Effects of Damping

Part 1 - What effect does damping have in a wall?

In the "Understanding Damping" page we learned that damping = energy dissipation. But what does that mean for typical sound isolation applications?

Well, in sound isolation applications, damping has one broad effect - it works to eliminate stored energy. This broad effect can be broken down into two basic effects that describe all of the benefits a product like Green Glue can supply. The first of these is the reduction of resonance problems. The second of these is the dissipation of vibration (sound) as it travels through a structure. Let's take a look at the value of these one a at a time, and at the bottom of this page, we will conclude with a chart showing the actual sound isolation performance of a low damping wall (no Green Glue), and a Green Glue wall. This data was taken at the same lab, and on otherwise identical walls, and illustrates greatly the advantages of damping.

Damping of resonances

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 to the left show the resonant response of two panels bonded with conventional drywall adhesive compared to the resonant response of two panels bonded with Green Glue.  By damping the resonance, Green Glue 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.

Poor damping

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.

Damping factor = ~.005

Dissipation of vibration as it travels over distance

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 damped drywall sandwich.  The difference is stunning.  In the undamped panel, the magnitude of vibration hardly changed at all.  In the Green Glue panel it falls so much that it is barely above the background noise in the test room.

What does this mean to 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 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, to reduce energy transfer can go a long way towards making real world performance more like lab performance in many wall types.

The Actual Results

The net result of these two simple functions is shown in the graph to the left. Both walls are identical in every way, and the wall with conventional drywall performs poorly, with STC=40 and poor low frequency performance around the walls resonance point.

The Green Glue wall, on the other hand, performs marvelously, with an 8dB improvement at the resonance point and enormous gains at higher frequencies. STC is now 55.

The first in this series of discussions was some talk about Understanding Damping.