Professor Nick Fang with the cylindrical acoustic cloak

Researchers from the University of Illinois – led by mechanical science and engineering professor, Nicholas Fang – have developed an acoustic cloak which renders submerged objects invisible.

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

The cloak is made of metamaterial – a class of artificial materials that have enhanced properties as a result of their carefully engineered structure. Fang and his team designed a two-dimensional cylindrical cloak consisting of 16 concentric rings of acoustic circuits structured to guide sound waves.

Each ring has a different index of refraction meaning sounds waves vary their speed from the outer rings to the inner ones. The sound waves propagate around the outer ring, guided by the channels in the circuits, which bend the waves to wrap them around the outer layers of the cloak.

“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.”

The researchers tested the cloak by submerging a steel cylinder in a tank fitted with an ultrasound device on one side and a sensor array on the other. When they placed the cylinder in the cloak, it disappeared from the sonar. They also tested other objects of various shape and density but it had little effect on the cloak’s ability to hide the object.

The cloak can cover a wide range of sound wavelengths, offering acoustic invisibility to ultrasound waves from 40 to 80 Khz, although it could theoretically be tuned to cover tens of thousands of megahertz.

The researchers now plan to explore how the cloak could influence other applications from military stealth to soundproofing to healthcare.