Acoustic Cloak Bends Sound

Spain does seem to be a hotbed of audio research these days; the Univesitat Pompeu Fabra of Barcelona brought us the ReacTable, and now engineers at the Polytechnic University of Valencia have devised a method of bending sound around an obstacle so that it effectively becomes inaudible…

Layering Different Sounds – And Crystals

Acoustic cloaking materials are created using alternating layers of two different materials, which direct sound waves around an obstacle so that they reform without distortion on the other side.

These ‘metamaterials’ do not exist in nature, but the team led by José Sánchez-Dehesa have figured out a method of creating arrays of sonic crystals that allow some sound waves to pass while blocking others.

Although the original research in this area was focused on designing light-maniupulation materials, it was discovered that the principles could be also applied to sound. In effect, sound waves would flow around a cloaked object like water around a rock. Anyone inside the cloak would not be able to hear these sound waves flowing around them.

The image above shows a planar sound wave moving around a cylinder covered with the acoustic cloak. The number of layers used is critical to creating an effective cloak – the image on the left shows the cylinder with a 50-layer acoustic shell, and the image on the right shows the same cylinder with a 200-layer shell. As you can see, the extra layers render the cylinder non-destructive to the progress of the soundwave.

An obvious application of this would be in making ships ‘invisible’ to sonar, but there are many more possibilities. Creating a perfectly silent recording studio is one that springs to mind here…