Pentamode Metamaterials with Independently Tailored Bulk Modulus and Mass Density

Metamaterials can be engineered to control not just light, but also acoustic waves. The authors fabricate a class of three-dimensional, linear elastic metamaterials for which the effective bulk modulus and mass density can be adjusted independently over a large range. This tuning is not possible in ordinary materials, but through geometric design this scheme allows for ``acoustic cloaking'', in which an object may be shielded from noise or other sound waves, such as sonar.