In this paper three approaches are combined to develop a structural-acoustic model of a rectangular plate-cavity system with an attached distributed mass and internal sound source. The First approach results from a recently presented analysis based on the Raylegih-Ritz method and is used to circumvent the difficulties in obtaining the natural frequencies and mode shapes of a plate with an attached, distributed mass. Furthermore, di erent plate boundary conditions can be accommodated. The resulting mode shapes are dened as continuous functions; this is advantageous as they can be directly used in the second approach, i.e., the classic modal-interaction approach in order to obtain the coupled equations of the system. Finally, in the third approach a group of point sources emitting a pressure pulse in the time domain is used to model an internal sound source. For the validation of the developed model an experiment was conducted in two congurations using a simply supported aluminium plate and a clamped plate coupled with a plexiglas box containing a loudspeaker. Good agreement was found between the analytical and experimental data.