Perfect acoustic bandgap metabeam based on a quadruple-mode resonator array

Abstract

Solid structures guide a multitude of elastic modes of different polarizations including both compression and shear, and the nature of the elastic constant tensor implies a much richer behavior than in optics. Here, we introduce a metamaterial in the form of a rectangular cross section beam of a single isotropic material that can simultaneously suppress all elastic-wave polarizations in the beam over a range of frequencies in the kHz range. This is experimentally achieved by machining replicas of a subwavelength unit cell in an aluminum metabeam based on a planar resonator with interconnected ribs, showing complex vibrational degrees of freedom that allow it to couple to compressional, in-plane shear, flexural and torsional vibrations, that is, all four existing mode types. The result is a lightweight structure that can forbid all possible acoustic modes over the metamaterial bandgap frequency range, an exotic behavior that opens up diverse applications in easily manufacturable vibration isolation structures and acoustic wave control.