Blowing across the top of an open, empty bottle or soda can produces a typical sound, formed when pressure waves are excited at a characteristic frequency. For an empty soda can, this so-called Helmholtz frequency is 420 Hz — just below that of a concert A.
Alexei Maznev and colleagues investigated an arrangement of 37 'hexagonally packed' empty soda cans, and showed that it could be used to focus sound. The authors drove six loudspeakers, arranged around this metamaterial in a symmetric way, at fixed frequencies in the range 360–430 Hz. They then measured the acoustic intensity profile just above the cans by means of a suspended movable microphone. Near the Helmholtz resonance frequency, the peak at the centre of the array narrowed down to 2 cm, around 1/40 of the sound wavelength.
Although this result suggests subwavelength focusing or resolution beyond the diffraction limit, the authors point out that this is not the case — such narrow intensity profiles cannot be obtained between 'lattice points'. But the study does make one think about the meaning of the diffraction limit in a periodic (meta)material.
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Verberck, B. Sound cans. Nature Phys 11, 377 (2015). https://doi.org/10.1038/nphys3333
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DOI: https://doi.org/10.1038/nphys3333
