Retrieving continuously varying effective properties of non-resonant acoustic metamaterials

We exploit non-resonant inclusions under the metamaterial scheme to enable full analysis of the effective properties with different shapes and lattice structures using the S-parameter retrieval method. Various constraints on the extraction of effective properties are investigated to quantify their dependence on the number of unit cells, the frequency, and the lattice structure. Moreover, distinct effective parameters are confirmed with respect to the size parameter and packing factor of the proposed metamaterials. By this approach, continuously varying effective properties in a broad range of frequencies are achieved, resulting in the realization of acoustic gradient-index devices with appropriate inclusions with non-resonant aspects. (C) 2019 The Japan Society of Applied Physics