What kind of STC rating can I expect?

[Tony Tracey]

I'm building a recording studio in my basement. I have four rooms framed using "floating room" construction. My rooms have a 12" separation gap between parallel walls. The walls are each 4" thick and will be filled with sand. What kind of sound isolation can I expect from this construction? Can anyone give me an estimated STC rating?

Thanks,
Tony

[johnbergstromslc]

Well, if you put insulation in between the walls, I would guess you could get an STC of 65-70 or so, but that much sand in the walls is a bad idea.  

To contain all that sand, you'd have to space your studs closer - 12" o.c. instead of 16", which would hurt acoustical performance - and to keep the walls from bursting you'd need to use 3/4" OSB/plywood sheathing screwed into the studs, on both sides, before any drywall.  This gives you a solid panel inside the wall cavity, which is generally a no-no.  

And having all that weight concentrated on a 3 1/2" wall plate, you'll probably crack your slab.  That wouldn't be good...

Also, it's kind of futile, since you can't have sand in the ceiling.  That's where all the noise is going to come from anyway.  You've already got thick concrete/block walls down there; you hardly need the kind of overkill you're suggesting.

If you want good isolation, go with a double stud wall, insulated, with 2 layers of 5/8" drywall on each side.  Mount the walls on a 1/4" layer of Mass-loaded vinyl (sold on this site) to reduce the flanking noise.  If budget allows, use Green Glue between the sheets to absorb more bass.(http://www.audioalloy.com)   Seal up the wall real good and, voila, STC 65.  Need more?  Slap on another layer of 5/8" drywall per side and you might get 68.  That should be good enough for an already quiet basement.

I think the 'room inside a room' idea is a good one, but do you have enough ceiling height to do that?

[Tony]

What kind of an increase in STC rating does having a 12" gap between parallel rooms provide?




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I'm more concerned about isolation between interior rooms than I am isolating the sound from the outside world.

                           

[Tony]

Sorry, the picture I tried to make abouve didn't come out right when posted.

But to try to further explain. No room has a common wall. Each room is an independent structure with it's own independent walls and ceiling. Each independent room is at least 12" away from any room next to it. So, I have 4 room-inside-a-room structures in a large open basement.  

[Tony]

Okay, I have changed the plans a little. The rooms will be separated by a 36 inch space between the south wall of room 1 and the north walls of rooms 2 and 3 and a 82 inch space between the west wall of room 2 and the east wall of room 3.

So, now my questions relate to 36 inch and 82 inch spacing between parallel walls, rather than 12 inch spacing. Estimated STC?

Thanks

[johnbergstromslc]

Tony:

I honestly have no idea what your STC rating would be in any of the configurations you mentioned. Acoustical labs test common, closely-spaced wall layouts and not unusual setups like that.  If, as you say, all the walls and ceilings are independent of each other and you build them with adequate mass, insulation and sealing, you can expect excellent airborne sound transmission loss - an STC in the 70's, to be sure.  But don't go overboard - most double parallel stud walls are only built with 1" spacing between the plates.  Increasing the airspace isolates more low frequency bass, but only to a point.  Save your floor area.

It sounds like the thing you'll have to worry about most is flanking noise from the slab.  The vibration can travel through the concrete, into and around your walls.  

You'll want to provide for some kind of 'floating floor'.  The ideal would be another slab of concrete on top of a thick, soft underlayment product, but you could also get good results with 1/2" closed-cell foam (or a foam+MLV product sold on this site) and 2 layers of 1/2" OSB/plywood glued and screwed together, with the seams staggered, sealed and taped.  Just remember to isolate the perimeter with foam too - don't let the wood rub up against the walls.  

Also, I mentioned to mount the walls on a layer of mass-loaded vinyl.  This will help kill some of the vibration that would travel into the wall plates and across the slab.