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DucksVersusWombats t1_j4v2ooo wrote

But space is filled with gasses of varying degrees of rarefaction. How dense does gas have to be to propagate sound waves?

Space is filled with periodic and aperiodic events; can't some of them be interpreted as sound?

What frequency or amplitude of vibration in a gaseous medium do we decide isn't sound?

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ISvengali t1_j4v9inl wrote

That a good point, though the answer is rarely. Theres something like 1 molecule / cm3 (though places can be less and more)

Every now and then one of those atoms will get close enough and the tuning fork will knock it away making 'noise'. Presumably if one hits at the right time itll add energy also.

Both events are going to be fairly rare.

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yak-broker t1_j4yygy9 wrote

There isn't going to be an abrupt cutoff between sound and not-sound, of course. But there'll be a range of pressures where it stops being as useful to think of the phenomenon as "sound" and starts being more useful to think of it as a vibrating thing occasionally imparting more/less energy to individual gas molecules.

My semi-educated guess is that's around when the mean free path of the gas molecules stops being small compared to the wavelength of the sound. But that's just a guess.

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