Understanding sound damping (with Green Glue) - AVS Forum
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post #1 of 21 Old 01-07-2013, 08:20 AM - Thread Starter
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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).
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post #2 of 21 Old 01-07-2013, 09:46 AM
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15.739 percent.
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post #3 of 21 Old 01-07-2013, 09:55 AM
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Originally Posted by BIGmouthinDC View Post

15.739 percent.

Well, sure, but only near room temperature at sea level... biggrin.gif

I'll let the experts answer, but think of a line of sponges and a wave of water hitting them. If there's gaps between the sponges, you get the idea. Any air "gaps" around drywall/joints are filled with caulk to prevent open air spaces. Same idea with putty pads, etc.

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post #4 of 21 Old 01-07-2013, 10:29 AM - Thread Starter
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Originally Posted by BIGmouthinDC View Post

15.739 percent.
Thanks BIG, now I see why you have 17k+ posts! wink.gif

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?
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post #5 of 21 Old 01-07-2013, 11:09 AM - Thread Starter
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Originally Posted by jautor View Post

Well, sure, but only near room temperature at sea level... biggrin.gif
I'll let the experts answer, but think of a line of sponges and a wave of water hitting them. If there's gaps between the sponges, you get the idea. Any air "gaps" around drywall/joints are filled with caulk to prevent open air spaces. Same idea with putty pads, etc.

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.
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post #6 of 21 Old 01-07-2013, 11:32 AM
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for the purpose of me deciding whether or not to buy $150 worth of acoustic sealant

Unless something has changed in the, ah, umm, SEVEN years since I've been working on my room, regular old 50-year caulk works as well as "acoustic sealant."

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post #7 of 21 Old 01-07-2013, 12:06 PM - Thread Starter
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Originally Posted by tlogan6797 View Post

Unless something has changed in the, ah, umm, SEVEN years since I've been working on my room, regular old 50-year caulk works as well as "acoustic sealant."

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.
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post #8 of 21 Old 01-07-2013, 12:23 PM
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I'd say if it's something that's going to eat at you down the road by way of "What If", then it's worth the time now to just take care of it.

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post #9 of 21 Old 01-07-2013, 12:26 PM
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Actually, I always say to make the room the best it can be. Going back and adding it would be REALLY hard. Even at $150, in the grand scheme of things, it's not so much.

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post #10 of 21 Old 01-07-2013, 12:40 PM - Thread Starter
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Originally Posted by tlogan6797 View Post

Actually, I always say to make the room the best it can be. Going back and adding it would be REALLY hard. Even at $150, in the grand scheme of things, it's not so much.

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.
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post #11 of 21 Old 01-08-2013, 11:06 AM - Thread Starter
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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.

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post #12 of 21 Old 01-08-2013, 04:36 PM
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Originally Posted by digi-head View Post

Thanks BIG, now I see why you have 17k+ posts! wink.gif
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).

Everything I know about drywall under the subfloor I learned from Ted. if he now says it isn't necessary then I guess I just have to rip down some ceilings and redo it. In my opinion if you have hard surface flooring above your theater add the damped mass it does make a difference. I can't say exactly how much but since you can detect it without measurement it must be significant. Check out the Merimount build thread and ask Snickers his opinion.
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post #13 of 21 Old 01-09-2013, 06:15 AM
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Sealant primarily seals for air penetration. Let's establish that.

No lab has ever, nor will likely ever, test such a nuance, so we can mentally spin this for years.

15 years ago, people used fiberglass instead of damping compound in a similar application and sealant was used at the edge of the drywall.

When this technique was adapted using damping compound, the use of sealant at the edge of the drywall was maintained. The thought at the time was that the sealant helped to bond the system together, allowing the floor being treated to act more as a single layer, rather than many sections.

I simply don't believe that happens to any significant extent, and because we don't promote products that don't have a clearly defined advantage, we don't promote sealant use any longer in this specific application. I've thought about this for years, have yet to see any data or theory that refutes my position, so we don't advocate the sealant, and are in the process of pulling the sealant reference from our illustrations.

Keep in mind we sell sealant, and we are not recommending it in this application. Certainly no harm in using it.
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post #14 of 21 Old 01-10-2013, 12:39 PM - Thread Starter
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Thanks for chiming in Ted.
Quote:
Originally Posted by Ted White View Post

Sealant primarily seals for air penetration. Let's establish that.
I definitely plan to seal up all places where plumbing, wiring, etc. are coming through the floor.

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No lab has ever, nor will likely ever, test such a nuance, so we can mentally spin this for years.
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.
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post #15 of 21 Old 01-10-2013, 12:42 PM
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Not sure what you want at this point.

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post #16 of 21 Old 01-10-2013, 12:51 PM - Thread Starter
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Originally Posted by Ted White View Post

Not sure what you want at this point.

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.
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post #17 of 21 Old 01-10-2013, 01:00 PM
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Well, one thing I've learned in the last 14 years is that what seems intuitive, obvious and straightforward often isn't. For example, while it seems reasonable that is 10% of your floor isn't treated, that the performance would drop 10%. That wouldn't happen. A light application of damping compound (1 tube per sheet) has 70% of the damping capacity of two tubes per sheet. Not 50%. Contrarily, going to three tubes per sheet doesn't get you 150% performance, it might only yield 107% improvement. Damping isn't a linear function. Green Glue would seem to be a good thing to use on the face of the stud or joist. Not the case at all.

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post #18 of 21 Old 01-10-2013, 01:19 PM - Thread Starter
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Originally Posted by Ted White View Post

Well, one thing I've learned in the last 14 years is that what seems intuitive, obvious and straightforward often isn't. For example, while it seems reasonable that is 10% of your floor isn't treated, that the performance would drop 10%. That wouldn't happen. A light application of damping compound (1 tube per sheet) has 70% of the damping capacity of two tubes per sheet. Not 50%. Contrarily, going to three tubes per sheet doesn't get you 150% performance, it might only yield 107% improvement. Damping isn't a linear function. Green Glue would seem to be a good thing to use on the face of the stud or joist. Not the case at all.
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.
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post #19 of 21 Old 01-10-2013, 01:25 PM
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Well anecdotally we know it works well. We know that if footfall is an issue we have to get a treatment on the offending surface. We know we need insulation in hollow cavities, and we know we need to decouple the drywall below. We could pony up $20K to tell us this, but...

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post #20 of 21 Old 01-10-2013, 01:29 PM - Thread Starter
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I'd be willing to do the test for $10k ;-)
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post #21 of 21 Old 01-10-2013, 01:31 PM
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You have a NVLAP approved lab? Who'd have thunk it?

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