You've probably heard of invisibility cloaks that can hide tiny objects, even those visible to the naked eye. Now, researchers have used the same types of metamaterials to create an acoustic cloak, a technology that renders underwater objects invisible to sonar and other ultrasound waves. A metamaterial is a class of artificial materials that have enhanced properties as a result of their structure.
“We are not talking about science fiction. We are talking about controlling sound waves by bending and twisting them in a designer space,” said Nicholas Fang, a mechanical science and engineering professor at the University of Illinois at Urbana-Champaign and leader of the project.
“This is certainly not some trick Harry Potter is playing with,” he said in a university release.
Fang and his team of researchers 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. The speed is slower at the outer rings so that's where sound waves propagate since greater energy is required for them to move to the inner rings. The specially structured acoustic circuits bend the sound 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.”
Tests revealed that the acoustic cloak can effectively bend the ultrasound waves around the hidden object. The researchers submerged a steel cylinder in a tank with an ultrasound source on one side and a sensor array on the other, then placed the cylinder inside the cloak and watched it disappear from their sonar. They also tested other various objects and found the same results.
“The structure of what you’re trying to hide doesn’t matter,” Fang said. “The effect is similar. After we placed the cloaked structure around the object we wanted to hide, the scattering or shadow effect was greatly reduced.”
It is also capable of hiding an object from a broad range of sound waves from 40 to 80 KHz and potentially tens of megahertz with modifications.
Applications for the acoustic cloak include military stealth, soundproofing, and healthcare. For instance, ultrasound and other acoustic imaging techniques used in medical practice produce images that are often marred by things in the body that can cause interference. A metamaterial bandage or shield could effectively hide a troublesome area so the scanner could focus on the region of interest.
While materials that can wrap sound around an object rather than reflecting or absorbing it have been theoretically possible for a few years, this is the first time it has been demonstrated.
The research describing the working cloak prototype appears in a paper for the journal Physical Review Letters.