Acoustic surface modes on metasurfaces with embedded next-nearest-neighbor coupling

We design, simulate, and experimentally characterize an acoustic metasurface comprising of a 1D array of open, sound-hard, cavities, modulated with beyond-nearest-neighbor (BNN) couplings in the form of additional connecting cavities embedded beneath the surface. The hidden complex structure is realized readily with additive manufacturing techniques (3D printing). The dispersive properties of the supported localized acoustic surface waves are influenced by competing power-flow channels provided by the BNN couplings, that generate extrema in the dispersion spectra within the first Brillouin zone. The structure supports negatively dispersing 'backwards' waves that we experimentally verify. Such structures thereby provide a route to enhanced acoustic sensing by acoustic metasurfaces.