In most cases of self-excited combustion instabilities, the feedback mechanism is acoustic in nature. For this reason, it is very important to understand the causes which give rise to the occurrence of self-excited combustion oscillations and this involves first taking a closer look at the acoustic properties of the combustion system. Of primary interest here are the possible acoustic resonance frequencies (eigenfrequencies) and the associated eigenmodes of pressure oscillation and sound particle velocity. Eigenmodes are defined as the standing waves associated with pressure oscillation and sound particle velocity and present under resonance conditions.

The following illustration shows an example of a simple tube, closed at its left end and open at its right end, and showing the 1st, 2nd and 3rd corresponding harmonic eigenmodes for sound pressure in a longitudinal direction. The final conditions are idealized here, showing a "rigid end" on the left and an "open end" on the right.

In contrast to this simple example, eigenmodes look very different in a real combustion system due to changes in cross section, the influence of flow conditions, changes in temperature in the combustion zone and the complex parameters associated with inlet and outlet apertures in the system.