  Architected materials that control elastic wave propagation are essential in
vibration mitigation and sound attenuation. Phononic crystals and acoustic
metamaterials use band gap engineering to forbid certain frequencies from
propagating through a material. However, existing solutions are limited in the
low frequency regimes and in their bandwidth of operation because they require
impractical sizes and masses. Here, we present a class of materials (labeled
elastic meta-structures) that supports the formation of wide and low frequency
band gaps, while simultaneously reducing their global mass. To achieve these
properties, the meta-structures combine local resonances with structural modes
of a periodic architected lattice. While the band gaps in these meta-structures
are induced by Bragg scattering mechanisms, their key feature is that the band
gap size and frequency range can be controlled and broadened through local
resonances, which is linked to changes in the lattice geometry. We demonstrate
these principles experimentally, using novel additive manufacturing methods,
and inform our designs using finite element simulations. This design strategy
has a broad range of applications, including control of structural vibrations,
noise and shock mitigation.
