![]() There is some debate among acoustic scientists as to whether this is the first time anyone has truly beaten the acoustic diffraction limit. The results are due to be published in Physical Review Letters. 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. 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. Normal waves scatter efficiently, so they disappear quickly. ![]() Thanks to a quirk of wave physics, the resultant waveform cancels out the resonance patterns everywhere-except above the chosen can.Īfter the playback, the can continues to resonate by itself, scattering out the sound energy left inside. 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. To focus the sound, the trick is to capture these waves at any point on the array. 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. 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. 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. Lerosey and his colleagues took a similar approach to an experiment they performed in 2007 and later described theoretically for electromagnetic waves. "Without being too enthusiastic, I can say is the first experimental demonstration of far-field focusing of sound that beats the diffraction limit," Lerosey says. 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'. Some acoustic metamaterials have been shown to guide and focus sounds waves to points that are much smaller than a wavelength in size. 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. Evanescent waves can reveal details smaller than a wavelength, but they are hard to capture because they peter out so quickly. 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. Scientists have attempted to overcome the acoustic diffraction limit before, but not using such everyday apparatus. 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. ![]() Try to focus it to a point much smaller than one wavelength and the waves bend uncontrollably-a phenomenon known as the diffraction limit. Sound, like light, can be tricky to manipulate on small scales.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |