Can I use tar paper, sometimes called felt, as part of a CLD(Constrained Layer Damping )system?

The automobile industry has used asphalt based sprays to dampen the metal parts of cars. This may be where the notion came from, that asphalt impregnated paper could be substituted as a dampening material in acoustic environments.  And this may be a more accurate use or description of the properties of this product.

But what happens when oils on the paper interact with the sheetrock, the most common type of sheathing used in acoustical environments?

"Oils and other petrols can penetrate into some materials and alter the behavior as well as jeopardize the bond between a material and the host structure, something which will be shown to be very important. Therefore, it is important to study the effects of these foreign elements on the behavior of the material which will be used in a particular application." [3]

One draw back is that in order for this system to work, the parts have to be laminated and the interior has to have the ability to shear which causes the energy to be lost during deformation of the panel.

The roof underlayment (felt) will not adhere or stick to the sheetrock so glue would have to be used to laminate the two together. Glues will not produce the results required in order to develop a CLD (constrained layer damped) system. If anything, the glue will more than likely create a poor connection which is opposite of what you are trying to produce.

"Adhesives, if used, must have a high shear stiffness. Shear strains in the adhesive will reduce the strains in the damping layer, reducing its effectiveness." [1]

A CLD constrained layer damping system is defined as:
"Constrained-layer damping (CLD) systems are usually used for very stiff structures. A “sandwich” is formed by laminating the base layer to the damping layer and adding a third constraining layer. When the system flexes during vibration, shear strains develop in the damping layer. Energy is lost through shear deformation, rather than extension, of the material."[1]

What is the damping agent made of, what are the properties required in order to have an adequate material for a constrained layer damping material?

"Viscoelasticity is a material behavior characteristic possessing a mixture of perfectly elastic and perfectly viscous behavior. An elastic material is one in which there is perfect energy conversion, that is, all the energy stored in a material during loading is recovered when the load is removed. Thus, elastic materials have an in phase stress-strain relationship." [4]

It does have some mass, assuming 30 # felt is in question, so in that respect it can be used for additional isolation material. Tar paper has somewhere in the area of .3 pounds per square foot of weight and as of this day, August 13, 2010 is about 15 USD per 400 square foot roll, so it isn't much mass and should not be relied upon as an isolation material.

To install the tar paper with no bonding agent, since it will not give the desired results of a CLD system but is more closely defined as dry friction damping[5], will only result in small added mass, small if any refraction due to dissimilar materials and questionable damping.

The answer is no.

Felt is not a qualified material to use in a CLD or constrained layer damping system.




References
1.Jennifer Renninger, "Understanding Damping Techniques for Noise and Vibration Control"

2.Harris Cycil M., Piersol Allen G., "Harris' Shock and Vibration Handbook", ISBN 978-0-0713-7081-3

3,4. Craig Allen Gallimore "Passive Viscoelastic Constrained Layer Damping Application for a Small Aircraft Landing Gear System

5.Douglas Thorby, "Structural Dynamics and Vibration in Practice", ISBN: 978-0-7506-8002-8

Additional references

Brian Ravnaas,Technical Innovation Manager,The Green Glue Company: My experience with Green Glue @ Recording.org