Let me start by saying that I am not trying to chew you out, but merely ensure that everyone is as informed as they can be. I enjoy discussions like this as I think everyone benefits from them.
Originally Posted by moneyisflying
The sound board is more dense than drywall. I don't see how they would sell sounbaord successfully knowing a piece of drywall would block the sound better.
Denisty is equal to mass divided by volume. A 1/2" sheet of soundboard and a 1/2" sheet of drywall have the same volume, thus the only factor that affects their densities in relation to each other is their mass. Drywall is heavier, and is thus more dense.
I tried to find an STC number for one layer of soundboard to compare it directly with one layer of drywall, but all the documentation relates to soundboard covered with a layer of drywall.
Addressing why they continue to sell it, soundboard is an older product that was made popular before the physics of soundproofing was better understood and before new technologies were made available that simply perform better for the money. I'd guess that they still sell it simply because of the name. Many people would see the word soundboard and immediately think that it is the best product.
Unfortunately we can't objectively judge how well a construction method works just by how it performs in one situation in someone's home. There's just too many other variables involved. The only way to compare things accurately is using certified lab data. Fortunately, we can do just that as tests have been done.
Lets look at two different wall structures and the isolation we get from each one. For simplicity sake, we'll say that each one is 4'x8', framed with 2x4 studs 24" on center, and contains R13 fiberglass insulation. Neither features any decoupling (though it should be mentioned that this is highly recommended as it only requires some extra 2x4's to frame a double wall). We'll also look at the costs of each wall, as that pertains to this thread.
2 layers of 5/8" drywall - $18
2 tubes of Green Glue - $20
1 layer of 1/2" soundboard - $10 (Though I've never seen it less than $20 I will give you the benefit of the doubt)
32 square feet of carpet (assume $2 a square foot) - $64
As we can see, wall 1 is significantly cheaper than wall 2.
Now lets look at some test data obtained at Orfield Laboratory, an NVLAP accredited lab. It must be noted that the walls tested do not directly match the ones that I have described as both sides of the wall were treated instead of just one. Also, the soundboard wall is covered with a layer of 5/8" drywall instead of carpet as I could not find any data on soundboard covered with carpet. However, the heavier weight of the drywall over the carpet would only improve the isolation numbers (there may be a very specific frequency where the carpet is actually better than the drywall in this case since whether you use drywall or carpet will affect the resonant frequency of the wall. This is looking really far into it though.) So while these numbers aren't a perfect representation of these two walls, it can give us a very good idea of how they compare to each other from an isolation standpoint.
If you look at the graph on the first page you'll see the transmission loss vs. frequency for each wall. A higher number means more isolation.
So we can see that wall 1 performs significantly better than wall 2 for almost half the cost. Even if you found a great deal on carpet and got it for $1 a square foot, the double drywall and green glue wall is still cheaper. The performance you get for your money just isn't there with soundboard and carpet.
I'm all for not doing stuff by the book. That's how we learn and come up with new ideas. In this case, however, the evidence just doesn't hold up to the physics that we know and the tests that have been done.
Once again, I'm happy that you are enjoying your room. It looks like you have a great space to watch movies in and I know that it will continue to serve you well for a long time. Edited by aaustin - 1/14/13 at 9:28pm