Abstract
Sound transmission through double-shell composite structures sandwiched with poroelastic cores in the presence of an external mean flow is studied analytically with the focus on the effects of random incidence in a diffuse field. The shell motions are described by Love's theory, and the poroelastic core is modelled as an equivalent fluid by considering only the dominant wave based on Biot's theory. The transfer matrix method is employed to solve the vibroacoustic problem with appropriate boundary conditions. The total transmission loss in the diffuse field is calculated numerically for three sandwich configurations over a wide frequency range, and the limiting incidence angle due to total internal reflection is analysed. The numerical results suggest that the effects of the direction angles and external mean flow on the transmission loss and characteristic frequencies observed for a fixed incidence direction have been averaged after integrating over all random incidence directions. Both the poroelastic core and air gap significantly improve the sound insulation performance particularly in the mass-law region, and airborne waves are preferable to frame waves to be insulated by the double-shell sandwich structures.