Invisibility cloak hides underwater objects from sonar

University of Illinois researchers have developed an acoustic cloak, making underwater objects invisible to sonar and other ultrasound waves.

“We are not talking about science fiction. We are talking about controlling sound waves by bending and twisting them in a designer space,” says mechanical science and engineering professor Nicholas Fang. “This is certainly not some trick Harry Potter is playing with.”

While materials that can wrap sound around an object rather than reflecting or absorbing it have been theoretically possible for a few years, the concept has been hard to put into practice.

Fang’s team designed a two-dimensional cylindrical cloak made of 16 concentric rings of acoustic circuits structured to guide sound waves. Each ring has a different index of refraction, meaning that sound waves vary their speed from the outer rings to the inner ones.

“Basically what you are looking at is an array of cavities that are connected by channels. The sound is going to propagate inside those channels, and the cavities are designed to slow the waves down,” Fang said. “As you go further inside the rings, sound waves gain faster and faster speed.”

Since speeding up requires energy, the sound waves instead propagate around the cloak’s outer rings, guided by the channels in the specially structured acoustic circuits. The circuits actually bend the sound waves around the outer layers of the cloak.

The researchers tested the cloak by submerging a steel cylinder in a tank with an ultrasound source on one side and a sensor array on the other. When they placed the cylinder inside, it disappeared from their sonar.

One big advantage of the acoustic cloak is that it can cover a broad range of sound wavelengths – from 40 to 80 KHz, says the team. And with a little modification, they say, it could theoretically be tuned to cover tens of megahertz.

Next, the researchers plan to look at applications for the cloaking technology, from military stealth to soundproofing to health care.