The radiation efficiency of baffled plates and strips

Abstract

The average radiation efficiency of point-excited rectangular plates, including those with a very large aspect ratio (‘strips’), is investigated by using a modal summation method based on the farfield sound intensity. By taking an average over all possible forcing positions on the plate, the cross-modal contributions average out to zero. The numerical results from the modal summation are compared with established formulae for rectangular plates. For wavenumbers where acoustic circulation takes place, it is shown that the previously published formulae are not applicable for predicting the average radiation efficiency for a strip. The analysis for the strip reveals that the radiation efficiency at frequencies below the first structural natural frequency of the strip is proportional to the square of the shortest edge length. At frequencies between the fundamental natural frequency and that of the second order mode of bending across the strip, the average radiation efficiency is found to be proportional to the structural damping loss factor, but differs from the results obtained from the usual model of the nearfield radiation from the forcing point of a rectangular plate. Approximate expressions for calculating the average radiation efficiency of the strip are derived. The maximum radiation efficiency around the critical frequency is found to vary less with Helmholtz number k c a, where k c is the wavenumber at the critical frequency and a is the width, than previously published models suggest.