JA> For both apps the sound insulation properties are 
 JA> excellent for the extra money involved. With the 
 JA> addition of a second regular type drywall - wall, with 
 JA> the studs completely separated from the panels by a 
 JA> mere 1 inch, the payback is suppose to be almost total 
 JA> soundproofing. That is quite a claim, so it would be 
Using words like "almost total soundproofing" suggests having very little 
idea what you really mean.  With "almost total soundproofing" I presume your 
walls will reduce a 140 dBspl on one side to a barely perceptible 25 dBspl on 
the other, across a wide frequency spectrum?  
For that you'd likely be talking a poured concrete outer room, and an inner 
room suspended from an overhead structural exoskeleton by shock mounts, with 
the inner room costing more than most homes.  Just ventilation might require 
several times the space of a large home theater for acoustic isolation.  
Are you working over a slab?  Joist supported floor?  Other?  
High frequencies are easy to manage with surface treatments.  A couple inches 
of add on materials can eat up 99% of what's bouncing around.  Low end 
control takes some combination of huge spaces, like several foot anechoic 
wedges, mass, like from multiple layers of 5/8 FC sheet rock, poured 
concrete, etc., isolation, as in multiple structurally isolated layers, and 
good seals, including caulking bottoms of walls, not making holes for 
outlets, and seals around heavy doors.  Space isolation can also be useful, 
such as designing a closet to allow a few feet between sets of walls.  
You're likely thinking of 50 dB of isolation as high, and 35 dB as average, 
but with different frequency distributions shifted a bit toward improved low 
end isolation.  If you want a basic starting point for casual use, it can be 
easier to 2x4 frame on 2x6 plates on 24" alternate centers and use heavy 
sheet rock (5/8 FC) than to add a 3rd layer in front of common house walls.  
Any such walls should be sealed to the floor, and insulated (special sound 
blankets prefered, though fiberglass batts help).  2 or 3 layers of sheet 
rock, or lead blankets under rock, increase low end isolation.  You'd also 
got to think about floors and ceilings.  It's common to bring isolated stud 
walls to an upper deck level with a solid ceiling or roof, and then drop an 
acoustic panel ceiling (such as 1.5" Armstrong knubby FG board) below that.  
Slabs are good for floors, though suspended raised ones or lightweight 
concrete overpours can be useful.  
Every little detail counts on overall quality.  In a home theater where you 
might want 110 dBspl, the above might make it tolerable near an active living 
room, but not near a bedroom without a lot more work.  
It is possible to get a 16 dBspl in a studio under 200' from I-95 (and has 
been done), but you probably don't want to think about the cost.  I can only 
think of one large commercial studio location that bothered acheiving that 
level of quiet.  
Terry
 
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