Researchers in the US have built the first practical device that can cloak an object from being detected by sound waves. The cloak, which is made from a cylindrical array of acoustic cavities, has been shown to operate in water at ultrasonic frequencies. The technology could in principle be adapted to cloak underwater objects from sonar.

Several research groups have already managed to build "invisibility cloaks" that hide objects from electromagnetic waves. Such cloaks are made from "metamaterials" – artificial structures with special optical properties such as negative indices of refraction. These structures are arranged such that incoming light waves flow smoothly around the cloak, joining up on the other side as if the cloak and object were not there.

The same principles can be applied to sound and in 2008 Nicholas Fang and colleagues at the University of Illinois at Urbana-Champaign in the US created an acoustic "superlens" using acoustic metamaterials. Now Fang and team claim to have built the first practical broadband and low-loss acoustic cloak.

Acoustic capacitors and inductors

The device comprises 15 concentric cylinders that vary in radius from 13.5–54.1 mm. Each cylinder contains an array of cavities connected by channels and these behave as acoustic capacitors and inductors. The size of the cavities and channels changes from one cylinder to the next and this gives the device the acoustic properties needed for cloaking.

To demonstrate their device, Fang and colleagues placed a solid object at the centre of the cloak and then fired sound waves at it. By detecting the sound that reached the other side of the cloak using a hydrophone, they showed that the cloak works across a range of acoustic frequencies from 52 to 64 kHz.

This "broadband" response is unlike many optical cloaks, which tend to function only over an extremely narrow frequency range. This narrow optical response is related to restrictions on how the speed of light can vary inside and outside the cloak. These restrictions do not, however, apply to sound waves.

Another benefit of the design is that that the metamaterial does not absorb much of the sound: if it did, the cloak might become apparent to an observer. This is unlike previous acoustic cloak designs, which relied on soft, absorbent materials.

Steve Cummer of Duke University in the US, who was not involved in the work, says that the research is an important breakthrough in the development of acoustic metamaterials. "This is the first real experimental demonstration that acoustic metamaterials that achieve the properties required by transformation acoustics can be designed and constructed using relatively simple building blocks," he says.

Should work in 3D

One shortcoming of the design is that it is restricted to cloaking objects from sound that propagates in 2D – making it impractical for many real-life applications. However, Cummer told physicsworld.com that "the underlying theory of transformation acoustics applies to both 2D and 3D and the required material parameters are similar in these cases. Honestly I don't see why it couldn't be extended with 3D cavities instead of 2D ones". The big challenge in making 3D devices, according to Cummer, would be fabricating the structure.

Although the design could be used to hide underwater objects from sonar, it does have its shortcomings. As Cummer points out, the cloaking shell is much thicker than the object it is cloaking: Fang's cloak needs a 40 mm thick shell to hide an object about 13 mm in diameter. Such a thick shell would be impractical for shielding a moving object such as a submarine, although Cummer admits that lessons learned from Fang's demonstration could lead to thinner cloaks.

The work is described in Phys. Rev. Lett. 106 024301.