I am wondering if anyone can tell me if 1inch of either closed cell or open cell foam would be a better acoustic barrier than R13 fiberglass batt insulation? If not, how about 2 inches of open cell foam? Thanks for the comments/suggestions.

How about two layers of 5/8" drywall.....MASS! http://www.stcratings.com/assemblies.html

I would think spray foam is a better acoustic barrier than fiberglass batts... cellulose would be my first choice.

Posted By Jere on 17 Feb 2010 08:44 AM
I would think spray foam is a better acoustic barrier than fiberglass batts... cellulose would be my first choice.

2lb foam (closed cell) works better than fiberglass or half-pound (open cell) foam, but not nearly as well as dense-packed (3.0+lbs/ft^3) cellulose.  Cellulose both has both higher mass and lower mechanical rigidity going for it compared to 2lb foam.

Mechanically dampening the gypsum/stud interface with rubbery materials on both sides also substantially reduces the coupling via the studs at mid-range & higher.  Doubling the mass of the gypsum by layers applied with sound dampening adhesives (GreenGlue, etc.) also has an effect, as does air sealing all penetrations & seams with sound-dampening caulks (many vendors.)

With staggered stud 2x3 double walls mechanical coupling from the framing is much reduced, and combined with dense-packed cellulose fill the gypsum-to-gypsum transmission is highly damped.

Concrete also works pretty well if you don't care about thermal insulating properties. It's mechanically rigid, but at ~150lbs/ft^3 it has that mass thing going for it oh so well...

What area is this for? If this is an interior to interior wall , one of the most cost effective walls to reduce sound some is double 1/2" drywall on one side, fiberglass batts in the middle, and one layer of 5/8" on the other side. If you are wanting to further reduce sound, you start looking at staggered or double stud walls, masonry, and using doors with seals on all four sides. If you look into any sound studio construction book, you will see that what goes into a cavity isn't nearly as important as decoupling or mass. Based of measurements given in an old book I have, filling a cavity with absorbent material will only give you about a 3db drop in STC.

Posted By aardvarcus on 17 Feb 2010 05:44 PM
What area is this for? If this is an interior to interior wall , one of the most cost effective walls to reduce sound some is double 1/2" drywall on one side, fiberglass batts in the middle, and one layer of 5/8" on the other side. If you are wanting to further reduce sound, you start looking at staggered or double stud walls, masonry, and using doors with seals on all four sides. If you look into any sound studio construction book, you will see that what goes into a cavity isn't nearly as important as decoupling or mass. Based of measurements given in an old book I have, filling a cavity with absorbent material will only give you about a 3db drop in STC.

Even 2.5lb density cellulose is worth 3-4 more STC points over standard density fiberglass in equivalent 2x4 framed structures according to stuff I've read. High density FG buys you a couple over the standard goods. Part of the reason theorized is density, but the theory is never as relevant as the measurement (and cellulose does test better, for whatever reason.)  There are many parts to the mechanical systems that transfer acoustic pressure from one side to the other, and no one silver-bullet.

But half-pound foam doesn't test as well as fiberglass, 2lb foam has different spectral characteristics, and concrete tests VERY well, whatever the reason.

(I expect a concrete filled cavity would be best, eh? )

dolphin;

I would use sound attenuation blankets http://www.certainteed.com/products...lls/317367

how about rock wool sound attenuation blankets? It has a higher fire rating, and isn't fiberglass (and cellulose) associated with dust and asthma triggers?

I understand that cellulose (and concrete) will work better, I am just saying (if this is an interior wall and R value doesn't matter) from a cost to benefit perspective, fiberglass batts are cheap(and DIY) and work fairly well. If this is an exterior wall, and you are trying to isolate the house from the outside, that is a completely different animal. When I hear a question like this, I think of trying to isolate noise from one room out of another room in house (my wife works night shift, I know all about that). The money "saved" by not choosing to use a more expensive cavity fill could be better spent on a solid core door with seals all the way around. That will really keep some noise out. If you wanted higher density fiberglass for extra points, you could just cram 2*6 or two 2*4 batts into the cavities. That would prevent you from having to special order anything.