In order to carry out relevant stability analysis of any combustion device, its acoustic boundary conditions must be clearly defined and precisely determined. This determination is usually performed in separate dedicated experiments in forced flow configurations. An alternative method is proposed in the present paper to circumvent this step by measuring on the fly the fuel and air feeding line impedances during self-sustained combustion oscillations. This is only possible when the flame oscillation provides enough coherent acoustic energy to properly analyze the signal. It is shown that, in order to estimate the reflection coefficient of the feeding lines, the Mach flow number should be taken into account because of the relatively high velocities encountered in these lines, but that viscosity effects may be neglected. Results also show that the reflection coefficient of the feeding lines may largely deviates from purely reflecting or non-reflecting conditions, but can be estimated with confidence, provided the impedance of the mass flow controller at the system boundary is known.