One diagram
Coming soon: cloaked submarines
Wednesday, 16 January 2008
by Joshua Cockfield
Cosmos Online
BRISBANE: They're already masked beneath the waves, but soon these silent killers could be harder to detect - even using sonar.
American scientists have proven that a cloaking device which would render submarines and ships invisible to sonar is theoretically possible.
"We've devised a recipe for an acoustic material that would essentially open up a hole in space and make something inside that hole disappear from sound waves," said Steven Cummer, an electrical and computer engineer at Duke University in Durham, North Carolina.
A 'three-dimensional (3-D) sound cloak' made of this material would render objects within it acoustically invisible by forcing sound waves around them. The discovery follows on from the development of working models of electromagnetic invisibility cloaks, which operate at microwave frequencies.
Bending sound
Sound waves are normally reflected by objects that lie in their path – and these reflected waves can be detected by sonar. The hypothetical cloaking device could prevent this by bending sound waves around an object, to continue moving along their original path.
To do this without disturbing the waves, artificial substances known as 'metamaterials' are required. These materials have unusual properties that are not found in nature, and which arise from their structure rather than their composition - much like how diamond and graphite have very different properties yet they're both made of carbon.
Waves travelling through a conventional material, like air, all behave the same way regardless of the direction in which they are moving. In contrast, electromagnetic waves passing through metamaterials can behave in different ways, such as travelling at different speeds, depending on their direction.
In a previous study, the Duke University team designed an electromagnetic cloak that could make microwaves pass around it. However, other experts doubted that the cloaking concept would be possible with sound waves.
Working cloak: three years away?
"In my mind, waves are waves. It was hard for me to imagine that something you could do with electromagnetic waves would be completely unachievable for sound waves," said Cummer.
His team wondered if they could come up with a specific set of properties for a metamaterial that would have the same effect on sound waves that previously designed cloaking technology would have on microwaves. So they used mathematical modelling to test the idea.
"And somewhat to our surprise, the answer was yes," he said, adding that a working model of the acoustic sound cloak might be possible within two to three years.
"The paper contains significant theoretical and numerical results," commented Sebastien Guenneau, an expert in metamaterials from the University of Liverpool in England. "I think the paper on 3-D acoustic cloaks will have a high impact."
The research is published in the journal Physical Review Letters.
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