Originally Posted by BllDo
This is absolutely not correct. The second layer of drywall needs to be secured to the structure holding the wall board whether it be channel or studs.
I'm afraid we disagree on this point. This is the installation technique I got from Quietrock and the people selling the products. And my own experience.
The logic is that sound waves hitting the surface of the board can transmit through the screw to the stud (this is limited to frequencies with wavelengths equal to or shorter than the length of the screw), thus allowing higher frequencies to transmit. By screwing to the first layer of DW only you are breaking this link.
The green glue people would also like to see you use a bead of green glue on the studs before installing the board which would further dampen this connection. It didn't look like BEF had used this bead on the stud, so I was limiting my comments to installing a second layer of board to an existing drywalled room which is, I believe, where Nightlord is proceeding from.
FYI...The issue with attaching DW to a new wood stud without green glue, is that the stud will probably shrink slightly as the wood acclimatizes to the room humidity. The screw holds fast in the wood and when the wood shrinks a gap can develop between it and the DW, allowing movement. If you have ever seen a "popped" DW screw, if you push on the DW, you will probably see movement at the screw. This is why. This can also be a major problem with old, very dry studs, if you drywall in the summer in an un-air conditioned room. The studs actually swell in the humidity and shrink once the building is sealed and the heat/ A/C drys the air. Now, if you have a layer of DW that can move, it acts as drum skin and makes the air in the stud space vibrate like a drum and those nasty lower frequencies have escaped.
Now, this is all pretty picky in the grand scheme of things, and as I have previously said, all for not, if you then start cutting holes through the DW for lights, wiring and ventilation without sound traps!
There will always be different opinions about how things should go together. What works in one case won't necessarily work in another. You literally have to look at all the details around a particular situation. I'm just trying to share my experience in trying to "minimize" sound transmission that I've learned through watching and listening to others in the construction industry and my own trial and error R & D as a contractor over the past 30 years.
...But, as always, I'm willing to listen to your argument.