We've heard a lot about how someday metamaterials -- composites that bend electromagnetic waves in bizarre ways -- could lead to invisibility cloaks. Also in the lab are acoustic cloaks that bend sound waves to hide objects underneath. Researchers from Duke University have just reported on their success making a small acoustic cloak that works in air, not just water as has already been demonstrated. Eventually, the metamaterial could be used to tweak acoustics in concert halls at a very high resolution or hide submarines from sonar. From Science News:
To manipulate sound waves in air, (electric engineer Steven) 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."You haven't heard it all"
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.
“We are still a long way from a full acoustic cloak, but this carpet cloak is a step in the right direction,” says Andrew Norris, a physicist at Rutgers University in Piscataway, N.J.