Abstract: We use broadband picosecond acoustics to detect longitudinal acoustic phonons with few-gigahertz frequency in three-dimensional supracrystals (with face-centered cubic lattice) of 7 nm cobalt nanocrystal spheres. In full analogy with atomic crystals, where longitudinal acoustic phonons propagate with the speed of sound through coherent movements of atoms of the lattice out of their equilibrium positions, in these supracrystals atoms are replaced by (uncompressible) nanocrystals and atomic bonds by coating agents (carbon chains) that act like mechanical springs holding together the nanocrystals...