How do vibration damping materials in metal sheets work?

Typical applications: chutes, hoppers, machine guards, panels, conveyors, tanks… Thin metal panels of all types.

Benefits of damping sheets: low cost – large noise reductions are possible – rugged – hygienic

Damping techniques: there are 2 basic techniques:-

• unconstrained layer damping materials: a layer of bitumastic, rubber or similar high damping material is stuck to the surface of the panel
• constrained layer damping: a viscoelastic damping material layer is sandwiched between 2 metal sheets to create a laminate. This is an order of magnitude more effective than unconstrained damping

Unconstrained layer damping involves sticking sheets of proprietary high damping material to thin metal panels. As the panel vibrates, it bends and stretches the damping material, causing some of the vibration energy to be dissipated as heat. This is not very effective unless the panel is free hanging – it has little effect on bolted panels. Constrained layer damping on the other hand, traps a layer of high damping material between two metal sheets to create a laminated sound deadened steel panel. As it vibrates, the whole volume of the elastomeric damping material is sheared, dissipating most of the vibration energy. As a result, constrained layer damping such as sound damped steel (SDS) is usually about 30 times more efficient than conventional damping treatments. Apart from much higher performance, SDS also avoids the hygiene, wear and “peeling” problems associated with stick-on damping materials.

How damping materials work

All materials exhibit varying degrees of inherent material damping i.e. a proportion of any vibration energy is dissipated within the material itself. This hysteretic damping is defined as the loss factor of the material that is proportional to the amount of energy that is dissipated for a given vibration amplitude. Low loss factor materials therefore “ring” as the vibration decays slowly. Structures also exhibit loss factors which depend not only on the materials used, but also on the construction method. Bolted and riveted joints add appreciable damping, whilst welded joints do not. Steel and aluminium alloys have typical loss factors of c 0.001; bolted joints c 0.03 and elastomers/rubbers c 0.1 to 1. Hence a high damping elastomer can dissipate up to 1,000 times as much energy as untreated steel. The following table illustrates the effects of different constructions on the damping ratio and the noise radiated by a steel panel or plate.

Construction              Loss Factor   Noise Reduction*
Suspended panel         0.001               0dB
Bolted panel                 0.02                 13dB
Elastomer coated         0.03                 15dB
SDS laminated panel   1.00                  27dB
* subject to a constant vibration excitation

This table illustrates that unconstrained layer damping (i.e. stick-on damping such as anti-drumming pads used on car body panels) is only effective where there are no other sources of damping such as bolted joints. Constrained layer damping (sound damped steel), however, is often up to x10 more efficient as shown by the plot below.

SDS x6 higher performance than conventional damping

This graph is an overlay of the impact noise signatures (noise level against frequency) for a 16swg bolted stainless steel panel for 3 configurations: standard panel; standard panel with stick-on bitumastic damping pad; laminated panel. The noise reduction achieved using the laminated is very substantially higher than for the stick-on damping material.

Effect of material thickness on damping

Damping efficiency decreases with panel thickness, particularly for unconstrained layer damping. Much above c 4mm thickness even the performance of laminates decreases rapidly. Thicker sections and structures can be damped by creating multi-layer laminates or by using more sophisticated and specialist damping techniques such as the dynamic vibration absorbers developed by the INVC for pipelines and large machine components.