Omnidirectionally impedance-matched phononic crystals for full-parameter transformation acoustics

In this work, we present an approach to realize full-parameter transformation acoustic media utilizing phononic crystals instead of metamaterials, which has the exceptional advantages of omnidirectional impedance matching and configurable dispersion surfaces. By employing a sandwich structure, we demonstrate phononic crystals that exhibit tunable shifted elliptical equal frequency contours and nearly omnidirectional impedance matching simultaneously. Two typical acoustic devices, including a beam shifter and an illusionary scatterer of different size, are demonstrated with the phononic crystal approach. Our work provides a practical platform for implementing full-parameter transformation acoustics, opening avenues for versatile applications such as advanced acoustic imaging and stealth technologies.