A plane wave is incident on a simply supported elastic plate covering a back volume; the arrangement is surrounded by a hard ba%e wall. The plate may be porous with a #ow friction resistance; the back volume may be "lled either with air or with a porous material. The back volume may be bulk reacting (i.e., with sound propagation parallel to the plate) or locally reacting. Since this arrangement is of some importance in room acoustics, Cremer in his book about room acoustics [1] has presented an approximate analysis. However, Cremer's analysis uses a number of assumptions which make his solution, in his own estimate, unsuited for low frequencies, where, on the other hand, the arrangement mainly is applied. This paper presents a sound "eld description which uses modal analysis. It is applicable not only in the far "eld, but also near the absorber. Further, approximate solutions are derived, based on simplifying assumptions like Cremer has used. The modal analysis solution is of interest not only as a reference for approximations but also for practical applications, because the aspect of computing time becomes more and more unimportant (the 3D-plots presented below for the sound "eld were evaluated with modal analysis in about 6 s).