A multi-layer active elastic metamaterial with tuneable and simultaneously negative mass and stiffness

All conventional acoustic/elastic media are restricted to possess positive constants for their constitutive parameters (density and modulus). Metamaterials provide an approach through which this restriction can be broken. By making these parameters negative and/or tuneable a broader range of properties becomes possible. This paper describes the first experimental implementation of an acoustic/elastic metamaterial in which the material parameters can be both simultaneously negative in a finite frequency band and the magnitude of the parameters independently tuneable on demand. The design is an active metamaterial (meta-mechanicalsystem) which is realized by directly applying feedback control forces to each layer within the metamaterial. The ability to tune the magnitude of the negative parameters has important implications for the use of a standard design that can be tuned to a particular application, or one which can adapt to a changing performance requirement. The implementation of the design is relatively large scale and low frequency, but the unit-cell length is significantly smaller than the wavelength in the double negative band. Importantly, assuming appropriate control hardware is available, the design can be both reduced in scaled and expanded to greater degrees of freedom.