This research presents the finite element formulation of a bulk-reacting sound absorbing material for use in interior cavity solutions. The bulk properties of the absorbing material are represented by complex density and complex propagation speed. Coupling between the vibrating cavity structure and the acoustic medium is considered. The continuity of sound pressure and the particle velocity at the interface between the acoustic domains having different properties is satisfied. Two case studies, a one-dimensional duct and a three-dimensional cavity, are considered. Analytical solutions and experimental results are compared to the finite element results. Excellent agreement has been achieved.