Optimization of metasurfaces for the design of noise trapping metadevices

The article deals with the design of a metadevice able to trap acoustic waves in a duct. The acoustic perturbation is produced by a source placed inside the duct. The aim is to limit the outgoing acoustic power and confine the perturbation inside the duct exploiting the unconventional reflection of the optimized metasurface. The metabehaviour is modeled by means of the generalized Snell's law for reflection from acoustically rigid surfaces. The realization of the device relies on a modular concept, which building set is made of eight elementary cells, able to induce a reflected field suitably phase-delayed with respect to the incident wave. The set spans the whole 0-2π phase delay range, and the anomalous reflection is obtained by the tailored design of the phase delay gradient profile on the metasurface. The cells are designed in order to extend the effective frequency range of the device, keeping the overall thickness of the metadevice smaller than a quarter of the design wavelength. The duct and the source are considered co-moving within the fluid at rest. The numerical analysis is performed in the frequency domain in a frame of reference rigidly connected to the duct, and considering several values for the Mach number