Proper Orthogonal Decomposition method was used to decompose the limit cycle pressure fluctuation measured using an array of 15 microphones, the pressure of the first 3 eigenmodes and acoustic modes were reconstructed.

The first 3 eigenmodes contributed more than 99% of total fluctuation energy. The dominant mode (half sine wave) contributed 85–95%, depending on the heater location. The eigenmode resembled a full sine wave contributed 13–2%, and the other eigenmodes contribute approximately 2–3%.

Three types of fluctuations were found. The types I and III fluctuations resembled the Fourier modes 1 and 2 of the pressure fluctuations in a unheated tube.

Type I fluctuations appeared to cause a second type of fluctuation with a fixed phase lag to the type I fluctuation. The phase lag between types I and III varied with the heater position suggesting type III was not a direct consequence of the type I fluctuation.