Acoustic metamaterials: From theory to practice

Abstract

Why have acoustic metamaterials appeared on the scene in the last few years, and what are they anyway? The original defining property of a metamaterial is that it achieves effects not found in nature, with the more effects the better. The acoustic cloak, justifiably the best known example, redirects sounds around the cloaked object regardless of the incident acoustic wave. The common theme of the cloak and of the many other devices that have been proposed is defiance of the intuitive laws of acoustical physics as much as possible. This requires strange concepts such as acoustic anisotropy, allowing sound to have speed that depends on direction, and transformation acoustics which is the basis for acoustic cloaking. Periodic structures, aka phononic crystals, play a central role in practical applications. The tutorial will provide a comprehensive introduction to these ideas without the need for detailed mathematics. While the field of metamaterials originated in electromagnetics before it migrated over to acoustics, there are fundamental differences between the two fields, the most obvious arising from the fact the acoustic wave speeds are not strictly limited, a distinction which leads to profound consequences. Acoustic wavelengths are also typically orders of magnitude larger than optical wavelengths, meters versus microns, which makes the acoustic problems easier, in principle. This practical advantage is perhaps the main reason for the intense interest in the field by funding agencies and researchers alike.