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Michael R. Haberman
Postal:
Applied Research Laboratories and Department of Mechanical Engineering The University of Texas at Austin P.O. Box 8029 Austin, Texas 78713-8029 USA
Email:
haberman@arlut.utexas.edu
Andrew N. Norris
Postal:
Mechanical and Aerospace Engineering School of Engineering, Rutgers University Piscataway, NJ 08854 USA
Email:
norris@rutgers.edu
Acoustic Metamaterials
Acoustic metamaterials expand the parameter space of materials available for new acoustical devices by manipulating sound in unconventional ways.
Introduction
Why have acoustic metamaterials (AMMs) appeared on the scene in the last few years, and what are they? The original defining property of a metamaterial is that it achieves effects not found in nature as a means to address long-standing engineer- ing challenges in acoustics. Can one, for example, create ultrathin acoustic barriers whose performance surpasses currently existing technology? Is it possible to elim- inate scattering from an acoustic sensor and minimize the influence the device has on the field being measured? Can spatially compact acoustical lenses be created whose resolution surpasses the diffraction limit? These are but a few examples of the problems AMM research strives to address. The common theme of the many AMM devices is an apparent defiance of the intuitive laws of physics, which of- ten require strange concepts such as negative density and negative compressibility. Negative effective properties underlie behavior such as negative refraction that, in turn, enables acoustic lens designs that beat the diffraction limit.
AMM research was originally motivated by parallel developments in electromag- netics, such as negative refraction and cloaking (Norris, 2015). The first to study these topics quickly found that the available materials were not up to the task of providing the necessary properties for cloaking or negative refraction. A straight- forward reaction to this problem was to simply create new materials. Although not simple, the creation of new materials has been, and continues to be, the fun- damental driving force for the study of AMMs. This has led to concepts that could not be anticipated from electromagnetics, such as pentamode materials (exotic materials that only resist one mode of deformation, analogous to how fluids only resist volumetric change), to address AMM challenges (Norris, 2015). The chal- lenge in developing new materials has been significantly aided by steady improve- ments in technology, primarily computer simulation and additive manufacturing. Those technologies combined with ingenious ideas from the acoustical research community have helped drive rapid advances in AMMs over the last decade, some of which we describe in this article.
For acoustical applications, dynamic material properties are of interest and may open the door to more exotic behavior. The focusing/beamforming from the fatty lobe of a dolphin, known as the melon, is a case in point (Yamato et al., 2014). Dy- namic properties are not the same as their static counterparts that we learn about in introductory courses on mechanics. Thus Figure 1 displays the design space of AMMs, a range that includes negative values for both density and compressibility, that is explained in detail in this article. At this stage, it is sufficient to note that the effective density and compressibility provide a helpful way to view the overall re- sponse of an engineered structure as an effective medium rather than using the im- pedance of a complex system. This is consistent with the metamaterial paradigm that seeks to expand the available parameter space available to people designing acoustic systems and devices to control acoustic waves.
All rights reserved. ©2016 Acoustical Society of America. volume 12, issue 3 | Fall 2016 | Acoustics Today | 31