Acoustic pressure fluctuations within the combustion chamber or the plenums of a liquid-rocket engine can induce temporal modulations of the propellants injection velocities. The dynamic response of a tran-scritical coaxial flame to such velocity modulations is here investigated as it is thought to be one of the mechanisms that can promote high-frequency combustion instabilities. A low-order model for this process is proposed, assuming that the modulation induces fluctuating flame stretch rates traveling along the flame front. This question is then addressed through Large-Eddy Simulation (LES). The annular fuel stream of a single element LOx/GCH 4 injector is acoustically modulated at several frequencies. The proposed model is in reasonable agreement with the flame response extracted from the LES in terms of both gain and phase.