This paper investigates the damping effect of loose particulate within the core of sandwich structures. Beam specimens fabricated from aluminum honeycomb core and IM7 carbon fiber face sheets with various amounts of loose particulate are experimentally examined. Both free vibration and forced vibration tests are performed. It is shown that a moderate amount of particulate can cause a large increase in damping. The effect of varying amounts of particulate is also investigated. Plate equations of motion with damping and inertia terms are derived to model the beam and compare with experimental results. Effective mass and effective viscous damping are generated by matching the theoretical model to the experimental data.