The study of the dynamic behavior of a structure coupled with a light fluid and excited by an external harmonic mechanical force needs a good understanding of the interaction between the solid and the fluid. As the fluid is not directly excited, the dynamic response of the fluid in the cavity depends only on the reciprocal influence of both media. This paper quantifies the fluid-structure interaction, as function of both the modal spatial coincidence between the solid and the fluid, and the frequency coincidence of the natural frequencies of both media with respect to the excitation frequency of the force. A simple method of comparison is developed to investigate the optimal contents of the modal basis. This makes the dynamic response of the cavity and the structure converge with a minimum number of modes. In order to address this problem, one has to develop criteria for defining the appropriate modes for each basis. The theory has been applied to four cases and the results show good agreement. Finally, some results are also compared with those obtained by a finite element model to demonstrate the usefulness of these criteria to find the optimal mesh size for both media.