Active Acoustic Metamaterials

Abstract

Considerable interest in the field of metamaterials over the past decade has focused on metamaterials' use as a viable means of redirecting waves. Such interest has stemmed from the unnatural characteristics and unique properties of metamaterials, which have enabled their use in numerous interesting and critical applications that cannot be achieved with natural materials. The term metamaterials was coined nearly a decade ago by Walser. The prefix "meta" originates from the Greek word for "after" or "beyond," and metamaterials were then defined as "macroscopic composites having a manmade, three-dimensional (3-D), periodic cellular architecture designed to produce an optimized combination, not available in nature, of two or more responses to specific excitation." Since then, a broad class of metamaterials has been developed with unusual optical, electromagnetic, and acoustical properties. The design features and performance characteristics of these materials have been extensively reported; for example, by Lapine, Shamonina and Solymar, and Gil et al. In most of these reported studies, the focus has been on passive metamaterials with fixed material properties. This considerably limits the potential of this class of materials as they can operate effectively only at specific operating conditions and their performance characteristics degrade dramatically if there is a need to operate at different conditions. This serious limitation is clearly indicated by the narrow frequency band characteristics of passive metamaterials.