We present the physical concepts of acoustic barriers which have the purpose of reducing airborne sound transmission for both paths: internal and external noise. We introduce the mass-spring model, the utilization of compound sandwiches, and the improvement of the Transmission Loss through the use of channels in the material working as spring. We show some results regarding the behavior of absorption materials exposed to environment/working conditions. In addition we discuss the effect of acoustic leaks and the way of evaluating these barriers in vehicles. We show examples based on experimental investigations.