Stiffener-induced stiffness/mass suppression of the exterior sound field in sandwich structures with open-cell foam cores: Periodically arranged stiffeners

Sandwich cylindrical shells with porous polymeric foam cores and periodically distributed stiffeners are attractive candidates for lightweight control of surrounding acoustic pressure, yet their acoustic scattering in realistic multilayered configurations remains poorly characterized. This study develops an analytical framework for predicting the total scattering cross section of a sandwich cylindrical shell under oblique plane wave incidence for a specific configuration combining a functionally graded (FG) outer facesheet, an FG open-cell porous polymeric foam core, an isotropic inner layer separated by air gaps, orthogrid (ring-string) stiffeners, and coupled interior and exterior acoustic fluids. Structural dynamics are modeled using first-order shear deformation theory and Hamilton's principle, while the foam core is treated as an FG viscoelastic medium with a frequency-dependent complex modulus. Continuity of acoustic pressure and normal velocity at the fluid-structure interfaces couples the interior and exterior acoustic fields and yields the scattered pressure and total scattering cross section. Numerical results show that periodic ring-string stiffening strongly attenuates low-frequency resonances and shifts the first major scattering peak to higher frequencies, thereby reducing low-frequency scattering levels. The stiffened configuration also reduces the peak magnitude of the surrounding acoustic pressure (from approximately ±1.5 Pa to ±1.0 Pa at 10 kHz), indicating a weaker scattered field.