Journal of Sound and Vibration (1999) 221(1), 143–167 Article No . jsvi.1998.1987, available online at http://www.idealibrary.com.on RADIATION DAMPING IN PLATES, INDUCED BY POROUS MEDIA† A. C School of Engineering , University of Hull , Hull HU67RX, U.K. H. J. R Department of Mechanical and Manufacturing Engineering , Trinity College Dublin , Dublin , Ireland  R. W School of the Built Environment , University of Nottingham, Nottingham NG72RD, U.K. (Received 17 February 1998, and in final form 2 October 1998) 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 of open-celled plastic foam, are presented along with numerical results from two independent analyses. The first is a coupled modal analysis incorporating 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 absorbent of semi-infinite extent. Numerical predictions from the two models and the experimental data are generally in reasonable agreement, and the roles of both the plate/absorbent spacing and the mode of vibration of the plate are highlighted. It is shown, for example, that the loss factor of the plate 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.  1999 Academic Press 1. INTRODUCTION The transverse vibration of a thin metal plate, mounted with low-loss boundary conditions, is normally lightly damped if the plate radiates into a gas. Acoustic radiation, internal losses and dissipation at the boundaries are the prevailing damping mechanisms, but the combined effect of these is nonetheless fairly small. If, however, a layer of porous, acoustically dissipative material is placed near the † A shorter version of this paper was presented at the ISMA21 Conference in Leuven, Belgium, 18–20 September 1996. 0022–460X/99/110143 + 25 $30.00/0  1999 Academic Press