Duke scientists have accomplished what was previously thought to be impossible.
Researchers announced today that they had found that, in theory, a cloak that would make an object appear invisible to sound waves can exist.
"This could be very useful for hiding submarines from sonar waves so that it would make them invisible to radar," said Steven Cummer, Jeffrey N. Vinik associate professor of electrical and computer engineering in the Pratt School of Engineering and co-author of the study to be published in the Jan. 11 issue of Physical Review Letters.
The sound cloak would be similar to an invisibility cloak created by Duke scientists in 2006, Cummer said.
"It would work in exactly the same way as the electromagnetic cloak," he said. "Sound waves would bend around an object smoothly so, instead of bouncing off that object, the waves would appear as though they were traveling through space."
Although the team of scientists have not actually created the acoustic cloak, Cummer said he expects someone to create a prototype of the cloak in the near future.
"You can bet that there will be a lot of people looking to create this," he said. "It will be harder to create than the invisibility cloak for light waves, but I would not be surprised if this were done by somebody within the next year."
There has been some doubt among other researchers, however, that developing the acoustic cloak was possible, said former Duke researcher David Schurig, an assistant professor of electrical and computer engineering at North Carolina State University, and a co-author of the study.
"A specific paper by [Graeme] Milton stated that doing this would require some mathematical constraints that were far too restrictive to make it useful," Schurig said. "What he didn't realize, however, that even with the constraints, it would still be possible to make something that would be incredibly beneficial to science."
Although others were skeptical about the acoustic cloak, Cummer said he originally set out to make a cloak that did not work perfectly but did work well enough to have some practical application.
"The conclusion before this was that creating a true sound cloak was impossible, but I thought we could still make something work reasonably well," Cummer said. "When we continued to chip away from the problem though, we kept finding solutions that were perfect."
In addition to hiding submarines from sonar, an acoustic cloak could be useful in the future for architectural design, Cummer said. Buildings could be created with hidden structural elements that would be acoustically transparent, he added.
The creation of an acoustic cloak could also mean the potential for developing cloaks to shield objects from other types of waves, Cummer said. In the future, scientists might be able to hide objects from seismic waves that are emitted during earthquakes or even ocean waves.
"Applying this type of cloaking device to acoustic waves, and maybe others in the future, is a completely new phenomenon," Schurig said. "This could be incredibly useful for science and for the public in the future."
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