Damping of flexural vibration using low-density, low-wave-speed media

Significant damping of structural vibration can be attained by coupling to the structure a low-density medium (such as a powder or foam) in which the speed of sound propagation is relatively low. We describe a set of experiments in which flexural vibration of aluminum beams over a broad frequency range is damped by introduction of a layer of lossy low-wave-speed foam. At frequencies high enough to set up standing waves through the thickness of the foam, loss factors as high as 0.05 can be obtained with a foam layer whose mass is 3.9% of that of the beam. We model the foam as a continuum in which waves of dilatation and distortion can propagate, obtain approximate solutions for the frequency response of the system by means of a modal expansion, and find that the predictions are in close agreement with the measured responses. Finally, we develop a simple approximation for the system loss factor based on the complex wavenumber associated with flexural vibration in an infinite beam.