Acoustic vibrations of ionic crystal exposed to a short impact pulse are addressed. The study evaluates the contribution of the Coulomb bond energy of ionic crystals to the dependence of moduli of elasticity on stress field induced by RMS electromagnetic field in the absence of the field at the initial time point. Dynamic vibration equations are written using the Maxwell stress tensor defined by the second electromagnetic stress field moment. It is shown that non-equilibrium Poison's ratio in ionic crystals describes qualitatively the experimental results. Keywords: impact loading, electrostatic interaction, Maxwell tensor, relaxation processes, Poison's ratio.