In this paper, the sound transmission loss (STL) through sandwich structure with pyramidal truss cores immersed in the surrounding acoustic fluids is investigated. The periodic model is established in two dimensions. Trusses are replaced by translational and rotational springs and the stiffnesses are calculated by the theory of Euler beam. Fluid-structure coupling is considered by imposing velocity continuity condition at fluid-structure interfaces. The dynamic equations are derived by using space harmonic expansions and the principle of virtual work. The sound transmission characteristics of structure are studied by using the numerical calculation. Then, the effects of incident wave, geometry of truss and material properties are discussed for thorough understanding and system design.