Novel materials: Sound waves traveling backwards

Acoustic waves in gases, liquids, and solids usually travel at an almost constant speed of sound. Rotons are an exception: their speed of sound changes significantly with the wavelength, it is also possible that the waves travel backwards. Researchers are studying the possibilities of using rotons in artificial materials. These metamaterials might be used in the future to manipulate or direct sound in ways that have never been possible before.

source https://www.sciencedaily.com/releases/2021/06/210610135559.htm