'Bubble' coatings can hide submarines from sonar

Bubble-filled rubbery coatings could make military submarines virtually undetectable to sonar, scientists say.
Military submarines are often covered with sound-absorbing tiles called anechoic coatings to avoid detection by sonar. These perforated rubber tiles are typically about 1 inch thick.
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 Universite Paris Diderot in France.
Leroy and his colleagues modelled 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, 'Live Science' reported.

This simplification helped them derive an equation that could optimise 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 per cent of the incoming sound energy and reflected less than 3 per cent of the sound energy.

The findings are published in the journal Physical Review B.