endorka wrote:It really is excellent. Do viscoelastic treatments like Green Glue act in a similar manner to damping or is it something different?
Thanks Jennifer!
Green Glue (and its lesser copy-cat wanna-be products, such as Quiet Glue) work on a different principle. I'll probably do an article on that at some point too, but basically they work on the principle of "shear". GG is a "Constrained Layer Damping" compound, or CLD, and as the name implies, it is meant to be used by being constrained as a thin layer between two panels of drywall in a leaf. It acts on bending waves: waves that cause the leaf to bend in and out, or that run along he panel itself. If the GG was not there, the two layers of drywall would normally slide past each other as they bend and flex: and they try to do the same in a GG wall... except that the GG is adhered to both surfaces, sort of like peanut butter inside a sandwich (yucky analogy, but the first one that came to mind!
). The peanut butter is stuck to both slices of bread, but it is not solid, and you can swirl and slush the two slices of bread around, you could even rotate one slice around comepltely on top of the other ... but it takes a lot of effort to do that!
And that's the point. In both Green Glue and Peanut Butter, there are are "shear" forces, working along the plane of the goo, as the constraining layers (drywall, or bread) try to slide past each other. Chemical bonds are broken and re-formed in different places inside the goo in both cases... and that takes energy... Which is what you want! Removing energy from the sound wave is the entire purpose of the goo, so you want to formulate it just right that many bonds can be broken and re-formed all the time, but not too easily, as the two layers of drywall (bread...) slide past each other. The shear forces along that plane are responsible for breaking those bonds, and their resistance to breaking, is the "damping".
Here's a graphic that might help understand the principles:
- CLD-graphic-explains-shear-forces-SML-ENH.jpg (55.23 KiB) Viewed 22917 times
- CLD-graphic-explains-shear-forces-SML-ENH.jpg (55.23 KiB) Viewed 22917 times
Some people think that Green Glue is like having a layer of rubber inside the wall, but it is very different. People who don't understand this difference think that you can use caulk, silicone glue, construction adhesive, or any other "soft" product to do the same thing, but you cannot. Those are all more like rubber, than Green Glue. They are very different in how they work, and GG is much more efficient.
Rubber damps in a different way, and there are no bonds being broken and reformed inside the rubber. Rubber is a solid: GG is a gel. GG never hardens: it always remains gooey and tacky (very technical terms, those...), so the sheer forces acting sideways on the GG can break and re-make bonds all the time, for an indefinite time (many, many years at least). Rubber flexes and stretches and bends, but the sheer forces inside a rubber sheet don't usually break the chemical bonds in the rubber: with Green Glue, they do. And that's why Green Glue is so very effective: It takes a lot of energy to break a bond, but only a little to remake it, so energy is lost with each bond break. Or rather, the energy is converted to low-grade heat, which is then dissipated within the material. That doesn't happen with rubber: imagine that rubber is made up of millions of tiny springs or elastic bands, all in parallel: the energy used to stretch out each spring , is then recovered when the spring contracts again. So energy is not lost, or at best only some of the energy is lost. But with GG, a lot more energy is lost, because the "springs" are design to snap apart, then joint together again. Imagine that the springs are magnetic, and each one has a break in the middle... it takes energy to "snap apart" the magnetic halves of the spring.... (OK, so now we have gooey springy magnetic peanut butter rubber in our walls... Umm... I think I need to look for better analogies!
)
I'm sure Andre can add a lot more to that, but that's the general gist of how visco-elastic polymers (such as Green Glue) work. It's all down at the microscopic level.
- Stuart -
