Soda Cans Beat Acoustic Diffraction Limit

I love experiments such as this. It is one of those experiments that uses ordinary objects, but produces astounding results, AND, gets published in PRL, of all places!

We have heard about optical metamaterial and how such materials can become left-handed and also defeat the diffraction limit. Well now, an acoustic metamaterial made up of soft drink cans (link open for free only for a limited time) has been made, and it can defeat the acoustic diffraction limit!

Lerosey and his colleagues took a similar approach to an experiment they performed in 2007 and later described theoretically for electromagnetic waves^1,2. 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. Thanks to a quirk of wave physics, the resultant waveform cancels out the resonance patterns everywhere — except above the chosen can.

The work will be published in PRL (I'll put a reference to it here when it is published).

Edit: A review of this work can be found on PhysicsWorld website.

Zz.