Re: why does a candle make noise when blown upon gently?

Hi Jim,

The short answer is that I'm not sure. When you blow *very* gently upon a candle, I notice that it flickers back and forth. The flame wants to rise straight up, but it pushed down by the air that you blow over it. My guess is that this competition causes back-and-forth oscillations, similar to those inside of a wind musical instrument (I can't find an on-line illustration of this, but one should be in any book on the physics of music), and that the sound might come from the back-and-forth motion.

The trouble is that when I tried it, I heard nothing from the candle when I blew very gently over it. The sound that I heard came when I blew almost, but not quite, hard enough to blow the flame out. I suspect that what is happening is that the flame is very briefly extinguished by the airflow, but re-ignites, and the sound that we hear is the repeated re-ignitions. This would be a great thing to look at with a slow-motion camera.

[note added by MadSci Admin: there is certainly turbulence that occurs when the flame is noisy. It may just be the high-frequency turbulence that is entirely responsible for the noise.]