## Metamaterials and cloaking |

Metamaterials are composite materials with bulk properties determined by their structure rather than their composition. The bulk properties enable manipulation of the way in which waves propagate through the metamaterial. They are normally composed of a host material with periodic arrays of inclusions with alternative material properties. The inclusions can be solid scatterers or can be locally resonant structures themselves.

Application of Bloch's theorem for periodic media can determine the effective wave properties of the metamaterial. Effective properties can be achieved that are not realisable in natural structures, such as negative bulk modulus. It is possible to realise specific required material parameters by adjusting the structure of the metamaterial, such as the period of the inclusions. These features of acoustic metamaterials mean that they are of great interest in many different applications.

One example application of metamaterials in acoustics are silencers. New silencing technology can be developed using metamaterials which can attenuate noise at lower frequencies in much smaller packages than previous silencer materials and methods.

Another application is that of acoustic cloaking. This works by surrounding an object with a specifically designed metamaterial that can manipulate the path of propagation of incident sound waves to travel around the object. This means that there is no scattering by the central object, and so the object becomes acoustically invisible.

The discovery of and the theory behind acoustic metamaterials is relatively recent and thus research is very current in the field. Many more applications are still to be realised and difficulties remain in the physical realisation of some mathematically designed metamaterials.

Application of Bloch's theorem for periodic media can determine the effective wave properties of the metamaterial. Effective properties can be achieved that are not realisable in natural structures, such as negative bulk modulus. It is possible to realise specific required material parameters by adjusting the structure of the metamaterial, such as the period of the inclusions. These features of acoustic metamaterials mean that they are of great interest in many different applications.

One example application of metamaterials in acoustics are silencers. New silencing technology can be developed using metamaterials which can attenuate noise at lower frequencies in much smaller packages than previous silencer materials and methods.

Another application is that of acoustic cloaking. This works by surrounding an object with a specifically designed metamaterial that can manipulate the path of propagation of incident sound waves to travel around the object. This means that there is no scattering by the central object, and so the object becomes acoustically invisible.

The discovery of and the theory behind acoustic metamaterials is relatively recent and thus research is very current in the field. Many more applications are still to be realised and difficulties remain in the physical realisation of some mathematically designed metamaterials.