generated by the microphone 103 which is of course directly indicative of the aggregate level of sounds having frequencies over about 12 khz generated within the envelope of the flame 15 and constantly adjusts the air control fuel control and flue gas control valve assemblies 45 58 and 83 in order to maintain optimality at all points along the firing rate of the burner 3 which firing rate varies in response to the heat demand that the furnace assembly 5 is subjected to fig4 is a schematic diagram of a manually operated alternate embodiment of the control system 1 of the invention in this embodiment the output of the filter amplifier 109 is electrically connected to a monitoring mechanism 110 that monitors the output of the sound intensity detected by the acoustical sensor 101 so that it can be compared to empirically derived sound intensities associated with optimality to this end the output of the monitoring mechanism is connected to a chart recorder 112 that records the sound intensities detected by the sensor 101 over time this embodiment preferably also includes a comparator circuit 114 for continuously and automatically comparing the detected sound intensities with the optimal sound intensities and an alarm circuit 115 for informing the system operator when the air and fuel valve assemblies 45 and 58 and flue gas control valve assembly 83 are.