such as phononic crystals and lattice structures can inhibit wave propagation in particular frequency ranges (see e.g. [@Vasseur; @Vasseur1]). Both phononic crystals and lattice structures are artificial materials which are characterized by periodic microstructures in which strong contrasts of the material properties can be observed. Also suitably ordered granular assemblies with defects (see e.g. [@Merkel; @Merkel1; @Kafesaki]) and composites ([@Economou]) have shown the possibility of exhibiting band-gaps in which wave propagation cannot occur under particular loading conditions. In all these cases, the basic features of the observed band-gaps are directly related to the presence of an underlying microstructure in which strong contrasts of the elastic properties may occur. Indeed, the rich dynamic behavior of such materials stems mainly from their hierarchical heterogeneity at the microscopic level which produces the mixing of the longitudinal and the transverse components of travelling waves. It is clear that the applications of such metamaterials would be very appealing for what concerns vibration control in engineering structures. These materials could be used as an alternative to currently used piezo-electric materials which are commonly employed for structural vibration control and for this reason are widely studied in the literature (see, among many others, [@Piezo1; @isola2; @isola6; @isola1; 