Y2Ray -
I'd rethink what you wrote...
Heavy curtains do nothing but absorb the very high end, and as for cardboard on the ground.... hahahahahahhahah... thats the most rediculous think i ever heard.
Look, I don't wanna flame you on this one, so I'll give you 10 out of 10 for effort, and having the guts on your first post...
but...
Things like heavy curtains and eggboxes are a total waste of time. The thickness of the material means it will only absorb the high endfrequencies within your room. Which will leave your mixing area bottom heavy, with muddy bass and mids.
aostudios:
What you really should consider doing is making your own sound proofing stuff. Some carpentry skills and brains is usually whats required.
When we talk about sound in terms of sound leaking in or out of a space, we must consider is as changes in pressure, not as waves. Tomi M is completely right when he says that things must be airtight, because when things are airtight, these changes in pressure will not escape the room (ideally...)
So,
The first step would be to put a seal on any doors, effectively making the door completely airtight when it is closed. That means the floor too. This can usually be done with a peice of rubber around the door frame and the door. When the door is shut, VOILA, the whole space around the door
should be airtight.
Depending on how much permanent change you want to do to the room, you may want to consider adding some acoustic treatments too, such as bass absorbers and low-mid absorbers.
John Sayer's work on a 'bedroom' studio type acoustic treatment should make a great start.
http://www.locall.aunz.com/~johnsay/HR/index1.htm
I don't know if you spent a lot of time lurking here before you started posting, but John is an accomplished studio designer, with many Pro establishments to his name.
Thankfully, he also wrote a Free guide to acoustic treatments and studio design, which you can find here:
http://www.saecollege.de/reference_material/index.html
This should be standard reading before asking any questions, as i'm sure once you've read it, most of the questions you have will be answered.
Welcome to the board (to both of you, y2ray and aostudios)
Rochey