Submarines could become invisible to sonar

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.