The Chinese Acoustics Research That Might Help Shield Submarines From Sonar

Staff, South China Morning Post
5 December 2016
 
Chinese scientists are developing a technique they hope will be able to make submarines invisible to sonar detection under the sea.
If successful, it would ultimately involve covering subs with special rings made of aluminum alloys.
The researchers at the Chinese Academy of Sciences in Beijing and Huazhong University of Science and Technology in Wuhan in Hubei province experimented with rings about 14 cm across and with periodically etched grooves.
They found that sound waves were guided around the rings rather than bouncing back, which would allow them to be traced by sonar detectors.
The grooves were able to steer the sound waves in a set direction like cars travelling on an expressway.
The researchers published details of their work earlier this month in the scientific journal Nature Communications.
The scientists were originally using the technology - called a topological insulator - to control the movement of electrons to reduce heating in computer chips, but they later realised it also had applications for sound waves.
Several rings could work together to direct sound waves in almost any direction, potentially hiding a submarine from sonar in the future.
Other researchers have been working on the technology, but the Beijing and Huazhong researchers said their system was the simplest.
A research team at Nanyang Technological University in Singapore constructed an array of cylinders creating similar effects last years, but they had to spin at high speed, about 400 revolutions per second, to keep the sound on a strict course.
The Singaporean team also claimed their technology could help submarines evade sonar detection, but planting a large number of spinning cylinders over the hull of the craft could prove an engineering nightmare.
“Our method is simpler. It does not require moving parts,” said one author of the Chinese paper, who asked not to be named.
However, he added that many problems remained to be solved before the technology can be used outside the laboratory on submarines or to reduce noise on aircraft.
Submarines now use used a rubber or plastic coating to absorb sound waves produced by sonar.
The anechoic tiles also reduce noises produced from inside the sub, but the technology is old, first used by the Germany navy in U-boats during the Second World War.
New materials have been developed over the decades to increase the absorption rate, but a powerful and sensitive sonar system can still pick up traces of vessels.
Yang Jing, associate professor of acoustics at Nanjing University, said the topological insulator could trigger a revolution in acoustic studies.
“It has borrowed many ideas from quantum physics, which shed new light on sound problems,” she said.
But the technology was still in its infancy with major problems remaining, said Yang, who was not involved in the rings research.
For instance, a submarine has to remain invisible from sonar beamed from different directions and at different frequencies.
The rings, however, are now only able to deflect sound waves coming from certain angles and within certain frequencies.