By superimposing two sound waves of the same wavelength, propagating in the opposite direction, we can create an intensity pattern having a characteristic scale equal to half a wavelength: it is the diffraction limit. Recently a group from the Institut Laue-Langevin in Paris1 has shown that it is possible to go beyond this limit by focusing sound waves on a spot whose size is much shorter than half of a wavelength. We explain here how to reproduce, at an elementary level, this experiment that illustrates fundamental properties of waves.
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