TITLE: Why does fire make very little sound?
QUESTION [82 upvotes]: Sound is air particles vibrating (thus hitting each other to make longitudinal waves) and heat is the vibration of air molecules. Because we can only assume that heat made from fire is a higher intensity of vibration than sound (because we don't burn ourselves when we speak) why doesn't it make an extremely loud sound?

REPLY [3 votes]: Apart from Chair's assertion that the air temperature would have to be enormous before the associated Brownian motion of the air molcules could be heard, it is important to note that the heat doesn't reach your ear at all. We only hear oscillating air in the ear proper; we cannot hear oscillating air far away, directly. We can see a glowing piece of metal; we can feel the infrared; but we are not exposed to air at 1000K.
That is obviously a good thing.
Why sound — a more or less oganized oscillation of air molecules — carries over long distances while the chaotic Brownian motion carries comparatively badly is a different question (and one I cannot immediately answer). But it's true for most materials, for some of them in an astonishing fashion: You can use a blowtorch on a ceramic heat shield and touch it on the other side; but its likely that banging at it with a hammer would carry through the whole space shuttle.
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