I had to write a quick note on this, as I was reminded about it while thumbing through a popular mixing book that had a 15 page section on speaker resonance and room sound proofing. I must say, a very exciting 15 pages! Ha!
Way too much is made out of room sound proofing! I can audio master songs to perfection in an untreated basement, a huge studio room, in a bathroom, anywhere. I've done it all before. I can even master in my own backyard, with no walls!
Now, I would have to listen to a few reference tracks to learn a new environment. And some client feedback might be required. But after a couple hours, I could master anything in any space. At least 15,000 of the songs I've mastered were done in an untreated room. And my clients LOVED them all!
The proof is in the number of highly satisfied clients I've serviced, over 7,500. And the THOUSAND who were totally blown away by what I did with their music. You don't need foam or sound proofing on your walls. The important thing is to learn your environment, whatever it is.
If you want some foam or acoustic treatment in your room, buy it. You'll still have to learn how the room sounds after you install it. My point is you don't need it for audio mastering.
I've gotten in hundreds of remaster jobs that were originally mastered at studios with thousands of dollars worth of foam on their walls, and these masters were HORRENDOUS! A mash of distorted noise trainwreck! The foam did a lot of good here. Ha!