Soft-Drink Cans Focus Sound Waves to a Point, Beating Diffraction Limit

To focus sound to a point, all you need is a thirst for fizzy drinks.

Jon Cartwright of Nature magazine

Sound, like light, can be tricky to manipulate on small scales. Try to focus it to a point much smaller than one wavelength and the waves bend uncontrollably–a phenomenon known as the diffraction limit. But now, a group of physicists in France has shown how to beat the acoustic diffraction limit–and all it needs is a bunch of soft-drink cans.

Scientists have attempted to overcome the acoustic diffraction limit before, but not using such everyday apparatus. The key to controlling and focusing sound is to look beyond normal waves to ‘evanescent’ waves, which exist very close to an object’s surface. Evanescent waves can reveal details smaller than a wavelength, but they are hard to capture because they peter out so quickly. To amplify them so that they become detectable, scientists have resorted to using advanced man-made ‘metamaterials’ that bend sound and light in exotic ways.

Some acoustic metamaterials have been shown to guide and focus sounds waves to points that are much smaller than a wavelength in size. However, according to Geoffroy Lerosey, a physicist at the Langevin Institute of Waves and Images at the Graduate School of Industrial Physics and Chemistry in Paris (ESPCI ParisTech), no one has yet been able to focus sound beyond the diffraction limit away from a surface, in the ‘far field’. "Without being too enthusiastic, I can say [our work] is the first experimental demonstration of far-field focusing of sound that beats the diffraction limit," Lerosey says.

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