New stealth for submarines: Bubble wrapping them

Charles Q. Choi, Yahoo News, Feb

Bubble-filled rubbery coatings may one day help make submarines virtually undetectable to sonar, researchers say.

To avoid detection by sonar, military submarines are often covered with sound-absorbing tiles called anechoic coatings. These perforated rubber tiles are typically about 1 inch (2.5 centimeters) thick.
In the past decade, research has suggested that the same degree of stealth could be provided by much thinner coatings filled with vacant cavities. When hit by sound waves, empty spaces in an elastic material can oscillate in size, "so it will dissipate a lot of energy," said lead study author Valentin Leroy, a physicist at the Université  Paris Diderot in France. [7 Technologies That Transformed Warfare]
However, figuring out how to optimize such materials for stealth applications previously involved time-consuming simulations. To simplify the problem, Leroy and his colleagues modeled the empty spaces in the elastic material as spherical bubbles, with each giving off a springy response to a sound wave that depended on its size and the elasticity of the surrounding material. This simplification helped them derive an equation that could optimize the material's sound absorption to a given sound frequency.
The researchers designed a "bubble meta-screen," a soft layer of silicone rubber that is only 230 microns thick, which is a little more than twice the average width of a human hair. The bubbles inside were cylinders measuring 13 microns high and 24 microns wide, and separated from each other by 50 microns.
In underwater experiments, the scientists bombarded a meta-screen placed on a slab of steel with ultrasonic frequencies of sound. They found that the meta-screen dissipated more than 91 percent of the incoming sound energy and reflected less than 3 percent of the sound energy. For comparison, the bare steel block reflected 88 percent of the sound energy.
"We have a simple analytical expression whose predictions are in a very good agreement with numerical simulations and real experiments," Leroy told Live Science. "I find it exciting and beautiful." 
To make submarines invisible to the sound frequencies used in sonar, larger bubbles are needed. Still, the researchers predicted that a 0.16-inch-thick (4 millimeters) film with 0.08-inch (2 millimeters) bubbles could absorb more than 99 percent of the energy from sonar, cutting down reflected sound waves by more than 10,000-fold, or about 100 times better than was previously assumed possible.
However, despite the possibilities, "making these samples will probably be tough," Leroy cautioned.