Surface waves in granular phononic crystals

Acoustic Tamm states of three-dimensional solid-fluid phononic crystals

In this work, the existence and propagation of acoustic Tamm states at the interface of air and a face-centered cubic solid-fluid phononic crystal composed of spherical air voids interconnected by cylindrical air channels are demonstrated. Supercell band structure computations via the finite element method reveal surface bands for Tamm states on (100), (110), and (111) surfaces of the phononic crystal. The states decay sharply into the phononic crystal so that only a two-row slab is sufficient to guide them over the respective surfaces without leakage, as confirmed by finite element simulations. In addition, surface wave propagation along the [10] direction of the (100) surface is experimentally demonstrated. Ability to confine the Tamm states in all three dimensions is a key aspect in designing few-layer-thick acoustic circuits. Low material filling fraction of the phononic crystal could be leveraged to realize lightweight all-acoustic systems where either bulk or surface states can be incorporated.

Broadband and tunable one-dimensional strongly nonlinear acoustic metamaterials: Theoretical study

This paper focuses on the dispersion properties and mechanism of the one-dimensional strongly nonlinear acoustic metamaterials (NAMMs) based on the homotopy method. The local bifurcation mechanism, which is different from conventional local resonance, is found. It is demonstrated that the local period-doubling bifurcation of multiple cells will induce chaotic bands in the NAMMs, which can significantly expand the bandwidth for wave suppression. The saddle-node bifurcation leads the system state jumping to the chaotic branch. Furthermore, the amplitude-dependent dispersion properties enable NAMMs to manipulate elastic waves externally. Study of broadband tunable abilities reveals that stronger nonlinearity (larger nonlinear coefficient or higher amplitude) presents a broader nonlinear band gap and larger transmission loss. Moreover, with less attached mass, a low frequency and broadband are achievable simultaneously. This research may provide useful approaches for elastic wave control.