Reduction of indirect sound transmission by additional layers

Acoustical linings and other additional layers are widely used to improve the sound reduction index of walls. Apart from the reduction of direct sound transmission, acoustical linings mounted on lateral walls act on flanking transmission. Here, additional construction features are to be considered, such as the influence of the junction between the flanking walls and the partition wall or the internal damping of the wall. Measurements in a full scale laboratory show, that the improvement of sound reduction indices is not necessarily the same for direct sound transmission and flanking transmission. The influence of the junction seems to be rather small as well as the influence of the different wave types involved in sound transmission. However, the structural damping of the basic wall has a clear effect.