Over the past decade, new tools have emerged that enable precise control over the propagation of electromagnetic waves, sound, and even heat. Using coordinate transformations, one can calculate the spatial variations in materials parameters, such as the dielectric constant and magnetic susceptibility or the mass density and bulk modulus, that will produce the desired propagation. Metamaterials provide a way to engineer the needed parameters from assemblies of appropriate subwavelength building blocks. (See the article by Martin Wegener and Stefan Linden, Physics Today, October 2010, page 32.) But the required materials properties can be quite complex, particularly in acoustics, for which the implementations have been limited to two-dimensional configurations.

Lucian Zigoneanu, Bogdan-Ioan Popa, and Steven Cummer of Duke University have now demonstrated a 3D acoustic device, shown here, that shields its interior region from sound waves coming from any direction. A so-called ground cloak, it reflects the waves...

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