Experimental tests of protective covers to obtain optimal transmission loss in porous materials

Porous resistive materials are widely used to increase the wide-band transmission loss and reduce shell vibrations and noise propagation. For industrial applications, porous materials need to be covered to protect them from environmental contaminants. The goal of our experimental studies is to determine suitable covers which are acoustically transparent and offer this needed protection. Expansion volumes were designed for transmission loss tests with two accurate modeled limits related to two rigid boundary conditions. Seven different covers were tested in this study tests. Best results were achieved using thin porous steel woven cloth. Tyvek, Mylar, thick steel woven cloth, and the Nomex were not as effective. The porous materials and covering materials were tested and then retested after being soaked in oil to explore transmission loss that might occur in actual industrial conditions. These independent measurements of the cover materials were used to determine the material property in the FEA models.