Cloak makes underwater objects invisible to sonar

Researchers have figured out how to make underwater objects invisible to sonar. This cloaking technology could be used by the military or enhance health care imaging.
Written by Boonsri Dickinson, Contributing Editor

Nick Fang of the University of Illinois created a two-dimensional acoustic cloak that can hide underwater objects by manipulating sound waves.

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

Sorry Harry Potter fans.

In the picture above, Fang is holding a cloak made with rings of acoustic circuits. The rings were designed to hide underwater objects from sound waves such as sonar or ultrasound.

The rings have a different index of refraction, and were made that way so the speed of the sound waves could be manipulated.

“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 in a statement. “As you go further inside the rings, sound waves gain faster and faster speed.”

As it turns out, objects can be acoustically invisible to ultrasound waves from 40 to 80 KHz. Although, the researchers claim that the range can technically be expanded.

Essentially, the researchers built an acoustic transmission line for this cloak, made from non-resonant circuit elements.

The researchers describe the details of their experiment in the paper Broadband Acoustic Cloak for Ultrasound Waves, which was published in Physical Review Letters.

So what is the real world relevance? Well, imagine if a submarine could be invisible to sonar. But that's far from the only application.

In health care, a cloak could help hide unwanted body parts so the ultrasound can focus on the hard-to-see regions.

Also, the researchers have a hunch the technology could be used for more practical things like soundproofing.

Photo by L. Brian Stauffer

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