Acoustic waves in gases, liquids, and solids usually travel at an almost constant speed of sound. So-called rotons are an exception: their speed of sound changes significantly with the wavelength, and it is also possible that the waves travel backward. Researchers at Karlsruhe Institute of Technology (KIT) are studying the possibilities of using rotons in artificial materials. These computer-designed metamaterials, produced by ultra-precise 3D laser printing, might be used in the future to manipulate or direct sound in ways that have never been possible before. A report on the researchers’ work has been published in Nature Communications.