Acoustic metamaterials manifest properties unachievable in bulk materials. One such property goes by the colorful name of rainbow-trapping: the ability to absorb sound perfectly and simultaneously over a wide range of frequencies.
As is the case with acoustic metamaterials’ electromagnetic counterparts, the route to rainbow-trapping and other exotic properties lies in incorporating or fashioning substructures at or below the wavelengths to be absorbed, refracted, or reflected. Optical metamaterials tend to be rigid arrays of metallic rings or other shapes. Acoustic materials need not be rigid. Indeed, their elastic response to sound provides another route toward tailoring properties.
Acoustic metamaterials also need not be made of substances whose density and stiffness remain the same throughout the material. Another route toward tailoring properties is to engineer gradients in one or more parameters.
Can the two routes—elastic response and engineered gradient—be combined? Yes, say Stephanie Konarski and Christina Naify of the US Naval Research Laboratory in Washington, DC. In a newly published paper, they report the results of simulating the passage of sound through various combinations of material properties.
Their adjustable acoustic metamaterial, whose shape is tuned with an external load, consisted of a semi-infinite sheet of virtual rubber punctured by a periodic square lattice of circular holes, which together accounted for 56% of the sheet’s total area. The 11 rubbers in the numerical experiment had different shear moduli and different sound speeds. For each rubber, two cases were evaluated: the effect of the waves in an unbuckled lattice of holes and a buckled lattice of holes. Gradients provided the final source of variation.
Konarski and Naify examined several configurations by combining different rubbers. For example, one combination arranged the rubbers from high shear modulus to low shear modulus. Another had the shear modulus increase and then decrease. Yet another had the shear moduli arranged randomly across the 11 regions.
The property that Konarski and Naify sought to optimize was the extent of so-called stop bands. Analogous to the band gaps in photonic crystals, stop bands are frequency ranges over which no transmission occurs. They found that the most extensive stop bands arose in sheets that underwent buckling and whose constituent rubbers had one or more gradients. Although the metamaterials that Konarski and Naify investigated were virtual, they can in principle be realized using computer-controlled additive manufacturing. (S. G. Konarski, C. J. Naify, JASA Express Lett. 1, 015602, 2021.)
