Random incidence transmission loss of a metamaterial barrier system

It has been shown previously that a panel comprising a cellular array can yield a normal incidence transmission loss in a specified low frequency range that is significantly larger than that of a homogeneous panel having the same mass per unit area. The cellular metamaterial considered consists of a periodic arrangement of unit plates held in a grid-like frame. However, when the incident sound field is diffuse, the relative advantage of the metamaterial barrier is reduced or eliminated. Here it will be shown through a sequence of experimental measurements that the relative advantage of the metamaterial barrier can be restored by creating a hybrid system consisting of a layer applied to the front surface of the material that causes sound to approach the barrier at normal incidence, and a layer on the rear surface of the material that compensates for the transmission loss minimum that normally follows the peak in a metamaterial barrier transmission loss. In the implementation considered here, the front layer consists of a lattice structure, and the rear layer consists of high performance glass fiber. The role of each of these components will be illustrated using measurements of transmission loss of a 1.2 m square panel system.