Properly designed and manufactured fibrous media can be very effective at absorbing airborne sound, and as a result, glass fiber and polymeric fiber media are used in a wide variety of noise control applications. The value of these treatments could be increased if they could also be used to damp the vibration of structures to which they are attached. The materials could then serve a multi-functional role, thus reducing the number of elements required to achieve a specified level of noise and vibration performance, and also reducing the total weight of a sound package, an outcome of particular interest at the moment. In past work, it has been demonstrated that layers of fibrous media placed on panels can effectively dissipate the panel motion by removing energy from the nearfield acoustical motion generated by the panel vibration. That is, subsonic panel vibration results in an oscillatory and essentially incompressible flow of fluid parallel to the panel surface. The magnitude of that motion decays exponentially with distance from the surface, but it has been shown that a fibrous medium placed close to the panel can dissipate significant amounts of energy by viscous interaction with the nearfield, which thus has the consequence of damping the panel vibration to which it is coupled. The question then is: How can a fibrous medium be designed to ensure optimal vibration damping in a particular application? The object of the present work was to address that question. First, a method of calculating the response of a panel with an attached fibrous layer is recalled and updated to allow layers of limp, fibrous media to be modeled. Examples of the results that can be obtained by using such a model are presented, and then it is shown that an optimal flow resistivity exists for a given frequency range and configuration of interest. Finally, based on a recently-developed theory for the flow resistivity of fibrous media, the optimal flow resistivity identified in that way can be translated into a particular fiber size, given the density of the fiber material and the desired superficial density.
Porous media, Vibration damping
Acoustics and Noise Control
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