Localization of ultrasound in a three-dimensional elastic network

After exactly half a century of Anderson localization(1), the subject is more alive than ever. Direct observation of Anderson localization of electrons was always hampered by interactions and finite temperatures. Yet, many theoretical breakthroughs were made, highlighted by finite-size scaling(2), the self-consistent theory(3) and the numerical solution of the Anderson tight-binding model(4,5). Theoretical understanding is based on simplified models or approximations and comparison with experiment is crucial. Despite a wealth of new experimental data, with microwaves and light(6-12), ultrasound(13) and cold atoms(14-16), many questions remain, especially for three dimensions. Here, we report the first observation of sound localization in a random three-dimensional elastic network. We study the time-dependent transmission below the mobility edge, and report 'transverse localization' in three dimensions, which has never been observed previously with any wave. The data are well described by the self-consistent theory of localization. The transmission reveals non-Gaussian statistics, consistent with theoretical predictions.