Understanding Damping

Damping is a very straightforward concept.  Simply put, damping is the rate at which something dissipates energy. The higher the damping, the higher the rate of energy dissipation.

The plots below illustrate different levels of damping in a pictorial form. In these plots, what you see is vibration, and the faster this vibration dies down, the higher the damping. Damping factor is a measure of damping performance, much like a batting average or a grade point average.

Extremely poor damping Damping factor = ~.006 This graph shows the response of a stainless steel cooking bowl to a light tap with a mallet. The damping of metals, such as steel, is lower than that of any other common material.
Poor damping Damping factor = ~.005 This graph shows the response of a freely hung piece of 1/2" drywall.  While superior to the steel shown above, damping is still poor.  The poor damping of common building materials such as drywall or wood products is responsible for a lot of the sound isolation problems that the world has today.
Good damping Damping factor = ~0.10 This graph shows the damping of two sheets of drywall laminated with an older damping technology. Damping is vastly improved relative to the raw drywall.
Excellent damping Damping factor = ~0.60 This graph shows the damping of 2 layers of 1/2" drywall bonded with Green Glue Compound.  Green Glue Compound is an extremely efficient damping material, and in this case raises damping relative to raw drywall by about 100 times.  To deliver this level of performance, the Green Glue itself has to exhibit damping much greater than critical damping.

These graphs were adjusted for ideal viewing by adjusting the time displayed on the X axis to compensate for different resonant frequencies in the materials tested. By doing this, the graphs truly reflect the relative damping of each system.

So damping is energy dissipation, but how do we improve damping? To raise the damping of some structures, we have to treat it with a material that has intrinsically high damping. Once this material is applied to the structure, it can transfer some of its energy dissipation abilities to the structure, and damping is raised. Essentially all high-damping materials in use today are viscoelastic damping material. A viscoelastic material exhibits traits in between elastic (springy, like a super ball), and viscous (like peanut butter).

Viscoelastic materials come in two basic types:

Extensional Damping

In an extensional damping system, the damping material is placed on top. Like a coating (paint) or damping pad on the sheet metal of a car door. Damping occurs when the damping material is bent/flexed - compression and extension.

When bent, the material is stretched (Extension) and compressed (Compression), and this allows it to convert some of the energy that it took to stretch and compress it into heat.

Extensional damping is not effective on stiff structures, and as a result is almost solely found in applications where thin sheet metal is to be damped.

In a constrained layer damping system, sometimes referred to as CLD, a damping material is sandwiched between two other (usually stiff/rigid) materials. An example is Green Glue Compound sandwiched between two layers of drywall. Damping occurs when the viscoelastic center of the "sandwich" is sheared.

When bent, shear forces pull and stretch on the damping material, allowing the damping materials to dissipate energy.

Constrained layer damping is effective on even the thickest/stiffest structures, giving it an enormous advantage over extensional damping in many applications.

At most frequencies, the mass of a wall offers resistance to airborne sound. The heavier the wall, the more difficult it is for sound to vibrate it. At some frequencies, however, it is extremely easy for airborne sound to vibrate the surfaces of the wall. These frequencies are called resonant points.

The graphs above show the resonant response of two panels bonded with conventional drywall adhesive compared to the resonant response of two panels bonded with Green Glue Compound. By damping the resonance, Green Glue Compound greatly mitigates resonance problems.

These graphs were generated with an accelerometer and oscilloscope, and show the vibration in the panel as frequency is gradually increased. Around the resonant points of the undamped panel, vibration is extreme.

The graphs above were taken on a conventional wall (2x4 wood studs), and show the vibration response of the wall with increasing frequency. Note the extraordinary reduction in resonant response caused by the Green Glue Compound.

If a material or structure has low damping, the vibration can travel across it for great distances. If a structure has high damping, then the vibration is dissipated quickly and cannot travel very far at all. The graphs below show the magnitude of the vibration resulting from putting a shaker onto first conventional drywall and measuring the vibration near the shaker and then 8 feet away, and then repeating the test on a Green Glue Compound damped drywall sandwich. The difference is stunning. In the undamped panel, the magnitude of vibration hardly changed at all. In the Green Glue Compound panel it falls so much that it is barely above the background noise in the test room.

What Does This Mean For Walls?

It means a lot. It means that in the common wood stud wall, far less energy can transfer to the other side via the rigid mechanical connection provided by the studs.

It means that in decoupled walls, such as staggered stud walls, the connections at the top and bottom aren't nearly as detrimental to performance. It means that in the real world, where walls built with resilient channel or sound clips wind up attached to the concrete by sealant, performance does not degrade. It means that short circuits in resilient channel walls don't cause enormous performance losses. It means that double stud walls sitting on the same concrete slab don't lose performance as a result, and much much more.

An additional - extremely important - benefit of this function of Green Glue Compound is an enormous reduction in structure borne sound. This can have a fantastic positive effect on flanking noise.

In lab tests, the drywall is typically "floated" well off the concrete, and the seams are sealed with thick, dense, high-damping putty or mastic materials. In the real world, drywall is typically on the floor, or separated by a much thinner, much stiffer layer of acoustic sealant. The ability of a damping material, like Green Glue Compound, to reduce energy transfer can go a long way towards making real world performance more like lab performance in many wall types.