We fabricate e-beam lithographied two-dimensional lattices of nanometric metal cubes of various sizes and lattice parameters. Using ultrafast laser acoustics, we time resolve two kinds of vibrations in such crystals: individual cube vibrations and lower frequencies strongly dependent on the lattice parameter. From complementary experiments and an analytical model, we assign these modes to acoustic collective vibrations (i.e., acoustic phonons) of the artificial crystal. Their unprecedented detection takes advantage of another periodicity effect acting on the optical field.