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

Soft-Drink Cans Focus Sound Waves to a Point, Beating Diffraction Limit: Scientific American: The group generated audible sound from a ring of computer speakers surrounding the acoustic 'lens': a seven-by-seven array of empty soft-drink cans. Because air is free to move inside and around the cans, they oscillate together like joined-up organ pipes, generating a cacophony of resonance patterns. Crucially, many of the resonances emanate from the can openings, which are much smaller than the wavelength of the sound wave, and so have a similar nature to evanescent waves.

To focus the sound, the trick is to capture these waves at any point on the array. For this, Lerosey and his team used a method known as time reversal: they recorded the sound above any one can in the resonating array, and then played the recording backwards through the speakers...

Normal waves scatter efficiently, so they disappear quickly. However, the evanescent-like waves are less efficient at scattering, and take roughly a second to make it out of the can--a prolonged emission that allows the build up of a narrow, focused spot...