solids, in addition to simple reflection at an interface, polarization conversion always occurs; making it difficult to form complete gaps, and a design strategy for optimal constructs is not yet available. The use of fluids (which support only longitudinal waves) makes it difficult to form gaps because of the conversion of shear modes into longitudinal modes at the solid-fluid interface.

The idea of using a structured material to influence the propagation of elastic waves is promising since in addition to creating a band gap, there is the possibility of creating a set of band gaps that would significantly block multiple frequencies. Moreover, phononic crystals permit the tailoring of the allowed modes and their wave speeds inside the material, such that the frequencies of various material loss regimes may be matched with the density of states and frequencies of the allowed modes to provide enhanced energy absorption.

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