Active Acoustic Metamaterials

Extensive efforts are being exerted to develop various types of acoustic metamaterials to effectively control the flow of acoustical energy through these materials. However, all these efforts are focused on passive metamaterials with fixed material properties. The emphasis is placed here on the development of a class of acoustic metamaterials with tunable effective densities and bulk modulus in an attempt to enable the control of wave propagation. More importantly, the active metamaterials can be tailored to have increasing or decreasing variation of the material properties along and across the material volume. With such unique capabilities, physically realizable acoustic cloaks can be achieved and objects treated with these active metamaterials can become acoustically invisible. Theoretical analysis and preliminary experimental validation of this class of active acoustic metamaterials will be presented. The performance characteristics are determined for an array of fluid cavities separated by piezoelectric boundaries. These boundaries control the stiffness of the individual cavity and in turn its dynamical density and bulk modulus. Various configurations are considered to achieve different spectral and spatial control of the density and bulk modulus of this class of acoustic metamaterials. Efforts are now exerted to build and test modules of these active acoustic metamaterials in order to build practical configurations of acoustic cloaks.