Hi again all
I'm facing having to lose around 1.6 inches in a single stud wall due to beefing up between the studs. This leaves only 2 inches to the inner drywall. Am I still better off than going with less mass and having the bigger airspace? Not sure what to do. Im looking at two layers of 18mm plywood between the studs for the outer layer. Two layers of 5/8 going on For the inside with GG.Thanks.
Between the studs mass v lost air space?
Originally posted at johnlsayers.com, topic 17592.
The general rule of thumb is to never have less than 4" of gap between the inner and outer leaf. Below 4" the resonant frequency gets too high, and trying to compensate with more mass implies a LOT more mass, which takes up even more space... so it's a vicious circle.
Can you move your inner-leaf walls to get back to a 4" gap? If not, you could maybe consider putting hat channel across the studs before you put the drywall on, to increase the depth.
You could also consider using fiber-cement board instead of plywood for your "beef". Fiber-cement is about twice the density of plywood, so you could use half the thickness and still get the same amount of mass, and therefore the same amount of isolation. It's also a lot more expensive, but you only need half as much.
- Stuart -
Hmmmm, yes I suspected as much Stuart. Well, if can possibly glue my plywood sheets to the thin metal garage walls I could have a 7 inch air gap. Don't know how the metal garage sheathing would go with that, maybe someone has done it? I take your point about the plywood but it's already bought cheap used. For the very rough stc Im going for, around low 50's I calculate I should be ok. I gotta find a way to increase The air gap though to get close to that.
Don't forget that STC is not a good rating system for designing studios! It does not take into account anything below 125 Hz, which is actually where most of your problems will be. STC is more for talking about speech isolation, in offices and houses, not music isolation in studios. So if you are aiming for, and achieve, a certain STC value, you might still be very disappointed by how much isolation you actually get in the low end. - Stuart -For the very rough stc Im going for, around low 50's I calculate I should be ok.