There are 3N = 12 bands distributed into 3 acoustic modes and 9 optical modes. The 3 acoustic modes start from zero energy (ω = 0) at the  point, center of the Brillouin Zone, up to a few terahertz. They correspond to the lattice rigid translation modes of the crystal (two transverse and one longitudinal). Then 9 bands can be counted to the highest frequency above 40 THz. The diamond phonon band structure shown in Fig. stresses furthermore the similarity with the frequency magnitude of ω ~ 40 THz also observed experimentally by Raman spectroscopy .