“Acoustic metamaterial” may sound exotic, but researchers in France have managed to assemble one from a few multipacks of cola cans. Arranged in a grid, the drinks cans act as a superlens for sound, focusing acoustic waves into much smaller regions than their metre-long wavelengths typically allow. The cans act as resonators, directing the volume of the sound to peak in a space just a few centimetres wide, and this heightened precision could improve acoustic-actuator systems.
Propagating light or sound waves diffract when they encounter an object, with the resulting interference preventing the waves from being focused to a spot smaller than about half their wavelength. However, the scattering process also involves evanescent waves, which prevent discontinuities in the electromagnetic field and fade away quickly – within half a wavelength of the reflecting object.
Superlenses pick up and amplify these evanescent waves and offer a way of beating the diffraction limit. Now, Geoffroy Lerosey, Fabrice Lemoult and Mathias Fink of the Institute Langevin in Paris have developed a system to build and control evanescent waves in order to tightly focus acoustic energy.