Porous materials are widely used in the passive noise control field as sound absorbers. Conventional models of porous materials are assumed to have a rigid frame and show finite bulk elasticity. However, in the case of acoustical wavescharacterized by high frequencies and small wavelengths -the effect of microstructure becomes significant. This effect of microstructure has resulted in the development of new types of waves, not found in the classical theory of elasticity. Generalized continuum theories include the construction of the linear theory of micropolar elasticity that consists of deformation and microrotation with six degrees of freedom, and hence can be used to study the acoustical characteristics of composites with a granular structure. In this study, we investigated transverse wave propagation and its reflection and transmission from a plane interface between two different elastic-micropolar porous interfaces in perfect contact. The micropolar porous composite was constructed using hollow glass microbubbles embedded in an epoxy matrix with six material constants that can be used as the acoustical absorbers. It was found that there are different wave types in a micropolar porous material for the incident SV (vertical transverse) or SH (horizontal transverse) wave. It was also found that these two coupled sets of transverse waves, when traveling with different velocities, are dominated by the critical value of microinertia, showing the influence of the micropolar porous characteristics.

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A granular composite that consists of stiff inclusions embedded in a weaker matrix produces a well defined micro-structure. This composite is extensively used in the passive noise control field in sound absorbers. In classical continuum mechanics, the basic assumption is that the micro-structure of a material does not govern the mechanical behavior. However, classical theory and experimental results have some discrepancies, revealing the potential importance of the micro-structure. For instance, acoustic waves are characterized by high frequencies and small wavelengths, particularly in microporous materials with various stiffness values. Consequently, unlike for a homogeneous isotropic elastic medium with only two elastic constants, linear elastic micropolar theory utilizes 6 degrees of freedom and its results can hence be regarded as being a function of the micro-structure. This investigation explores the reflected and transmitted wave fields of an incident longitudinal plane wave that propagates in elastic–microporous–elastic interfaces. The numerical study demonstrates a through transmission in the sandwiched problem in situation of interest. This property of the half-wave microporous layer allows it to be regarded as a frequency or direction filter. Two sets of the forward and backward transverse plane waves are present in the sandwiched microporous layer. The specified work frequency and the width of the intermediate microporous layer affect all of the described phenomena.