The name singing flames was given to this apparatus because when the flame is burning and the length of the flame and the position of the burner in the pipe are properly regulated, a pure sound is obtained and the flame vibrates with the same frequency as the sound emitted; sometimes the sound is so loud and the vibrations of the flame so violent that the flame is extinguished.
On analyzing the sound emitted we find that it is the same that the pipe would emit in absence of the burner and with the air inside it at the same temperature.
If we start from the situation in which the flame is quite short and sings spontaneously, and then slowly increase the flow of gas, the flame lengthens regularly and arrives at a situation of unstable equilibrium in which the flame sings when a sound of sufficient intensity and with the same frequency as the one the pipe emits spontaneously is introduced in the environment.
If sounds different from the pipe's characteristic sound are introduced, singly or together, the flame remains uninfluenced no matter what their intensity; if together with these sounds we also introduce a sound in unison with the one characteristic of the pipe the flame begins to vibrate and, if the intensity is sufficiently high, the pipe emits only that sound.
In the first experiment we demonstrate the phenomenon of resonance in an open pipe and in the second the general property of a resonating body to decompose a complex sound coming into contact with it into its components, responding only to the one whose frequency is equal to the frequency of the vibration of that body.