The objective of this investigation in a laboratory combustion system was to understand why a premixed propane flame is acoustically excited with particular acoustic boundary conditions, in particular regions of the frequency spectrum. It was demonstrated that the phase of the heat release at various heights in the flame relative to the burner exit velocity indicated a disturbance with a time-delay related to convection by the mean flow from the moment that it left the burner mouth. This gave rise to both positive and negative contributions to acoustic energy flux (the Rayleigh integral) from different heights in the flame, in some cases over as many as seven cycles. Thus, some thermo-acoustic eigen-frequencies were not strongly excited, even when local contributions were high. Conversely, the coincidence of a Helmholtz resonance (low acoustic pressure fluctuations associated with high acoustic velocity fluctuations) in the premixture supply system with a combustion chamber quarter-wave resonance (high acoustic pressures) made it impossible to stabilise the flame.