Role and Limitations of Damping in the Control of Vibration and Noise

Abstract

The applications of damping are directed toward the reduction of vibration and noise. In this paper, the limitations and possibilities of achieving useful reductions are discussed. This is done by separating the problem into response and radiation behavior. The prime consideration in the assessment of damping effectiveness is its relative role in affecting the so-called forced and free wavemotions of the structure. The details of the effects depend on pure tone vs random excitation and correlation behavior both in time and space. Thus, in acoustic noise-reduction studies where the forced waves can dominate the behavior, the effect of damping can be quite different from the case of boundary-layer-noise transmission where the free waves dominate the motion. In some cases, it is not sufficient to merely describe the damping by a loss factor. It may be of some significance whether the losses occur at the joints of the structure or occur in a distributed way over the panel. A statistical description of the structure is governed by three parameters: the structural mean free path, the bending wavelength, and the loss factor. The combination of products of these dimensionless factors determines the damping effectiveness. These factors also control certain aspects of the division of radiation damping of the structure. A tabulation of these regimes of behavior is presented. [This research has been supported by the National Aeronautics and Space Administration.]