Radiation damping in plates, induced by porous media

Experimental data on the damping properties of a simply supported aluminium plate mounted in a rigid baffle, and separated by an air gap from a slab o f open-celled plastic foam, are presented along with numerical results from two independent analyses. The first is a coupled modal analysis incorporat ing a finite-thickness layer of absorbent and a baffle of finite dimensions . The second is a plate radiation model with an infinite baffle and an abso rbent of semi-infinite extent. Numerical predictions from the two models an d the experimental data are generally in reasonable agreement, and the role s of both the plate/absorbent spacing and the mode of vibration of the plat e are highlighted. It is shown, for example, that the loss factor of the pl ate is very strongly dependent on the air space and that the damping varies quite markedly between different plate modes. The damping is also strongly dependent on the steady flow resistivity of the porous medium. Predictions of the power dissipation per unit volume in the absorbent (with no air gap ) and of the intensity distribution over the plate surface (with an air gap ) illustrate some interesting features of the behaviour of the system. (C) 1999 Academic Press.