Abstract
Abstract Phononic crystals and acoustic metamaterials are periodic structures whose effective properties can be tailored at will to achieve extreme control on wave propagation. Their refractive index is obtained from the homogenization of the infinite periodic system, but it is possible to locally change the properties of a finite crystal in such a way that it results in an effective gradient of the refractive index. In such case the propagation of waves can be accurately described by means of ray theory, and different refractive devices can be designed in the framework of wave propagation in inhomogeneous media. In this paper we review the different devices that have been studied for the control of both bulk and guided acoustic waves based on graded phononic crystals.
Highlights
Phononic crystals and acoustic metamaterials enable to achieve innovative properties for the propagation of mechanical waves inapproachable in natural materials
Over the past two decades, we witnessed the rapid growth and development of phononic crystals and metamaterials as well as the recent emergence of metasurfaces to deepen the understanding of wave physics
The effective index is a homogenized result from infinite periodic structures, it is still proper to define a local index in a gradient structure, being a bridge between infinity and locality
Summary
Phononic crystals and acoustic metamaterials enable to achieve innovative properties for the propagation of mechanical waves (air-borne sound waves, water-borne acoustic waves, water waves, elastic waves, surface acoustic waves and Lamb waves) inapproachable in natural materials. The last section will present some conclusions and future challenges
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