FWIW, an attic is the worst spot in most houses to try to achieve significant noise reduction.
1. Air paths carry leaked sound, and the attic is designed to breathe. (BTW, if you block up all the attic spots that exchange air, you'll have issues.)
2. The attic's high, and tends to run to the house perimeter on all sides. Any other space in a typical house provides more massy attenuation above and on at least two of its sides.
3. Room-within-a-room treatments (plus multiple rocking out adults, + amps and other gear) = significant extra load. Especially if your house design has vaulted ceilings below the attic (or otherwise involves tall walls transfering vertical loads from the attic), be thoughtful about whether the building is designed to transfer the extra weight.
A basement usually offers far better sound attentuation. Next best is a first-floor room with its exterior walls on the opposite side of the house from the closest noise-sensitive neighbors. But if the attic is the only available space and you must cut the leaked sound to appease the neighbors, go e-drums and DI instruments.