Using nonlinear theory as guidance, the expected (both reported and unreported) nonlinear response modes of a laminar diffusion flame to external acoustic excitation are investigated at different frequencies (6-100 Hz). The flame oscillating frequency is analyzed using digital signal and image processing techniques, while the hot gas dynamics are visualized by a high speed schlieren imaging system. The natural flame flickering frequency of the burner is 7.8 Hz. It has been observed that both the excitation frequency and amplitude play a role in the resultant dominant flame oscillation frequency. In this study, four nonlinear modes are identified, including frequency division, frequency doubling, sum/subtraction of the excitation and natural buoyancy frequency, and frequency amplitude increasing with enhanced excitation signal. From the schlieren images, it is found that all the nonlinear phenomena observed are due to the coupling between buoyancy and acoustic excitation near the nozzle exit field. The resultant change in hot gas flow structure and evolution affects the flame frequency behaviour subsequently. (C) 2012 Elsevier Ltd. All rights reserved.