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Propagation of Longitudinal Waves in Microporous Slab Sandwiched between Elastic Half-Spaces

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.