Originally Posted by aaustin
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.
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.
Not trying to offend anyone, but I would like to add my two cents. Money has a setup that lends itself towards sound absorption, and by effect that can contribute to sound proofing. When you are talking about the different densities, and referring that more density is better for sound proofing, that isn't exactly the whole picture. What makes soundproofing effective is isolation and dissipation. When the sound radiates from your speakers, it is generally traveling at 1100 ft/second, when it hits the wall, it actually speeds up. I don't have the exact measurement of how much faster it goes in drywall, but I look at it like a solid. The numbers I remember from the classroom are about 4800 ft/ sec in water and over 16000 ft/sec in steel. This is where density comes in. The faster you can accelerate sound, not only does it consume more energy, but it also doesn't want to slow back down. In fact, making it change speed more frequently, actually consumes more energy than almost anything else. The best example I have of this is when a submarine uses a sound wave to find another submarine. As the sound propagates through the water, say its traveling at 4800 ft/sec, it hits the other sub, but it doesn't reflect off the sub like most people think, it hits the hull made of metal and speeds up as it goes through the hull, then it hits the inside of the sub which is all air, and that is like running full speed into a wall, it can't slow down fast enough, most of the sound is reflected off the air directly back, and some is reflected at 45 degrees back towards the direction it came, there are many other angles of reflection, but those angles have generally consumed so much energy that they dissipate quickly. This is why green goo is so effective, silicone would work as well, but regardless of which is used, it changes the speed as the sound hits it, ( slowing it down), then the sound changes speed again as it encounters the next piece of drywall, ( speeding up). In order for it to propagate to the other side of the wall, it has to change speed yet again, only this time it's just like my example, and most of the sound can't slow down, so it will be reflected back towards the source, only to endure the same transitions. Odds are by the time it gets back to the original piece of drywall, it won't have enough energy to transition to air. This is also why isolation is so important, sound doesn't lose a lot of energy when it hits the solid, be it a wall, I beam, framing, or whatever, it will ride the path as long as it can, so walls attached to ceilings tend to transfer sound in both directions.
In Money's scenario, the carpet not only absorbs a lot of the smaller amplitude dissipations, but it also acts as an item that changes the speed. Further more, this item, (carpet), is kinda performing isolation as its not attached as ridgedley to the wall as the drywall is. An over simplified way to think of this, would be to listen to sound in a room with hardwood floors and then with carpet, the carpet most definitely will not have as many echoes, and the sound will not propagate as far.
Oh, and Money, as I said earlier, your surround sound prob sounds awesome, it doesn't mean that music won't sound good, but your going to have a difficult time getting it to sound like a symphony hall...but why would you want it to, in that room your only concern should be Theater!