Due to the large fluctuations in sound field parameters in systems with self-excited oscillation sources, a simple determination of eigenmodes involving measurement of local amplitudes and phases did not yield any meaningful results. For this reason, a special evaluation process based on correlation metrology was developed. In simple terms, this involves placing a microphone at a reference point and, with a second microphone, defining the amplitude and phase relationship within a frequency range. This evaluation process and an additional averaging process in the frequency range achieve outstanding results.