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). But the number of explicit bands is subjected to the lattice symmetry, i.e., the higher the symmetry, the more dispersion curves showing degeneracy are found for a given frequency, and therefore an apparent reduction in the number of dispersion curves is observed. In all three panels the energy range is the same for the three phases, i.e. from 0 to 25 THz, stressing furthermore their similitude but with a much lower magnitude than observed for diamond by Raman spectroscopy: ω ~40 THz .