I recently finished building a studio in my former two car garage, roughly 18x19. Now it's more like 17x18. I put insulation into the walls, then resilliant sound channel followed by a layer of plywood, then a layer of 5/8" sheetrock, then 1/2" sheetrock. I also built a 5x8 isobooth using much the same method.

The beauty of it is, you cannot hear it when you walk 10 feet away from the garage. So now I can record at all hours of the night and not bother the neighbors.

But here's the rub with what you mention...

Egg crate doesn't work. Mattresses don't work. Plain old carpet isn't going to do a great job either. Sound proofing is not an easy task, nor one to be taken lightly. I failed in my first attempt, mostly out of attempting to save money and cut corners. After all was said and done I had to go back and do it again and do it right.

You should have a minimum of two layers of wall. Best if separated with resilliant sound channel or mounted on sound channel. It will need to be sealed around the cracks. Sound channel is pretty cheap and easy to install. It takes a little advanced planning but done right it will make a really good sound proof studio.

Best of luck.

Jody

Jody Whitesides
A Funky Audio Lap Dance For Your Ears!
www.jodywhitesides.com