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We are getting ready to begin a multi-room renovation project and are thinking about sound deadening. We want to either add sound deadening insulation or acoustical drywall to the interior walls but are not clear as to the advantages/disadvantages to each. This would only be for the interior walls. We are not concerned with thermal properties, only acoustical.

Can you provide some tips on which is better?

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If you're only going with one or the other, my personal opinion would be to go with accoustical drywall and seal the perimiter with accoustical caulk. Insulation goes between the studs, so you could still have vibrations traveling through the studs themselves and the top and bottom of the wall's frame. If you use sound-deadening drywall like QuietRock or use a double layer of regular drywall with Green Glue in the middle, you can avoid that flanking much easier since you're covering the entire area of the wall's frame. Of course, worst-case sound situations could still allow sound to flank through the floor or ceiling, under or over the frame, and through the floor or ceiling of the neighboring room, but I'd think that would be an extreme case.

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Personally I'd go for the accoustical drywall over the insulation, too. I'd add that my understanding is that what stops sound transmission is 1) mass, 2) density changes, 3) removing mechanical coupling. When I built a recording studio, each side of the stud walls was faced with 5/8" sheetrock, then 3/4" MDF, then another 5/8" sheetrock. The density changes help stop the sound. Mounting the sheetrock with some sort of a resilient system rather than simply screwing to the studs can help, too. –  mac Nov 7 '12 at 16:13
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Adding density is definitely effective, but the way Green Glue and QuietRock work is via viscoelasticity which converts sound waves into heat as opposed to using density to stop sound transmission. Decoupling the drywall from the frame as you said is a great method, but it is laborous. I've hung a double layer of drywall with Green Glue on RSI clips, though, and I think the STC on this wall is off the hook. –  oscilatingcretin Nov 7 '12 at 16:18
    
I've no data to back this up, but I suspect each technique is more effective at certain frequencies than at others, and they are dissimilar. Thus the best technique would be to do both! But as indicated above, if you do not deal with flanking paths, it's not worth doing anything. –  bcworkz Nov 7 '12 at 20:58
    
Green Glue boasts good performance at lower frequencies, as does Pliteq who manufactures Genie Clips for a floating wall assembly. –  oscilatingcretin Nov 8 '12 at 13:54

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