The impingement flowfield in a ducted combustor with one axial inlet and dual side inlets was characterized qualitatively by smoke flow visualization as well as quantitatively by laser Doppler velocimetry and microphone measured velocity and wall pressure spectral characteristics, respectively. The baseline momentum ratio MR of the axial inlet to side inlets was chosen to be 0.27. In this case, a resonant feedback frequency of 365 Hz, which prevailed in the entire combustor, and the second and the third harmonics, which appeared near the impingement region, were observed. The effect of MR on the flow oscillations was further studied, and the results showed that a constant Strouhal number, based on the axial inlet velocity and width, of 0.013 was found for MR < 0.94, and Reynolds number of 103-104 and the frequency of the oscillations varied linearly with the MR. From the investigation of the measured spectra, the feedback mechanism with appropriately proposed feedback distance and convective velocity, and of the chamber resonance curve, the onset of the oscillations was interpreted to be triggered by the coupling between the disturbance feedback frequency and the acoustic frequency of the chamber.