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Coke cans used to build sound-wave 'superlens'

French boffins in tinny acoustic focus triumph

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Focused sound waves aren’t just the domain of children fooling around with long-distance microphones. They’re also important in ultrasound machines, and in biomedical laboratories, for “acoustic actuators” using sound to sort cells.

Sound focusing has its limits, however – the diffraction limit, roughly one wavelength of the sound being focused. A 20 KHz tone has a wavelength of around 1.7cm in air, which is one of the reasons that ultrasound pictures look fuzzy and indistinct to the untrained eye. To improve the focus beyond the diffraction limit, you need expensive acoustic lenses – or an array of Coca-Cola cans.

In an experiment reported at Nature News, a French researcher has built a “superlens” out of a 7x7 array of soft drink cans with the tabs removed.

Coke can array focuses sound. Image: Fabrice Lemoult

The acoustic lens relies on a phenomenon called “evanescent waves” produced by the diffraction of the incoming tone. Those evanescent waves can be focused much more tightly than the original sound – according to researchers Geoffroy Lerosey, Fabrice Lemoult and Mathias Fink of the Institut Langevin in Paris, the focus can be as tight as 1/12th of the original tone’s wavelength.

To fine-tune the focus, the researchers used eight speakers surrounding the array, and recorded 49 pressure distributions of the waves emerging from it. The recorded sounds were then played back through the array backwards to identify where waves were reinforced and where they cancelled out.

This yielded a focus of one-quarter the diffraction limit. Equalization to amplify sounds lost in the array made the focus even more sensitive – down to around 1/12th the original wavelength.

The experimental setup worked at the 420 Hz resonant frequency of the Coke cans, but El Reg presumes the array could be scaled down to work at frequencies more useful to commercial and scientific applications.

The research will be published in Physical Review Letters. ®

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