Nonlinear numerical simulation of insertion loss for structures having a compressed softened porous layer sandwiched by double walls under acoustic excitation

We perform nonlinear numerical simulation of insertion loss for structures having compressed porous layers sandwiched by double walls under acoustic excitation. The double walls are consisted of a steel base plate and a plastic cover plate. We compare the insertion loss including Urethane foam with felt. These porous layers are compressed by a heavy frame around the cover plate. As increasing weight of the frame, Urethane foam has soft-hardening characteristics in its restoring force. If we set appropriate weight of the frame to get 33% compression, lower rigidity is obtained for Urethane foam. This leads to softer porous layer for the double walls. And vibration and sound decrease from the base plate to the cover plate. For felt, the hardening characteristics appear. This leads to the higher rigidity and the more transmission. In the numerical calculation using LS-Dyna as a nonlinear FEM code, geometric nonlinearity is considered. For compressed porous materials, Storaker model is adopted in consideration of material nonlinearity with hysteresis. Further, under compression, we also investigate the reason of the softening mechanism for the Urethane foam using another model of microscopic structures. We reveal that the softening mechanism is due to geometric nonlinearity by using multiple Kelvin cells.