VermontDale wrote:How would using two layers (2 mm) of this butyl material between two layers of MDF compare to using Green Glue?
They are different.
I once used 30# roofing felt (A.K.A. tar paper, asphalt-saturated felt/paper, or underlayment) between two layers of plywood on a deck.
It's 1.57mm (0.062 inches) thick.
I did this to prevent rattles.
Green Glue is a viscoelastic compound designed for things around the mass/stiffness of drywall including plywood.
Green Glue, combined with something on both sides such as drywall/gypsum, forms a sound damping/absorbing system known as Constrained Layer Damping - i.e. when it bends it resists bending in a way that absorbs the energy.
Let's chat about constrained layer damping
Green Glue is not glue in the traditional sense. Glue in the traditional sense creates a rigid inflexible bond between two surfaces. Green Glue is a flexible Viscoelastic membrane that converts two layers of drywall into a Constrained Layer Damping system.
This absorbs energy in the wall, not anything that's reflected, increasing TL a bit.
It would reduce the coincidence dip. It would reduce the resonance dip (YEA ! YEA! YEA!)
It would also reduce energy traveling down the wall (in a single layer of gypsum).
As the wall bends, the Green Glue stretches like an elastic (shear force), and converts the energy in the flexing wall to heat.
The concept of a viscoelastic material can be broken down into two parts:
A) Visco comes from the word viscosity meaning how easily a liquid pours. For example, water is not viscous and pours easily, but honey is a viscous liquid and pours very slowly. A viscous material absorbs energy when it is forced to change shape. It takes work to deform a viscous material. A lump of clay has much viscosity. It takes work to change its shape and then it holds that shape.
B) Elastic means that something can be deformed and it returns to its original shape. A rubber ball is elastic because it returns to its original shape, even after bouncing off the floor. It takes work to deform an elastic material but the material is like a spring, and stores the energy. This is why a ball bounces back.
A) + B) = C) Viscoelastic means it takes work to deform the object and also that the object returns to its original shape. But, it just doesn't spring back like a rubber ball.
If we have three balls, one of each type; visco, elastic and viscoelastic, and then we throw them one at a time on the ground, each behaves differently. The visco ball of clay hits the ground and flattens out like a pancake. The elastic ball hits the ground and bounces right back up off the ground and as round as ever. The viscoelastic ball however takes the middle road, it hits the ground and doesn't bounce up but it also doesn't flatten out, just sits there on the ground, still a completely round ball.