abbey road d enfer said:
There are many unknowns in your question, starting with how much is enough.
A .5" plank weighs about 6kg/m²; according to the experimental law of mass, that would result in less than 20dB wide-band attenuation. That would probably be somewhat higher at mid-high frequencies and significantly lower at low frequencies. There would be a point were the resonance of the panels may result in increased attenuation, but it's hard to control. Doubling the thickness would not result in much greater attenuation (6dB in the better case), but making a sandwich with a layer of soft material would be beneficial.
OTOH, there are many losses due to the door and aeration ducts that would actually be the limiting factor.
If doubling up won't do much extra, I might as well spend the extra money on some extra insulation for treatment. The boards are super cheap at $10 each which is why I was planning on using so many, and I'll only be saving $100 if I don't double up (but it'll be a much easier and much more "portable" project) I was planning to fill the inside with moving blankets and then additional treatment.
Out of curiosity, how much is 6 dB actually when we're talking about attenuation of external noise?
EDIT: Looks like 10 dB is how much louder a sound has to be to appear twice as loud (and 6 dB is double the sound pressure, correct me if I'm wrong, I found this via Google), so if I could achieve 20 dB of attenuation, does that mean that I should be about to cut down on most outside noise (neighbours, cars, etc.) to a reasonable level?
I decided to use OSB because it's cheap and I'll be able to unscrew the booth when I move and reassemble it. I know it not being actually air tight (not sealing gaps between the wood, or does that "soundproofing tape" actually work for that?) will cut down on the effectiveness (unless there's an easily removable method of doing it, I'm not planning to move for another year or two until I have the money I want saved up)
I'd really just like to cut down on noises from outside (and noises from neighbours, so not being able to hear kids outside, neighbours' music, cars, lawn mowers, etc) enough that I can use a noise gate and still have a really clean enough recording.