I'm assuming you mean you want to improve the sound in the room where you're practicing.
-The thickness of any absorber is directly related to what frequencies it will absorb - you can't get away from that. If you get that half-inch thick stuff, it'll absorb freqs from upper-mids and up, and you'll be stuck in a boomy, woofy room that'll drive ya nuts. There's no way around having to use thick material to evenly absorb a wide spectrum of frequencies. There is nothing better for the money than regular pink fiberglass; you build a frame out of SIX inch thick boards - like 1X6s (four-inches of fiberglass isn't the worst thing in the world, but some low-bass will still bounce back out from the wall behind), stuff'em with regular cheap fiberglass, cover'em with something like either chicken wire and cloth, or just the cloth, mount'em to the basement wall (or better, just build it against the wall in the first place), and whatever sound hits the face of them won't come back out! If you make them thinner than that, then some amount of lows will still bounce back out into the room.
-an absorber of any type can never absorb more sound than an 'open window'. Some people have this impression that certain absorbers can somehow 'bend' or 'suck' soundwaves into them or something - this isn't true. No matter what you use, it won't absorb more sound than if it were just a 'hole' of the same area.
-You can't make anything QUIETER than it IS if it's line-of sight in the room with you. This gets back to the sound-sucking myth - a drum set can't be made any quieter than it would be on, say, a raft in the middle of an ocean, unless you put a barrier between you and it. If you were to completely pad every wall - making your basement a complete 'dead room' (which is pretty irritating to work in - you want some amount of reverb!), then it'll be no different than just playing in the middle of a big hayfield or something.
-The main reason it helps to put up absorbers on the walls is that it lowers the reverb time, and 'tightens-up' the overall sound in a room. It is a lot better playing in a room with absorbers on the walls, but it doesn't make the overall volume level terribly much less, really - although this tightening of the sound allows you to hear individual instruments more clearly - more seperately - and it definately reduces 'hearing fatigue', especially over a long rehearsal.
SO... What you want to do, I think, is to make a bunch of these box-absorbers. What I've found works well is to put them from the floor (I say that because you can just support them on the floor - you don't have to mount them on the wall to support their weight) to somewhere over your heads (ya can't beat floor-to ceiling, but anywhere past your ears is pretty good). Make them like between two and four feet wide, and stagger them, so that across the room from each one is bare wall - this has the added benefit of squashing irritating 'flutter reflections'. I think that the optimal thing to do is to cover half the wall surface, so it's like three foot wide box, three foot of wall, three foot box, etc. If you make them too wide, and space them even, then as you walk through the room you can hear reflections from the wall getting louder and softer as you walk by; I like tall, two-foot wide absorbers best. If you don't want to make that many, than just any amount will help - go 2ft-wide absorbers that have 4ft between them or something. If you get all the way down to one quarter coverage - like 2ft boxes with 6ft between them, you'll still get a nice improvement, but much less than that, and I don't know if you'll be able to tell the difference very much - half-coverage with broadband absorbers, on the other hand, will result in a nice, tight room.
There can be lots more involved, of course, but the biggest first step is just to get some of those hard, reflective walls out of the acoustic picture!
That half-inch stuff would suck, unless you stacked it up to four or five inches, and you can't really beat pink-fluff anyway. These boxes can be slapped together real quick. Also, if it's an unfinished basement with ceiling joists, then get some fiberglass and cloth up in there too - that makes a huge difference because the ceiling is so close to you.
Joe
