ACOUSTIC PROPERTIES OF AN INFINITE ELASTIC PLATE BACKED BY MULTIPLE LAYERS

In this paper the reflected sound field from an infinite elastic plate backed by multiple layers is analyzed theoretically to clarify the ef fects of a multiple layer backing on the acoustic properties of the el astic plate. In order to apply the theory to almost all practical case s, the cavity is modelled with three layers of arbitrary media in the analysis. A closed form expression of the strict solution for the refl ected sound field is derived. The theory also can be applied to predic t the sound absorptivity of panel absorbers. The theoretical results a re compared with existing measured data to verify the accuracy of the theory. The theoretical predictions are in fairly good agreement with the measured results. Furthermore, the effects of the change in positi on, thickness and flow resistivity of the absorptive layer in a cavity on the absorptivity of panel absorbers are discussed with theoretical results. A few examples of the calculated oblique incident reflection coefficient, which is the most fundamental and important to consider the effects of early reflections, are shown and discussed. By the effe cts of the absorptive layer, the low frequency component is reduced mo re significantly than is seen in the case of air back cavity.