NORMAL INCIDENCE SOUND-TRANSMISSION THROUGH DOUBLE-PANEL SYSTEMS LINED WITH RELATIVELY STIFF, PARTIALLY RETICULATED POLYURETHANE FOAM

In this paper we present calculations of the normal incidence sound transmission performance of double-panel structures when lined with an elastic porous material, in particular, polyurethane foam. The predictions have been made using a theory for one-dimensional wave propagation in elastic porous media that makes allowance for two longitudinal wave-types, i.e. airborne and frame-borne waves. The boundary conditions that occur at foam surfaces in noise control treatments are then discussed in detail. To make the theory representative of typical noise control foams, certain parameters of the theoretical model (the bulk modulus, the loss factor, the flow resistivity and the structure factor) were determined by matching the theoretical and measured transmission impulse responses of an unfaced foam layer. The transmission impulse response measurements also clearly revealed the presence of both the airborne and frame-borne waves predicted by the theory. Theoretical predictions of the transmission loss of foam-lined double panels are then presented; the configurations considered were chosen to simulate aircraft fuselage treatments. The most important conclusion drawn from the theoretical predictions is that the foam-panel boundary conditions have a substantial effect on the composite panel performance. In addition, it is shown that a lining arrangement in which the foam is bonded directly to one panel, but is separated from the other by a thin air gap, appears to be optimum under practical circumstances. That configuration minimizes the effect of the 'mass-air-mass' resonance that usually impairs the low-frequency transmission loss, and offers good performance at higher frequencies.