One layer drywall on channel- walls act as large diaphrams. Help lower decay times of lower frequencies due to surface area. Less stc performance. Potential for rattles.
Two layers on channel w/green glue- neutral assembly. Great stc.
One layer with wood backer- extremely stiff wall. Bass decay longer. Potential for reinforced nodes.
Two layers with wood backer- might as well be concrete, but without the stc performance. Not a great use of budget, but people do it.
I was thinking about what you said here and I may have a solution.
But first a few thoughts ...
There are two different goals which maybe fighting one another:
- sound isolation (keeping the wife happy & minimizing ambient noise)
- taming modal reflections in the low frequencies
STC does not cover frequencies below 125Hz ... so a better STC does not necessarily translate to better low frequency isolation. My understanding was that three 5/8 sheets were better than two because the heavier large area on clips would work to better dissipate the low-end. But now you have me wondering if the OSB is too stiff to let the double wall green board sandwich do its job (which possibly make STC worse ... )
Wish I had data on all this but ... An interesting data point on ply/green-glue isolation is that green glue is often sandwiched between plywood sheets in ceilings and floors. Also, I've seen many write-ups where a wall board green glue sandwich was attached to the ply under the floor to do the same.
The 2nd issue you pointed out is that there are benefits to not stiffening the wall too much as the double wall green glue sandwich absorbs modal energy. I wish I knew more about this as I was considering the resilient/walls to only be for isolation w/o benefit to sound inside the room.
I have no practical experience on this topic ... but it makes sense to me that for the green glue to do it's job the wall board needs to flex. So, without any data to back this up, I'm thinking of putting 1/4" wood strips between the OSB and the double drywall. It would allow the double wall board to vibrate and do it's job while still having a backer board to screw into. It also provides the same mass wall on the clips which might help dissipate the lower frequency energy (yet another "I think it might" unknown: does it help, hinder or do nothing). It's my hope that it will will help (especially in the ceiling) with the low end and reduce flanking.
Anyway, I'd like to get any feedback on the above and especially on implementing the 1/4" spacing idea.
