“…Acoustic metamaterials are defined as engineered, artificial, periodic composites enabling altering mechanic properties of materials; thus, allowing properties that are not registered naturally [16]. In particular, metamaterials with controllable and negative bulk modulus were demonstrated, exploiting the effect of the negative mass [10,[11][12][13]17]. A number of experimental exemplifications of the "negative mass effect" were already demonstrated [11][12][13][18][19].…”