A new material created by researchers can refocus sound around certain objects and effectively render them sonically invisible to sonar. No natural material can do this, so man-made "metamaterials" must be created in order to toy with the laws of physics to essentially bend sound back on itself. Mind blown yet?
The technique, which bends light well beyond its natural tendencies, has been used for years in lenses used ultra-high-resolution imaging and has also been applied to real visible-light cloaking devices. The same theory is in use here, making a sort of acoustic lens, a material covered by a set of cavities which, when filled with water, work together to resonate with each other and refract the sound.
While the concept of this may be bending your brain backwards to understand, we contacted Nick Fang, assistant professor at the University of Illinois in charge of the project to help simplify:
"The sonar can spot out ships by detecting the echoes scattered from the ship. To cloak ships from sonars, we need to trick the sound waves to bend around the ship, like water flows around a smooth rock in a stream without wakes, so it cannot be detected. In order to do that, we are inspired by the similar optical effects such as mirages.
Quite often we can notice a highway mirage in sunny days, where we see a displaced image of distant object into the sky, while the real road with warm air on its surface seems immersed in water. This is because light travels with different speed in the warm air and cooler air; such gradient in light speed causes light to bend away from the road surface. In acoustic, we can bend sound wave in a similar fashion by changing the sound speed in different depth of the cloak. However, this is not a trivial job, as we have to make sure the sound waves from all angles are bent smoothly without scattering."
When the material is ready for real-world applications, not only will we never find any military ships, but high-resolution clinical ultrasound imaging will be greatly enhanced.
This would be defeated if the detecting vessel where above or below the cloaked vessel because there would be a big hole in sound reflecting off the surface of the water or the floor of a shallow sea. It will only be time before flash lidar augments sonar on submarines anyhow.
The submarine covered in this metamaterial will not appear as a hole because the metamaterial makes the sound waves go *around* the vessel, allowing the waves to continue on past it.
It would only appear as a "hole" if the submarine was covered in a sound *absorbing* material, which is what modern submarines are covered with already.
If the sound waves are indeed being bent around the object, the "shadow" effect you just described wouldn't happen at all: The sound would bend around the vessel, be reflected off the ocean floor, and then be bent around the vessel a second time on the way back.
You are right about one thing, though; unless the bending of sound was perfected to the point that it was undetectable, there would be some disruption of the sonar 'image,' similar to the lens effect described in the mirage scenario. With the mirage, although you cannot see the road you can see and recognize the lens effect.
So, you are partially correct; with a backdrop for the sonar image, you would notice the vessel... but you would be noticing the "lens" effect and not a "shadow."