-borne LF vibration, such as the sound of a refrigerator above. But at night, the SPL A weighted is about 28 dB . . . very quiet!
When construction is done, you treat the room as you would any other audio room, with products from GIK, ASC, RPG, Vicoustic, or other acoustics vendors.
I hope that’s helpful and that it’s not TL;DR material.
