Page 40 - \Winter 2015

Andrew N. Norris
Postal:
Mechanical & Aerospace Engineering School of Engineering Rutgers University Piscataway, NJ 08854 USA
Email:
norris@rutgers.edu
Acoustic Cloaking
It might drive bats batty, but there is no fundamental physical limitation on developing acoustic cloaking devices.
Introduction
An acoustic cloak is a shell surrounding an object so that sound incident from any direction passes through and around the cloak, making the cloak and the object acoustically “invisible.” We do not experience acoustic cloaking because the mate- rials required are exotic and, as far as we know, not found in nature. Yet there is no fundamental physical restriction on acoustic cloaking. Implementation is a matter of developing metamaterials with very unusual properties. Acoustic cloaking is in fact more likely to be achieved before its electromagnetic (EM) counterpart. The reason is that the cloaking material must have structure on sub-wavelength scales. Specific examples will be explained below. Acoustic wavelengths are typically orders of magnitude larger than optical wavelengths, meters vs. microns, which makes the acoustic problem easier, in principle.
This review attempts to explain the physics behind acoustic cloaking. No com- plicated mathematics is necessary to understand the concept of transformation acoustics, which defines the type of metamaterial required. We will see that acous- tic cloaking is not an analog of EM cloaking but has unique features. Other cloak- ing methods based on passive and active wave cancellation are discussed. Practical realizations are reviewed. This survey does not discuss some related topics, such as negative dynamic properties. However, comprehensive technical reviews are avail- able: Chen and Chan (2010) provide an early overview; Kadic et al. (2013) review acoustic metamaterials; Fleury and Alù (2013) give a recent review of cloaking and invisibility. Detailed reviews specific to acoustics can be found in (Craster and Guenneau, 2013).
Cloaking is an admittedly fantastic concept, well represented in popular culture (The Invisible Man, Invisible Woman, Harry Potter, etc.) using ingenious “technol- ogies.” The Cloaking Field Generator in Star Wars is an example of an active de-
Figure 1. Some space is made available in (b) by shrinking a region of (a) into the transformed green material. As explained in the text, the original and transformed ray paths in (c) imply that the transformed medium must display acoustic anisotropy. The wave speed in the hori- zontal direction is unchanged while the vertical speed is lower than the original.
38 | Acoustics Today | Winter 2015, volume 11, issue 1 ©2015 Acoustical Society of America. All rights reserved.