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.
