One‐dimensional wave propagation in relatively stiff, partially reticulated acoustical foam

Abstract

Recently the authors have conducted measurements of the acoustic properties of a relatively stiff (in vacuo bulk modulus of elasticity greater than air) partially reticulated (i.e., some of the cell membranes remained intact) polyurethane foam of the type often used in noise control applications. The results were interpreted by using a theory governing one‐dimensional wave propagation in elastic porous materials [G. S. Rosin, Sov. Phys. Acoust. 19, 60–64 (1973)] which was extended to account for frequency‐dependent viscous and thermal effects. The parameters of the theoretical model (flow resistance, structure factor, frame bulk modulus of elasticity, and loss factor, etc.) were determined by matching the predicted and measured impulse response of a 25‐mm‐thick layer of unfaced foam. Reasonable agreement was obtained between the measured and theoretical impulse reponses, reflection coefficients and surface normal impedances of the foam in two configurations (faced and unfaced). The theoretical work has indicated that the frame wave (associated with the bulk mechanical properties of the foam) plays an important role in determining the acoustical properties of relatively stiff, partially reticulated foam.