The influence of added damping on sound generated by vibrating sandwich plates
This report considers the effect of unconstrained damping layers and an edge-supporting stiffener on the sound power radiated from a mechanically-driven honeycomb sandwich plate. The free wave equations of coupled flexural plate motion are first set up, taking into account the orthotropic core shear stiffness. Solutions of these equations have been found and computed wavespeed dispersion curves have been presented. The plate boundary conditions at an edge supported on a flexible beam have been established and the response of the plate to a point harmonic force on that beam has been analysed and computed. The sound power radiated from the baffled plate surface has been found from the pre-determined plate response by numerical evaluation of Rayleigh's integral. A computational study has shown that the orientation of the core principal axes of orthotropy relative to the edge beam can profoundly influence the level of the radiated sound power. The sound power reductions obtainable when unconstrained damping layers are added to the plate surface have been predicted and explained. Full account has been taken of the effect of the damping layers on the radiation efficiencies and total loss factors of the plates. Experiments to validate the theory have been conducted. A supporting rig for sandwich plates has been constructed which simulated three simply-supported and one stiffened edge condition. Response and radiated sound power measurements were made on three different plates. Tests were conducted with and without damping layers on the plates and the results compared favourably with those predicted from the theory and computer programmes. Measurements were made of the damping material properties from tests on solid plates. These properties led to accurate predictions of the natural frequencies of simple sandwich beams, but the measured flexural loss factors of the beams were considerably different from predicted values.