A sonic crystal is a metamaterial made of a periodic arrangement of rigid or soft cylinders in air (or vice versa). When the acoustic wavelength is comparable to the lattice constant, Bragg scattering creates destructive interference — a phononic bandgap. Below the bandgap: waves pass. In the gap: evanescent decay. Discovery by Kushwaha et al. (1993) and experimentally demonstrated with marble cylinders by Martinez-Sala et al. (1995, "Music and Science").