See, now this thread should be in Studio Building!
Don't use egg cartons, they don't work. Your choices are:
- acoustic foam. Be sure to buy proper material; Auralex et al. There is cheap, untested foam on the market that doesn't work properly and is a potential fire hazard. Caveat emptor.
- rigid fiberglass insulation. This is what real pro studios use. You can either DIY by sourcing it at a local commercial insulation supplier, or buy prefab panels from a number of vendors.
Those are all room treatment materials, which are very important for good sound inside the studio. But isolation requires construction. Unless you're ready for some 2x4s and lots of drywall, stick with room treatment.