This camouflage coating hides submarines from sonar

William Herkewitz, Popular Mechanics
27 March 2015

Imagine a material that wicks sound across its surface like water droplets sliding over a windowpane. For submarines, such a coating would mean an entirely new way to slip past sonar without detection as sound waves pass harmlessly around the vessel.
Physicist Baile Zhang and his colleagues at Nanyang Technological University in Singapore think they may have found a way to design such a coating, which could work for any 3D shape—sharp corners included. In a new research paper published in the journal Physical Review Letters, he describes why this theoretical material could work and what you'd need to make it.

How it works

Zhang says that when sound waves like sonar hit his proposed coating, they strike an acoustically tuned material called a phononic crystal. That crystal bends the waves so that when they bounce off the hull, they loops around—smacking right back onto the surface to bounce over and over again. Zhang likens the process to a professional soccer player curving the ball.
Theoretically, the shape of the material you've coated doesn't matter. As you can see above, the curving sound waves will bounce past sharp corners and flat surfaces alike.
Zhang says that while this new surface is still just a theoretical prospect, he sees no reason why he and his colleagues can't build and begin experimenting on the coating within the next few months.
As for the future promise it might hold for sonar camouflage: "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," he says.

Many ways to hide a sub

Avoiding sonar detection is just a game of making sure you don't let incoming sound-waves bounce back to where they came from, Zhang ssays. That means there are plenty of other (at least theoretical) cloaking methods that also could do the job. So how does Zhang's approach compare?
Valentine Leroy, a physicist at Paris Diderot University in France, has developed a different method of sub camouflage. He's proposed a way to almost perfectly sound-proof a submersible. "The general idea goes back to Germany during WWII," Leroy says, "the idea then was to use some coating material like rubber to dampen the sonar [bounce-back]," making a submarine harder to detect, he says.
Rather than rubber, Leroy found that that a thin sheet of bubble-filled material (think of it like Bubble Wrap) works even better. Why? When the sonar wave smacks the bubbly coating, the energy of the wave is transformed into the vibration of the tiny bubbles, which which soaks up and disperses sound. In practice, a 4-millimeter film of such a material could dampen a sonar signal by as much as 99 percent, Leroy says.
There are other even crazier sounding ideas for acoustic camouflage. One concept would use an array of underwater speakers blast back a synchronized sound wave (with the exact opposite amplitude) whenever sonar hits a ship. In theory, the deflected sonar would be cancelled out into silence.
The undersea cat-and-mouse game continues.