Silent Sub
June 15,
2009 -- 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
journalPhysical Review
Letters.
Watch
video: Scientists work on invisibility tech for soldiers of the
future.
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.
Soldiers
may someday have the capability to become
invisible.
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.
Acoustic Shield
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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