This works aims to build an equivalent model of multilayered structures with imperfect interfaces for vibro-acoustic characterization. To do so, new boundary conditions containing the imperfections are implemented. The dispersion relation is derived from the equivalent model and the bending wavenumber is obtained. In this work we applied the methodology to a sandwich-structured composite with sliding interfaces. We computed the flexural rigidity and the damping of the flexural waves to see their effects.