Wouldn't that be the resonant frequencies of the tube?
Not the fire itself? Because you are creating standing waves in the tube..which is one environment which the fire (gas) is reacting to..bigger tube, longer tube, different wavelength and different standing waves...
Cool none the less //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif