Special coating could make submarines undetectable to sonar

The Daily Caller
31 March 2015

Physicists at the Nanyang Technological University in Singapore have published a new paper which lays the theoretical foundation for how submarines, with a special coating, could deflect sound.
This means submarines would be completely undetectable by sonar.
Authored by physicist Baile Zhang and his colleagues, the paper, published in Physical Review Letters, proposes that the same technique could apply to any 3D object. Normally, sonar waves hit the hull of a submarine and the sound scatters, which creates an echo, revealing the location of the sub.
Under Zhang’s system, sound waves sent out by sonar would hit a special material called a phononic crystal. This material is acoustically tuned and would manipulate the sound waves, such that they would bounce off and on the submarine in a loop, until finally the submarines passes through undetected.
For now, the coating is just a theory. But unlike other more fantastical proposals, Zhang says he and his team will get right to designing the coating in just a few months, as the project is very feasible.
“In principle, if a sound wave can be smoothly guided around the submarine without reflection, it can escape detection from sonar, because the sonar works by detecting deflected signals,” Zhang said, according to Popular Mechanics.
Zhang’s approach is particularly interesting because it avoids traditional problems which have plagued stealth engineers for decades. So-called “irregular protrusions” from objects ruin cloaking abilities, but a coating of an acoustic topological insulator, according to Zhang, would work on an object of any shape. Phononic crystals could also potentially be used to make hearing aids function more effectively in channeling sound through the ear canal.
Another method of stealth submarine technology is underway at the Paris Diderot University in France. Physicist Valentine Leroy  thinks that bubble-filled material could kill a sonar signal by as much as 99 percent. How it works is that the bubbly coating absorbs and then virtually dissipates the sound.