Originally Posted by erkq
That's just drywall, right? That won't do anything. Perhaps the rockwool will help sound isolation from the rest of the house more than regular insulation, but not much. And regular insulation isn't worth much to start, perhaps an added 3db of attenuation over an open void in the wall. As far as having drywall as the final layer in the media room... that's what it will sound like... drywall.
Agreed. You won't really see any great benefit at all. Probably not even noticeable without some way to test what it would have sounded without the rockwool. The room inside will still be very "live", and whatever inherent internal acoustical issues you have without the rockwool will still be there when you are done. You might get a touch of sound absorption as it passes through the normally void cavity between the two sides of the same wall, but the fact that the whole wall is connected to itself (drywall inside the room attached to a stud, wood or metal doesn't matter a whole lot, and then connected to the drywall outside the room) means that sound will just transfer right through that wall anyway. Look up "double stud wall" or "staggered stud wall" Those help to "decouple" the inside of the room from the outside. Put your rockwool in the cavities in one of those (just one side of a double stud wall or staggered stud wall so that the rockwool itself doesn't "connect", called couple, the drywall sides again, and you will have a decent core to some sound proofing that you can build off of. Then add more mass (a second layer of drywall inside the room, and the thicker those layers, like 5/8" vs regular 1/2" sheetrock) the better. Put some green glue between the 2 layers, and you will really be doing well. Again, for sound proofing.
None of that helps with in room acoustics. Acoustics as you are saying it (not that you meant it that way) should deal with the sound quality inside the room. Acoustical treatments in that aspect happen AFTER the room is done. Soundproofing is what the designer sounds like he is working towards since it is the room itself that he is working on, and for what he is recommending, you might as well not pay the extra $$ for the rockwool or anything beyond regular construction because, like I said, you won't notice much of a difference at all.
So if you don't care about sound escaping or entering the area, focus on real acoustical treatments INSIDE the room such as bass traps, first point reflection panels, etc. This will clean up the quality of the sound quite a bit, of course in a non-dedicated room, you need to take some care to get things to look good too.
If you want to improve the sound in the room for the least amount of money, have it constructed as the rest of the house is, and spend the money on some decent treatments. This will still be a lot cheaper than the cost of the extra thick walls, rockwool for in the walls, the thicker and double layered drywall, GreenGlue, etc. Plus, those only really help if you have an enclosed space and a nice heavy door to close it all off.
I went for the soundproofing and acoustical treatments for my room that is (almost) finished and absolutely LOVE the results. I can go in the room, shut the door, and if I don't watch a movie, or put on some music, it is just so nice and quiet in there. Great for power naps!
Let us know what your goal is, and people will chime in. Some of the above is just my opinion and others will say that you need to do soundproofing to improve the sound in the room (which IS true), but if you have open paths for sound to escape, then it, in my opinion, isn't worth the cost/effort to end up with a flawed design. Sort of like buying a beat up but expensive sports car and only driving it a couple of blocks at 20 MPH to and from work. You don't get to utilize the performance under the hood, and since it is beat up, it isn't pretty to look at either (like the soundproofing - you can't see it, so people don't even appreciate the effort/cost of it)...