Duke University's plastic plates of invisibility

Star Trek fans may be delighted to learn that cloaking technology—which makes an object invisible to sensors—is no longer merely the product of sci-fi creativity. Researchers at the Duke University Pratt School of Engineering have developed the first 3-D acoustic cloaking device, which can make itself and an object underneath it undetectable when “pinged” with sound waves, according to a recent news release.

The device is the first to work multi-directionally, said Bogdan Popa, a research scientist in electrical and computer engineering who helped design the model. It doesn’t matter where the sound comes from or where the observer is located, the object is undetectable by sonar waves. Not only does the technology work in all directions but it also works for a broad range of frequencies, from 0 to approximately 3,000 hertz.

The researchers employed concepts from the developing field of metamaterials, which uses synthetic composites that have properties not usually demonstrated in natural materials. In this case, they used plastic plates punctured with a repeating pattern of holes, stacked in pyramid. When sound waves hit the device, the material’s design interacts with the acoustics to give the illusion that the sound waves are reflecting off an empty surface.

The technology may be useful for a variety of stealth and commercial applications. It could be used to concentrate sound in a more directive beam. For example, audio communication between submarines and ships goes out in all directions, meaning it’s more likely to be picked up by others. But using the cloaking technology, the sound waves could be concentrated in one direction, requiring less power and making communication more secure.

Acoustic cloaking may also be useful for commercial applications like controlling sound waves in auditoriums and concert halls. If a structural beam in a room interfered with acoustics, it could be cloaked with the device.

The acoustic cloaking model does not work with the electromagnetic waves of radar. Some researchers are working on a similar design for electromagnetic cloaking but they have not yet found a way to hide an object in a broad range of frequencies.

Other researchers are working on designing a model for optical cloaking—which would work like Harry Potter’s invisibility cloak—but that is still a long way in the future. It is conceivable that some day acoustic and optical cloaking could be combined to prevent detection in both modes.

The acoustic cloaking device is fairly inexpensive to make.

“It only requires plastic with holes poked in it,” Popa said. “It can be done very easily by anyone.”