Duke engineers have created the world’s first three-dimensional acoustic cloaking device, an innovation that distorts sound waves to give the impression an object is not present.

University researchers in the electrical and computer engineering departments set out to discover how they could best manipulate sound. The device is the first of its kind able to acoustically cloak sound coming from any direction and researchers are considering various applications for the object.To make this technology possible, Steve Cummer, professor of electrical and computer engineering, said two goals had to be met—the device needed to absorb all waves in contact with it and not cast an acoustical shadow from the absence of sound waves.

“This is totally fundamental research," Cummer said. "We’re not aiming this at any particular application, we’re just trying to show the kinds of things that can be done.”

Although this device is the first of its kind, Bogdan Popa, a research scientist in electrical and computer engineering, noted that the findings build on a series of advancements in several disciplines.

The research started with electromagnetics, Popa said, citing a theory developed over the course of 2005 and 2006 stating that with the right materials, scientists could make objects invisible. Cummer decided to extend this theory to acoustics and work began in this direction.

Cummer added that, although the researchers are not the only ones working on techniques to manipulate sound, they created much of the original theory and discovered the types of material properties needed to achieve this effect.

“This has been a problem we’ve been working towards for the past five years, starting with the theory, and then sitting down with computer simulations and figuring out how to design the materials," Cummer said.

To achieve these goals, the research team applied a series of mathematical design tools to the problem to ascertain what kinds of material properties were needed to produce an effective cloak, Cummer said. They the engineered a structure made of plates carefully spaced apart, placed at particular angles and perforated with holes of specific diameter.

“These properties allow the material to intercept the sound waves, redirect it around, and make the sound come out with the same phase and time delay that it would have had if the object hadn’t been there,” Cummer said.

To test whether the device actually worked, Cummer and his colleagues placed a small sphere under the cloak on top of a flat surface and directed short sound bursts at it from various angles. They then mapped how the waves responded to the object under the cloak and compared this map to those generated by the object without the cloak and ones generated by the flat surface without the object or cloak. The results showed that the cloak was able to almost completely mimic the map generated by the naked flat surface.

“To make something invisible in this way you need to surround it with a shell that can take the incident wave energy, soak it up, bend it around itself, and then fill in the region behind it so that it looks like the wave’s traveled straight through the object when in reality it’s traveled around it.” Cummer said.

The device has diverse possible applications, but Cummer said the cloak was not designed with any particular implementation in mind.

Popa and Cummer named broad potential uses of the application, ranging from acoustic lenses for underwater communication devices to hiding objects from sound and sonar detectors. More work is required, however, before the device can be made commercially available.

“It’s not perfect yet, but we’ve just proven that this technology is possible,” said Lucian Zigoneanu, research associate in electrical and computer engineering.

Zigoneanu added that the cloak is two times the size of the object underneath it and would ideally just be a thin coating not increasing the object's size.

Ideas for how best to apply the object are still in the works, Cummer said.

“Now that we know that this can be done and we know something about how we can do it, we’re talking to lots of people who have specific ideas about applications,” Cummer said.