There are 3N-3 optical modes at high energy and 3 acoustic modes. The two body-centered orthorhombic (Imm2) and tetragonal (I4m2) allotropes were accounted for using primitive cells with 12 carbon atoms, whence the larger number of bands versus trigonal C 6 (Fig. ). The acoustic modes start from zero energy (ω = 0) at the Γ 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 30 bands are expected in Figs. and, and 15 bands in Fig. . But knowing that the higher the symmetry, the more dispersion curves showing degeneracy are found for a given frequency, an apparent reduction in the number of dispersion curves is observed. In the three panels the energy range is the same for all three phases, i.e. from 0 to 40 THz, stressing furthermore their similitude with the magnitude observed for diamond by Raman spectroscopy: ω ~ 40 THz .