Sound Cloaks for Subs

[Navy_Blue]

http://www.canada.com/topics/technology/science/story.html?id=badcd680-2713-4a85-991c-307fcbd0dd11

'Sound cloak' could make subs invisible
Tom Spears ,  CanWest News Service; Ottawa Citizen
Published: Thursday, January 10, 2008
OTTAWA - U.S. and British physicists have devised - though only in theory - a "sound cloak" that would hide an object from sound the way an invisibility cloak makes Harry Potter invisible.

The idea is that sound waves would flow around the cloak and re-form on the other side, without being muffled or distorted. This might make a submarine undetectable by sonar, since the sonar (sound) waves would just pass around it. Or it could prevent a structural beam from messing up the acoustics in a concert hall.

The same team showed in 2006 this is possible for invisibility, and actually made a working model in two dimensions. That cloak works by bending light around an object without distorting it, so the viewer sees whatever is on the far side of the cloak. One researcher called it "similar to river water flowing around a smooth rock."

This time they have figured, in theory, how to do the same with sound waves instead of light. The catch: No such sound-bending materials exist yet.

But the team thinks it knows how to engineer something. "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 project leader Steven Cummer, who teaches electrical and computer engineering at Duke University in North Carolina.

The new project spun out of the 2006 invisibility cloak - a small model about 12 centimetres wide - which combines copper rings and wires on a fibreglass base to bend light waves around it by an electromagnetic field.

At the time, everyone said the same trick wouldn't work with sound waves. Cummer wasn't so sure.

"Waves are waves," he said. The secret lies in "metamaterials" - composite materials engineered to react a specific way to sound.

He doesn't yet know what those will be, but his team has published the theory of how they could bend sound in this week's issue of a major physics journal, Physical Review Letters. "The electromagnetic (invisibility) cloak was pretty interesting in and of itself," he said Wednesday. "The natural follow-up question was: Can that be adapted to other forms of waves," such as sound waves, ocean waves or even seismic waves? A prominent math analysis last year concluded: No way.

Cummer wasn't convinced. "There are some very, very general properties of all kinds of waves ... and some things are just inherent to the idea of waves, (such as) carrying power in certain directions and how you can influence them."

He and his partners found first one, then another instance where sound waves did turn out to be bendable in theory.

Bendable with what, though? Some form of composite material, he says.

The trick is to design a material that reacts differently when sound comes at it from different directions, the way a one-way mirror acts like a mirror from one side, and a window from the other.

Natural materials such as metal or glass usually don't act this way. He hasn't designed the material yet.

But the concept looks like a ball filled with liquid or gas, and a lot of little metal or ceramic pieces inside "that scatter the sound in a very specific way."

So far there's more known about directing light this way than handling sound.

Cummer hasn't heard much reaction to his theory yet. Cloaking, it turns out, is a highly competitive field, and he has been keeping his ideas under a cone of silence until they were ready to publish.


© CanWest News Service 2008

Sounds cool excuse the pun  ;D We need to get ours working good Tho.

 

[CougarKing]

A related update:

Scientists at the University of Illinois in the United States have developed and successfully demonstrated a sonic cloak that could make submarines completely invisible to sonar.

The team of scientists, led by mechanical science and engineering professor Nicholas Fang, announced earlier this month that they have produced and demonstrated an acoustic cloak that makes underwater objects invisible to sonar and other ultrasound waves, finally proving what has long been speculated.

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

Rather than absorbing sound waves, the new material bends sound waves around the object, making it appear invisible.

The researchers tested their clock by wrapping it around a steel cylinder and submerging it in a water tank with an ultrasound device on one side and a sensor on the other. The cloaked cylinder did not show up on their equipment and proved invisible to a broad range of sound waves.

They then tried testing objects with different materials and densities, with similar results.

The cloak itself consists of 16 concentric rings of acoustic circuits that channel sound waves. Sound waves vary their speed from the outer rings to the inner ones, becoming faster further inside the rings. Because speeding up requires energy, the sound waves flow around the cloak’s outer rings, guided by channels in the acoustic circuits. The circuits bend the sound waves to wrap them around the outer layers of the cloak.

The metamaterial cloak can cover a wide variety of sound wavelengths, from 40 to 80 kHz, although in theory it could be tuned to cover tens of megahertz. Military sonar systems operate anywhere from 1000 Hz to 500 kHz, according to an Australian government report. This would make submarines covered in the cloak invisible to sonar, as the sound waves pass around the vessel rather than bouncing back to the sonar detector.

"This is not just a single wavelength effect. You don't have an invisible cloak that's showing up just by switching the frequencies slightly," Fang said.

The technology could also be applied to other areas of submarine stealth, such as cavitation, whereby small bubbles form and implode around fast moving objects like propeller blades. Fang and his group believe they could harness their cloak’s properties to balance energy in caviation-causing areas. Thus, the cloak could be used to keep noise from getting out of a submarine in addition to stopping sonar waves from bouncing back to their source.

Another benefit of the breakthrough would be in the medical industry, where the metamaterial could hide unwanted body parts during an ultrasound scan. The development could also lead to higher definition ultrasound scanners.

In addition, the technology could be used for soundproofing and improving the flow of sound waves in auditoriums.

The use of metamaterials, which have properties that do not exist in nature, has produced a lot of theoretical papers, but realising those ideas has been a challenge until quite recently. In 2008 a report by a research team suggested that a 3D acoustic cloak couldn’t work, but this has been proved otherwise, Science Daily reports. At the moment scientists are well on the way to creating an invisibility cloak that works on the optical level, the BBC reports.

So far, metamaterials have successfully been used to make objects invisible to other wavelengths. In 2007, scientists and professors at Duke University and Imperial College, London, demonstrated an invisibility cloak that could shield a small area from microwave detection, EE Times reported. This could lead to improved stealth fighters that don’t rely on radar absorbent materials to hide from radar.


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By Guy Martin, defenceWEB reporter

link

 

[George Wallace]

Sound is but one of their problems.  Wonder what they are doing to get rid of the heat signatures?

 

[Occam]

Sound is but one of their problems.  Wonder what they are doing to get rid of the heat signatures?

I could see IR/heat signature being a problem while surfaced, but at anything more than a couple of metres they're surrounded by the world's largest heat sink.

 

[aesop081]

The idea is that sound waves would flow around the cloak and re-form on the other side, without being muffled or distorted. This might make a submarine undetectable by sonar, since the sonar (sound) waves would just pass around it. Or it could prevent a structural beam from messing up the acoustics in a concert hall.

So it works against active. Thankfully that leaves passive, MAD and a few other things.

Hardly invisible.