i have a band and 3 of us are architects. this has its advantages. if push comes to shove, you could do as we did: call acoustic foam manufacturers and ask for samples for a 'project' you're working on. we found a warehouse in town that gave us a whole bunch of the stuff for free and then some extra for a small fee.

You have to separate the issues of sound isolation from those of tuning the space - ie reverb, decay, profile of the sound spectrum.

It sounds like you want to isolate the sound so that others can't hear you on the outside. You will probably never be able to cut out the low frequencies but you should be able to cut the hi and middle.

You might want to create another wall and leave an air space between the new wall and the old of about 1/2 inch. Ideally set the floor plate of the new wall's floor plate onto neoprene so that it doesn't connect rigidly to the floor diaphram or slab (i don't know what you actually have in your space). This is a generic solution. There are a lot of solution that should arise out of the particularities of your particular practice room - I'd have to see it and know the building your in to really tell you.

Another simple solution would be to add another layer of sheet rock hung on resilient channels onto the existing walls so that the new layer of sheet rock "floats", then caulk the seams with flexible acoustic sealant.

Good luck

a cheap way is to sandwich layers of carpet between two pieces of plywood creating an interior wall making your room about 1' thinner in both directions. The air pockets between all the carpet traps sound well between the harder plywood in exterior sheetrock...the change of materiality also really helps kill any sound leaks. I had a studio in central austin in a garage all the wall done as I mentioned and a false floor two feet high for all amps and drums to sit on...we could jam 24/7 no problems.