A two-dimensional model for wave propagation in elastic porous materials has been used to investigate the effects of edge constraints on the surface normal impedance of elastic porous materials when placed in standing wave tubes. The elastic porous material model used was based on the Biot theory and takes into account the three wave-types known to propagate within elastic porous materials. Previous work has indicated that the acoustical behavior of typical noise control foams can be very sensitive to small mounting details. Consequently, it is reasonable to expect that the surface normal impedance measured by placing foam in a standing wave tube will depend on the degree to which the sample is constrained at its edges when it is placed in the tube. Here a modal model was used to predict the surface normal impedance of a foam sample fully constrained at its edges. A comparison of that prediction with the surface normal impedance of an unconstrained half-space of the same material has shown that the principal effect of the edge constraints is to stiffen the sample at low frequencies, as might be expected on physical grounds. It was concluded that the impedance of an elastic porous material placed in a standing wave tube may not be the same as that of a large area of the same material at frequencies below the "cut on" of the first shearing mode within the solid phase of the porous material.
Standing wave tube, Poro-elastic materials, Elastic porous materials, Surface normal impedance, Edge constraint, Absorption coefficient
Acoustics and Noise Control
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