An analytical model is proposed in this paper for predicting time-averaged energies of boundary structures and enclosed sound field. The sound field is directly driven by an acoustic source and the structures are excited through acoustic-structural coupling. The present model is based on the classical modal coupling method but is developed in such a way to improve the computational efficiency in estimating the bandlimited response of acoustic-structural coupled systems particularly in the medium frequency range where a large number of acoustic and/or structural modes is involved. In this frequency range, one often has to deal with manipulation of large complex matrices in order to obtain full mathematical solutions to the system response using the classical modal coupling method. However, this is avoided if the present bandlimited model is used. This paper describes the mathematical development of the model. Numerical examples for a panel-cavity system are presented and the proposed model is compared with the classical modal coupling method in terms of computational efficiency and accuracy in the prediction of the system energies.