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Courtesy of L. Brian Stauffer/Univeristy of Illinois.

Nicholas Fang shows off the device he used to demonstrate sonar cloaking. Using the same technology, he is beginning to engineer new materials that have sonar cloaking properties.

Too true for science fiction: New device promises underwater invisibility cloak

by Chelsea Whyte
Jan 13, 2011

WhyteImageSonar-No Cloak Correct Size

Courtesy of Shu Zhang/University of Illinois.

Without the invisibility cloak, an object is detectable by sonar because it interrupts the sound waves.

WhyteImageSonar-With Cloak Correct Size

Courtesy of Shu Zhang/University of Illinois.

This demonstration of the sonar cloak shows sound waves traveling around the cylinder of the cloak and out the other side seemingly uninterrupted. That means sonar can't detect it.

A new acoustic cloaking technology sounds like something straight out of science fiction. And it is - minus the fiction.

A team of University of Illinois at Urbana researchers, led by mechanical engineering professor Nicholas Fang recently demonstrated the acoustic cloak that makes underwater objects undetectable by sonar. Fang recently joined the faculty of the Massachusetts Institute of Technology where he will continue the research started in Illinois.

The possibilities for applying this technology range from military to commercial, Fang said. The device is made of artificially-engineered material designed to apply optical properties to acoustics. The sonar cloak developed and demonstrated by Fang’s team looks like a two-dimensional disc with a hole in the center, similar in shape to an LP.

“The concept is adapted from optics,” Fang said. His “aha” moment came four years ago at Illinois, when he had the idea to apply the principals of refracting light to his work on the possibility of bending sound waves.

“Light will always find the easiest way to travel through the core of a fiber optic cable. The same concept is borrowed for acoustics. We ran a set of fibers, or channels, around something we wanted to hide,” Fang said.

When an object is placed in the center of the cylinder of channels under water and sonar is aimed at the object, the sound waves hit the cloaking device and move around the cylinder and out the other side as if they had not been interrupted.

“The acoustic wave will travel through the channels because it’s easier to travel through there,” Fang said. “So if we use an array of those channels, the moment sound hits the cylindrical structure, it will travel around the shell instead of traveling through.”

The sonar cloaking device pictured above is described in a paper accepted for publication recently in the Physical Review Letters journal. “The paper only presented a very crude toy, I would call it, that demonstrates the basic physics,” Fang said. “But it’s possible to make a flexible structure. Fabric is certainly a possibility.”

As for creating something like a paint that cloaks objects from sonar, Fang said that having overcome the basic challenge of refracting sound waves, anything is possible. “How we can make it a paint? I would consider this more of an engineering issue rather than a fundamental challenge.”

As part of MIT, Fang will be working with the Woods Hole Oceanographic Institute in Woods Hole, Mass., to test sonar cloaking in a natural atmosphere. “We’re closer to the ocean, so we have a lot more momentum to test our idea in a real environment. There are some critical challenges for marine systems,” Fang said.

Jim Lynch, Senior Scientist in Applied Ocean Physics & Engineering at Woods Hole, said this advancement in acoustic cloaking is different from technologies used to shield objects from sound in the past. Previously, said Lynch, acoustic cloaking was achieved by using either absorptive material, like the porous acoustic tiles one finds in ceilings, or anti-reflective surface coatings, such as those one finds on camera lenses.

But these older technologies can be limiting. “Dr. Fang’s innovation is in using the ‘sound speed’ to direct the sound energy around the object,” Lynch said. “As the sound gets deeper into the ‘cloaking ring,’ the sound speed increases so that it is pushed away from the object being cloaked.” This pushing away of the sound reflects the waves in a way that look uninterrupted to sonar detection.

The first challenge his team will tackle is the problem of cavitation, a term for shock waves from propeller motion that can damage moving parts of machines or boats. The team will attempt to engineer a material that will mask the sound waves to result in lower cavitation. Fang said he would also like to test applications such as military stealth and improvements in acoustic imaging used in medical practice.

This cloak adds to the understanding of invisibility possibilities for both acoustic and optical cloaks. Professor Cheng Sun of Northwestern University, who unveiled an optical invisibility cloak last year, said, “This is an interesting kind of advancement. It’s from a similar concept we’re all working on called transformation optics.”

Transformation optics, said Sun, is the science of manipulating the light or sound waves around an object to mask its appearance. “Consider that you have a small stone on the surface of a lake that generates the waves. When we see an object or hear a sound, we are perceiving the waves around it. If we see something that exists, it’s based on how the object is changing the waves in its environment,” said Sun.

The same basic science is behind both types of invisibility, though Sun said that the acoustic wave cloak has a more complicated design because sound waves incorporate two kinds of waves while, overall, light waves are more uniform.

Sun’s work in invisibility has progressed since his first demonstration of an invisibility cloak that initially hid objects very hard to see with the naked eye. “We’re getting more convincing results now. At this point we’re able to hide realistic sized objects.”

With a team at MIT, Fang will begin practical testing on sonar cloaking technologies at the Woods Hole Oceanographic Institute this spring.