Two teams of scientists have created new materials to hide submarines from their enemies’ underwater sonar systems – one that transforms the vessel into a “chameleon”, and the other a prototype of a Harry Potter-like invisibility cloak.

The chameleon-like ceramic-type material, created by researchers at the Chinese Academy of Sciences’ Shenzhen Institutes of Advanced Technology, manipulates sound waves that come into contact with it, such as pulses generated by anti-submarine vessels that can identify underwater threats.

This ability means sonar operators analysing the submarine’s acoustic pattern can be fooled into thinking it is a whale, a huge shoal of fish, or even a friendly submarine.
Researchers call such materials “phononic” crystals. In recent years, various forms of phononic crystals have been developed to control, direct and manipulate the transmission of sound in gases, liquids and solids, but they all suffered one limitation.
Once created, their physical properties were fixed forever, giving the enemy tracking it the opportunity to trace its acoustic traits.

But the Chinese team, led by Professor Zheng Hairong, solved the problem by making it possible to control the crystal’s ability to change its acoustic pattern in a way similar to a chameleon changing its colour.
In the journal, Physical Review Applied, Zheng’s team demonstrated that the new material could change its acoustic properties in different temperatures. Raising the temperature by 20degrees Celsius, for instance, could cause a 20 per cent shift in its sound frequency pattern.

Research on phononic crystals has been carried out in many countries because of their potential applications in military and civilian sectors. Earlier this year, researchers in Singapore reported that it was theoretically possible to hide a submarine from sonar detection by coating it with phononic crystals.
But a second team of Chinese scientists could be a step ahead thanks to the huge government funding for technologies with military uses.

Wu Jiuhui, professor of mechanical engineering at Xian Jiaotong University, said his team had developed the prototype for an “invisibility cloak” for submarines. Its coating material could render a smartphone-sized object undetectable to sonar, even at low frequency.
To remain undetected, a submarine not only has to dodge the enemy’s active sonar beams, but also prevent its own low-frequency generated sounds, such as its engine or crew members’ voices, from reaching an enemy’s listening devices.
“No submarine nowadays can escape low-frequency detection. [But] our research will change the game of seek-and-hunt in the oceans,” Wu said.
“The military wants the simplest solution because it will be the most reliable … They may prefer straightforward invisibility, rather than camouflage.”
Stephen Chen. SCMP ““Harry Potter and the new-age stealth submarines: Chinese researchers create ‘cloak of invisibility’”


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