Negative acoustic index metamaterial

Acoustic metamaterials utilizing periodic deep subwavelength resonators can attain negative acoustic properties unavailable in nature. We have developed a negative acoustic index metamaterial for water that combines Helmholtz and rod-spring resonators to control effective bulk modulus and mass density, respectively. Effective properties extracted from full-wave simulations of our metamaterial show that negative real components of bulk modulus and density occur simultaneously, resulting in a negative real component of the acoustic index. Experimental measurements on a sample of this metamaterial confirm that the real components of the acoustic index and bulk modulus attain negative values, but the density does not become negative. The primary causes of this are identified and potential solutions are presented.