Schlieren optics allows for air movement to be seen. (Blue Dot)
the hot air flow above a candle.
We captured the movement of the Schlieren Optics the fan reacts to the movement of the schlieren optics. a lot of movement reduce the blowing force. little movement increase blow. the delay of the air traveling from the fan to the candle delays this infinite feedback loop.

The air movement also gets interpreted as sound effects.

by Adrien Kaeser, Mathilde Colson and Me