Understanding sound damping (with Green Glue)

I've read up on sound isolation techniques, and I understand the physics behind sound damping with products like Green Glue, which essentially converts sound energy to heat when it flexes, thereby reducing the energy in the vibration. It's easy to see how a room with all surfaces damped would do a good job containing the sound. But when installing the damping material and drywall against the subfloor, between the joists, the joists themselves are left as sound transfer paths. There will also inevitably be gaps between the drywall and joists, as well. BUT, if I understand the contribution of the Green Glue correctly, it doesn't HAVE to be a continuous, unbroken surface for the job it is doing. For example, sound entering the joists will transfer to the subfloor above, but the damping material against the subfloor in the surrounding area will still absorb and convert some of that sound energy to heat. The same goes for sound entering the subfloor in the gaps. Now, obviously it would be better to dissipate the sound BEFORE it reaches the subfloor, but the real question is HOW MUCH of a difference can actually be heard between the continuous surface scenario and the non-continuous surface scenario?

Edit: Copied from my post below. Clarifies my question.

To clarify my question, consider this video, which demonstrates the effect of Green Glue "sandwiched" between two ceramic tiles versus conventional adhesive. When striking the tile sandwich with Green Glue using a hard object, the resulting sound is significantly quieter than when striking the tile sandwich with conventional adhesive. Now imagine cutting a strip, about 1/5th the width of the tile, out of the middle of one of the tiles with Green Glue, then performing the test again, striking the tiles in different places, on both sides. This is obviously an experiment I could do myself, and perhaps I will, but I know Green Glue takes about 45 days or more to fully cure, and I can't really wait that long to move forward. Regardless, as I said, I also want to start a discussion about how Green Glue works in this type of application. My prediction would be that the sound would still be significantly dampened, regardless of where you strike the tile. I would think that only the amount of surface area near the source of the vibration that has Green Glue between tile would determine how much damping occurs. So in the drywall under subfloor scenario, I would think that the damping effect of the Green Glue would be essentially the ratio of (drywall and Green Glue covered surface area) / (total floor surface area).
Edited by digi-head - 1/7/13 at 2:27pm

Thanks BIG, now I see why you have 17k+ posts!

I understand that the question can't really be answered with an exact number. But BIG, you yourself said that I should use soundproofing sealant in the gaps between the drywall and joists, but when talking with Ted White, he suggested that it is not necessary. I appreciate that BOTH of you have a lot of experience, and expertise. And I respect both of your opinions. So I'm just trying to start a discussion to try to get a better feel for exactly how the damping compound works. This is both for the purpose of me deciding whether or not to buy $150 worth of acoustic sealant, as well as a general quest for knowledge on the physics of how the damping material works, and specifically the difference between the two different installation scenarios (continuous versus non-continuous).

To clarify my question, consider this video, which demonstrates the effect of Green Glue "sandwiched" between two ceramic tiles versus conventional adhesive. When striking the tile sandwich with Green Glue using a hard object, the resulting sound is significantly quieter than when striking the tile sandwich with conventional adhesive. Now imagine cutting a strip, about 1/5th the width of the tile, out of the middle of one of the tiles with Green Glue, then performing the test again, striking the tiles in different places, on both sides. This is obviously an experiment I could do myself, and perhaps I will, but I know Green Glue takes about 45 days or more to fully cure, and I can't really wait that long to move forward. Regardless, as I said, I also want to start a discussion about how Green Glue works in this type of application. My prediction would be that the sound would still be significantly dampened, regardless of where you strike the tile. I would think that only the amount of surface area near the source of the vibration that has Green Glue between tile would determine how much damping occurs. So in the drywall under subfloor scenario, I would think that the damping effect of the Green Glue would be essentially the ratio of (drywall and Green Glue covered surface area) / (total floor surface area).

Anyone have an opinion on my analysis? Am I missing anything?

Ah, but I think what you're talking about is not really anything to do with the effect of the Green Glue, but rather just preventing "sound leaks". If you're strictly looking at the effect of the green glue, and not the effect of the mass that you're adding on top of the green glue to create the "sandwich", then I think my logic above regarding the effectiveness of the non-continuous application should hold.

So there may be some value in sealing these gaps, but would it really be adding to the effectiveness of the Green Glue? It seems to me that using acoustic sealant in places where the subfloor is punctured, for plumbing, electrical, ductwork, etc., would be extremely valuable, since it would prevent open air pathways. I'm not convinced that using a case or more of it to seal the gaps in the drywall would be worthwhile, especially considering that it still won't make a continuous constrained damping layer (Green Glue sandwich) due to the floor joists.

Actually, you're probably right, but according to The Green Glue company, their acoustic sealant is actually cheaper than silicon caulk. I haven't actually run the numbers on that, and I'm not sure if their product is actually any better than standard caulk or not, but I doubt it's any worse, so if it really is cheaper, then I'll go with it. The reason I said $150 worth is because to seal every gap on each side of every joist, along with all other gaps, I'm looking at a crapload of caulking.

I hear ya. And that's what sucks about this soundproofing stuff, is that you can't know exactly what effect some things are going to have until you do them, there's no way to test before you commit. I originally had planned to do the drywall and GG under the subfloor as the ONLY soundproofing measure due to a somewhat tight budget, but now that I've invested as much time as I have in installing it, along with the $800+ I've spent on it, the thought that it might end up not making much of a difference scares me into wanting to do more!

Like i said before, though, part of the reason for this discussion is just to try to understand how this product works. I'm curious if any of the experts can confirm that my logic is sound, or can tell me what I'm not understanding.

I think that this info on damping (Dissipation Of Vibration Over A Distance) somewhat demonstrates the concept that I'm talking about with the non-continuous application of Green Glue (bottom side subfloor installation). Even though the data doesn't address a non-continuous application, I think the dissipation over distance from the noise source (especially for "impact noise", which has a distinct center point) will be relative to the surface area of constrained layer damping (CLD) between the noise source and the measured point. I would think gaps in the CLD would only reduce the effectiveness ratiometrically.

Thanks for chiming in Ted.
I definitely plan to seal up all places where plumbing, wiring, etc. are coming through the floor.

Are you referring to the question of how well damping works when the damping compound is not continuous? This seems like a very worthwhile test for people that are considering drywall on subfloor with Green Glue as a soundproofing measure.

You said we could mentally spin this for years, but knowing the basic physics behind the damping compound (converts vibration to heat), this seems to be a basic conservation of energy equation. For any section of the floor that has drywall and damping compound under it, if that section vibrates, then the damping compound in that section will convert some of the energy from the vibration into heat, thereby reducing the energy in the vibration. The only effect that the missing sections of GG and drywall should have is that there will be less total damping compound, and therefore less sound energy will be converted to heat. But it should only be less by the ratio of (GG covered floor area / total floor area). Unless the damping compound is somehow more efficient in larger, continuous sections.

Haha, sorry if you think I'm pressing you for something Ted. You've already given me your advice on the sealant, and I'm good with that, and I appreciate it. I guess I"m just wanting you to comment on what I feel is sound logic that I laid out in my previous post. Not to help me make any decision, but just to make sure I understand how this stuff works.

Fair enough. I can see why you'd be hesitant to agree with my assessment, and how it could very well be an incorrect assessment. Thanks for your input.

I do think it would be a worthwhile test, though, since this method seems to be fairly common, and often recommended by companies like yours. It would be nice to have some data to compare it against other methods to determine which is the better bang for the buck.

I'd be willing to do the test for $10k ;-)