A quiet tube with a heated grid inside. The voltage on the grid is suddenly raised, and the tube starts to hum, sustaining itself. This is not a malfunction but a transition to another stable state, like a ball rolling over a hill into a neighboring hollow.
Inside the tube, sound and heat amplify each other: air oscillations change the heating of the grid, and the heating drives the oscillations. With weak heating, the system is calm, but once the threshold is crossed, a self-sustaining hum emerges. In the experiment, the threshold was crossed not by slowly turning a knob but by a sharp jump in voltage.
Such a sharp transition is called tipping. It depends on the strength of the impact and on how hot the grid already is. Understanding this mechanism can help make technology safer: knowing in advance at what impact a system will slip into an undesirable regime.
The horizontal Rijke tube is the simplest thermal sound generator: a heated grid inside makes the air oscillate, and the tube sings like an organ.