Sound transmission loss of the barriers determines significantly operational interior noise level of the automobile. The interior sound level in an automobile is determined mostly by the sound transmission loss through the walls (STL) and its absorption properties. In a well acoustically designed car, the sound transmission loss (STL) through the walls should be as high as possible.This paper presents theoretical and experimental results of the STL through double wall systems. The coupling properties of the material placed between sound transmission loss barriers have been studied experimentally and theoretically. Of particular interest, are the physical and acoustical properties of the coupling material, including the fraction of bounded area between barriers and substrate.