“…As far as the materials are concerned, a number of studies investigated the effects induced by the contrast in Young's modulus, Poisson's ratio, and the density between the matrix and inclusions in the case of a binary composite [5,6]. On the other hand, as far as the topology is concerned, which indeed represents one of the most-important features affecting the band gap properties, research has been conducted at the unit cell level, focusing on the lattice parameters to attain an optimal design [7][8][9] through topology and parametric optimization techniques [10][11][12][13]. For instance, in [14,15], an optimization method was proposed on the basis of a closed-form estimation of the band gap width and of the starting frequency as a function of a number of key geometric parameters; such an approach resulted in being useful in obtaining the optimal bang gap and the material design, to achieve better properties.…”