Origin of negative density and modulus in acoustic metamaterials

This paper provides a warning to the acoustic metamaterials community to be careful not to mistake the sign or value of the effective parameters, i.e. the density and modulus, of acoustic metamaterials for want of an overall picture of the physics involved. The authors here provide a new picture of the functioning of acoustic metamaterials based on hidden forces and hidden sources of volume. The new ansatz is tested on some established acoustic metamaterials with elements based on membranes, Helmholtz resonators, springs, and masses. It should provide the basis for a clearer vision of acoustic metamaterials and a faster route to real-world applications.