The laminar diffusion flame response to external acoustic excitation is investigated at low frequencies. Two typical responses are observed: the flame oscillates either at the same or half the excitation frequency. The flame and vortex dynamics of the two typical cases are visualized by high-speed schlieren and stereo images. A "vortex merging" phenomenon is observed and is responsible for the flame flickering frequency reduction to half. The 3D flame structures in the two cases are reconstructed based on stereo images. The 3D flame topology shows great variances from different view angles, which indicates the necessity of 3D visualization.