The dynamic response of a turbulent premixed flame to acoustic velocity perturbations was experimentally determined in a swirl-stabilized lean-premixed gas turbine combustor. $CH^*$ chemiluminescence intensity and the two-microphone method were used to measure heat release rates and inlet velocity fluctuations, respectively. Using the n-$ au$ formulation, gain and phase of flame transfer functions were incorporated into an analytic thermoacoustic model to predict instability frequencies and modal structures. Self-excited instability measurements were performed to verify eigenfrequencies predicted by the thermoacoustic model. Instability frequency predicted by the model is supported by experimental results. Results show that the self-excited instability frequency of ~220 Hz results from the fact that the flames amplify flow perturbations with f=150~250 Hz. The other instability frequency of ~350 Hz occurs because the whole combustion system has an eigenfrequency corresponding to the $^1/_4$-wave eigenmode of the mixing section.