This work studies the sound transmission loss across composite material in order to understand the sound-insulating capacity at various frequencies. A so-called specially orthotropic laminate was examined. The material properties in thickness direction are different from those in in-plane directions but it is isotropic in the laminating plane. Thus the specially orthotropic laminate can be analyzed by two-dimensional acoustic analysis. The sound transfer matrix of specially orthotropic single layer is determined by stress and velocity. The boundary conditions between different interface planes and the transfer matrix related to these conditions yield the mathematical model of the sound wave that propagates in composite materials. The experimental and numerical results of sound transmission loss are compared. Arrangement of various material properties and their sound penetrating characteristic were investigated and discussed.