Acoustic cloak from metamaterials

Devin Powell, in an article in ScienceNews, describes an "acoustic cloak" that can bend sound. The cloak is made of a metamaterial, developed by researchers at Duke University as reported in the June 24 Physical Review Letters. Using metamaterials to make a cloak that can guide sound waves was first proposed by Duke team member Steven Cummer, an electrical engineer, and a colleague in 2007 in the New Journal of Physics. Powell writes:

"To manipulate sound waves in air, Cummer's team designed and built a cloak that sits atop an object like a piece of draped carpet. By layering simple metamaterial building blocks — ordinary strips of perforated plastic — the researchers hid a triangular wooden block a couple of inches high and more than a foot long at its base. Sound waves over a range of high but audible frequencies slowed and changed direction cleanly after striking the holey plastic. Most reemerged appearing to have traveled all the way down to the flat surface beneath the block. The prototype is two-dimensional — both the speaker generating the sound and the microphone recording it must be in the same plane above the object. But Cummer believes he could make a 3-D version that would cover an entire bump on a log, not just a slice."