By 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.
Lerosey and his colleagues took a similar approach to an experiment they performed in 2007 and later described theoretically for electromagnetic waves. 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.
After the playback, the can continues to resonate by itself, scattering out the sound energy left inside. 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. In fact, Lerosey's group found that the focused spot could be as small as just 1/25th of one wavelength, way beyond the diffraction limit. The results are due to be published in Physical Review Letters.
Beyond the limit?
There is some debate among acoustic scientists as to whether this is the first time anyone has truly beaten the acoustic diffraction limit. Mechanical engineer Nicholas Fang at the Massachusetts Institute of Technology in Cambridge thinks that the results are a first because the focal point is away from the lens, in the far field. But John Page, a physicist at the University of Manitoba in Winnipeg, Canada, who has published evidence for sub-wavelength focusing in the near field, disagrees. "Super-resolution is super-resolution, no matter in what regime it is obtained," he says.
Still, Page calls the Lerosey group's work "a very important accomplishment" and believes it could find many applications, such as feeding energy to tiny electromechanical devices so they can operate.
Lerosey himself thinks that the simplicity of the apparatus is what bodes so well for applications. "To me, this experiment says, 'we can do it easily, even with Coke cans,' and it opens a door."
This article is reproduced with permission from the magazine Nature. The article was first published on July 8, 2011.