Synthetic jet actuators are of interest for potential applications to active flow control and thermal management. Resonant piezoelectric-diaphragm-type configurations are commonly considered. Modeling of such actuators remains a challenge due to complexities associated with both electro-elastic and fluid-structure coupling, as well as potential non-linearities in both. In this paper, the assumed-modes method provides an energy-based low-order model which captures electro-elastic and acoustic-structure couplings with adequate accuracy. Tri-laminar circular plates under simply-supported boundary condition with boundary joint spring and 1-DOF (assumed uniform pressure) acoustic mode of the cavity-orifice system were modeled using the assumed-modes method. Transverse motions of the two diaphragms were considered separately to allow potential phase differences. Acoustic resonance frequencies were predicted using an improved Helmholtz resonator model. Model predictions for the nominal design were compared with experimental results, and great agreement for the frequency response functions was found with the acoustic damping ratio appropriately tuned. Various combination of design variables were used in the optimization. Device configurations were obtained for various polycrystalline and single crystal piezoelectric materials driven at 10% of their coercive fields as well as fixed applied voltage. The relative merits of individual materials were also discerned from the optimization results. Future works are developing method to address nonlinearities in the orifice outlet and diaphragm deformation.