Understanding Damping
Part 1 - What is 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
Excellent damping. This graph shows the damping of 2 layers of 1/2" drywall bonded with Green Glue. Green 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.
Part 2 - What Are Viscoelastic Materials?
So damping is energy dissipation, but how to we improve damping? To raise the damping of some structure, we have to treat it with a material that has intrinsically high damping - an intrinsically high rate of energy dissipation. 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 flavors:
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.
Constrained Layer Damping
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 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.
Next up in this series of discussions, we look at the "Effects of Damping" on soundproofing in real walls.