Shape of things to comeBreakthrough: Acoustic cloak theoretically possible

Run silent, run deep. It is good not to be wedded too stubbornly to received wisdom. Here is a case in point: Contrary to earlier predictions, Duke University engineers have found that a three-dimensional sound cloak is possible, at least in theory. Such an acoustic veil would do for sound what the “invisibility cloak” previously demonstrated by the research team does for microwaves — allowing sound waves to travel seamlessly around it and emerge on the other side without distortion (see Adrian Cho’s article, listed below). “We’ve devised a recipe for an acoustic material that would essentially open up a hole in space and make something inside that hole disappear from sound waves,” said Steven Cummer, Jeffrey N. Vinik Associate Professor of Electrical and Computer Engineering at Duke’s Pratt School of Engineering. Such a cloak may hide submarines in the ocean from detection by sonar, he said, or improve the acoustics of a concert hall by effectively flattening a structural beam.

As in the case of the microwave cloak, the properties required for a sound cloak are not found among materials in nature and would require the development of artificial, composite metamaterials. The engineering of acoustic metamaterials lags behind those which interact with electromagnetic waves (that is, microwaves or light), but “the same ideas should apply,” Cummer said. The report by Cummer’s team will appear in a future issue of Physical Review Letters.

We should recall how we got to this important juncture in cloaking studies. In 2006 researchers at Duke and the Imperial College London used a new design theory to create a blueprint for an electromagnetic invisibility cloak. Only a few months later, the team demonstrated the first such cloak, designed to operate at microwave frequencies. Cummer and David Schurig, a former research associate at Duke who is now at North Carolina State University, later reported in the New Journal of Physics (see reference below) a theory showing that an acoustic cloak could be built. Alas, that theory relied on a “special equivalence” between electromagnetic and sound waves which is only true in two dimensions, Cummer said. A report by another team had also suggested that a 3-D acoustic cloak could not exist. It appeared the researchers had reached a dead end.

Cummer was not convinced. “In my mind, waves are waves,” he said. “It was hard for me to imagine that something you could do with electromagnetic waves would be completely undoable for sound waves.” This time, he started instead from a shell like the microwave cloak his team had already devised and tried to derive the mathematical specifications required to prevent such a shell from reflecting sound waves — a key for achieving invisibility. On paper, at least, it worked. “We’ve now shown that both 2-D and 3-D acoustic cloaks theoretically do exist,” Cummer said. The theory used to design such acoustic devices so far is not as general as the one used to devise the microwave cloak, but the finding nonetheless paves the way for other acoustic devices — for instance, those meant to bend or concentrate sound. “It opens up the door to make the physical shape of an object different from its acoustic shape,” he said.

The existence of an acoustic cloaking solution also indicates that cloaks may possibly be built for other wave systems, Cummer said, including seismic waves which travel through the earth and the waves at the surface of the ocean.

-read more in Adrian Cho, “Voilà! Cloak of Invisibility Unveiled,” Science 314, no. 5798 (20 October 2006) (DOI: 10.1126/science.314.5798.403): 403 (sub. req.); and Steven A. Cummer and David Schurig, “One Path to Acoustic Cloaking,” New Journal of Physics 9 (2 March 2007) (doi:10.1088/1367-2630/9/3/045): 45;