Cloaking device is able to hide underwater objects from sonar

Scientists in the US have succeeded in making a cloaking device that hides underwater objects from sonar.

Previous studies have shown that cloaking was theoretically possible but a team at the University of Illinois has built a metallic disc that can bend underwater sound waves around an object.

The technology could help make medical ultrasound scanners more effective, as well as providing a way for military vessels to avoid detection.

The team, led by Prof Nicholas Fang, based its design on an electronic circuit, using ring-shaped channels to transfer the sound waves from one side of the disc to the other and avoid the object hidden in the centre.

Because of an array of cavities in the channels, sound waves travel faster through the inner rings than the outer one. But as this requires more energy, the sound waves naturally stay in the outer rings.

‘The cavities are arranged with different sizes along the radial directions,’ Fang told The Engineer.

‘They are smaller at the outer rings so the sound waves take more time, and bigger at the inner rings so the sound waves run faster. Now for a sound beam coming from outside the shell, they prefer to avoid the very centre.

‘This is like a process of transmitting light through optical fibres. Light will not leak out of the fibre for similar reasons.’

The disc is made from a specially engineered substance called a metamaterial. The channels and cavities are drilled into the disc to create the specific conditions needed to transfer the sound waves underwater.

‘The most important idea in our work is the simplified structures using circuit analogue to electronics,’ said Fang.

‘By doing so the design is quite straightforward and more flexible. You can lay down the cavities and channels similar to designing a electronic circuit board.’

The device forms a layer of protection for an object no matter what its structure, shape or density. It protects against sound wavelengths between 40 and 80KHz, although theoretically it could be tuned to cover tens of megahertz.

But one disc only bends waves travelling within its two-dimensional area of space. ‘On the other hand, if we are able to stack the cloaks with many similar layers, 3D objects will become invisible too,’ added Fang.

The researchers now plan to investigate possible uses for the technology, including military stealth capabilities, soundproofing and medical imaging.

For example, the technology could be used to shield parts of the body that would otherwise interfere with ultrasound scanners and mar the resulting image.