A class of piezoelectric energy harvester is presented to harness the vibration energy from coupled acoustic-structure systems such as those existing, for example, in aircraft acoustic cabin/flexible fuselage systems. Generic idealization of any of these systems involves the interaction between the dynamics of an acoustic cavity coupled with a flexible structure. Pressure oscillations inside the acoustic cavity induce vibration in the flexible structure and vice versa. Harnessing the associated vibration energy can be utilized to potentially power various vibration, noise, and health monitoring instrumentation. In this paper, the emphasis is placed on harnessing this energy using a special class of piezoelectric energy harvesters coupled with a dynamic magnifier in order to amplify its power output as compared to conventional harvesters. A finite element model (FEM) is developed to predict the performance of this class of harvesters in terms of the mechanical displacements of the flexible structure, the pressure inside the acoustic cavity, and the output electric voltage of the piezoelectric harvester. The FEM is formulated here to analyze a two-dimensional (2D) energy harvesting system which is composed of a rigid acoustic cavity coupled, at one end, with a vibrating base structure to which is attached the piezoelectric energy harvester. The developed FEM is exercised to predict the output electric power for broad interior pressure excitation frequencies. Numerical examples are presented to illustrate the behavior of the harvester and extract the conditions for maximum electric power output of the harvester. The obtained results demonstrate the feasibility of the dynamic magnifier concept as an effective means for enhancing the energy harvesting as compared to conventional harvesters. The presented model can be easily extended and applied to more complex fluid–structure systems such as aircraft and vehicle cabins.