A new invisibility cloak for sound could help doctors find tiny tumors or hide submarines from enemy sonar.
"Our focus is not about dampening noise, but to guide sound waves around structures," said Nicholas Fang, a professor a the University of Illinois at Urbana Champaign and coauthor, along with Shu Zhang and Leilei Yin, on a paper that appears in the journal Physical Review Letters.
For example, "if we have a coating on a submarine that bends acoustics waves before they hit the surface, guiding them around the submarine smoothly, then you won't be able to detect a submarine using sonar."
The same technology that could render a military submarine invisible to sonar could also be used to create high-definition, in-utero baby pictures or detect previously undetectable, tiny tumors.
Invisibility cloaks, whether for sound or light, both manipulate waves. Harry Potter-style invisibility cloaks manipulate beams of light. Acoustic invisibility cloaks manipulate waves of pressure. Whatever the wave type, the principle is basically the same; bend a wave around an object without breaking it.
The concept is the same, but the design is quite different. One of the many differences between the two types of waves is size.
Sound waves are larger than electromagnetic waves. To manipulate either wave requires structures many times smaller than the size of the wave. Because the properties of the material are determined by their physical structure and not their chemical make-up, as they traditionally are, they are called metamaterials.
"If you need to build an ultrasonic metamaterial, the dimension of the physical structure is tens or hundreds of microns," said Fang. "Compare that with optical metamaterials, and you are talking about hundreds of nanometers. That makes it a lot more amenable for research."
The sonic invisibility cloak works a lot like a musical instrument.
Musical instruments amplify sound waves using shaped cavities. The sonic metamaterial uses cubes and octagons to create holes that can then bend the wave around the structure. The most obvious application would be as a coating for submarines that want to avoid detection from enemy sonar.
Besides bending waves around a structure, the CD-sized metamaterial can also focus sound waves into a sub-millimeter-sized area, an area smaller than traditional ultrasound machines can currently see. Known as a super lens, it could enable doctors to see babies in utero in much higher definition or detect tumors that are currently too small for ultrasonic detection.
"We have seen some very exciting demonstrations," said Fang. "But to make this as a practical structure we need another three to five years."
Other researchers are excited by the development.
"This is one of the very first metamaterials that has been designed to work and actually do something useful for sound waves instead of electromagnetic waves," said Steven Cummer, a professor at Duke University who helped develop the original invisibility cloak in 2005.
"I would say that the Illinois design for two-dimensional acoustic metamaterials is pretty straightforward to manufacture. I certainly hope that it can be transitioned to an actual application," said Cummer.
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