The acoustic phonons of the epsilon polymorph of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazatetracyclo[5.5.0.05,9.03,11]dodecane(ε-CL-20)have been studied using Brillouin scattering spectroscopy. Analysis of the acoustic phonon velocities allowed determination of the complete stiffness te...