Since the 1950s, physicists have known that a very disordered material can trap electrons. This is because of the electron's wave behaviour. When the degree of disorder is sufficient, the electron waves can become 'localized' in a single spot.
This trapping behaviour is complex, but John Page at the University of Manitoba in Winnipeg, Canada, and his colleagues have created an analogous but simpler system for sound waves. They fused small aluminium ball-bearings together and, using ultrasound, were able to observe sound waves become trapped in the disordered aluminium structures. They believe that their work may improve the understanding of localization, which could have applications in electronics.
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Condensed-matter physics: Sound trap. Nature 455, 1152 (2008). https://doi.org/10.1038/4551152e