Experimental study of nonlinear acoustic bands and propagating breathers in ordered granular media embedded in matrix

We experimentally study acoustic pass and stop-bands, and propagating breathers in harmonically excited, ordered granular chains composed of steel beads embedded in three different types of matrix—namely PDMS, polyurethane and geopolymer. Both single and coupled granular chains are tested and the bands and breathers are robustly detected in each case over varying frequency and amplitude ranges. Low-frequency acoustic pass-bands are characterized by pulse-like transmission in the granular media and negligible effective compression, whereas high-frequency stop-bands are characterized exhibit complete filtering of granular media dynamics and strong effective compression. At intermediate frequency ranges we observe the propagation of propagating breathers in the granular chains, in the form of wavetrains of localized wavepackets separated by silent regions. We study the effects of the matrix and the distance between coupled granular chains on the propagation of the breathers, and confirm the robustness of these highly nonlinear responses in this highly discontinuous class of acoustic metamaterials. Moreover, we relate our results to energy transfers between coupled granular chains. Finally, we propose a simplified theoretical model that fully recovers the experimentally detected responses. To the authors’ knowledge this is the first experimental report of acoustic filtering and experimental breathers in practical acoustic nonlinear metamaterials.