# What is the lowest density necessary for sound to propagte through? **URL:** \ **Category:** Factual Questions\ **Created:** [July 30, 2011, 4:14pm UTC](https://boards.straightdope.com/t/what-is-the-lowest-density-necessary-for-sound-to-propagte-through/590836 "2011-07-30T16:14:51Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [July 30, 2011, 4:14pm UTC](https://boards.straightdope.com/t/what-is-the-lowest-density-necessary-for-sound-to-propagte-through/590836/1 "2011-07-30T16:14:51Z")
Title says it all. I know sound cannot travel in a vacuum. But even deep space is not an absolute vacuum ([roughly 40 atoms per cubic meter](http://en.wikipedia.org/wiki/Outer_space#cite_note-19)). I am guessing as density lowers higher frequency sound waves cannot pass but lower ones can. Even so, seems there would be some hard limit unless there is a low enough sound to pass through deep space (which would certainly be beyond human range to hear but could it be detected?). For this question I would be interested in both the lowest density necessary for an average human to be able to discern as well as a bottom floor overall. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [July 30, 2011, 4:18pm UTC](https://boards.straightdope.com/t/what-is-the-lowest-density-necessary-for-sound-to-propagte-through/590836/2 "2011-07-30T16:18:04Z")
> [@Whack-a-Mole](#): > > For this question I would be interested in both the lowest density necessary for an average human to be able to discern as well as a bottom floor overall. For the first part, you’ll have to state what distance you are from the source. The second part won’t have an answer, since there is no point where all of sudden pressure waves cease to exist. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [July 30, 2011, 4:41pm UTC](https://boards.straightdope.com/t/what-is-the-lowest-density-necessary-for-sound-to-propagte-through/590836/3 "2011-07-30T16:41:43Z")
> [@John\_Mace](#): > > For the first part, you’ll have to state what distance you are from the source. Good point. I did not consider attenuation. Assume 1 meter (no particular reason…just seems good for this). > [@](#): > > The second part won’t have an answer, since there is no point where all of sudden pressure waves cease to exist. Are you sure? At 40 atoms per cubic meter I am hard pressed to see how a pressure wave will manage to knock one atom into another and then again and again to keep a pressure wave going. I suppose in this case attenuation would be really, really fast. Your pressure wave will putter out in short order. ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [July 30, 2011, 4:47pm UTC](https://boards.straightdope.com/t/what-is-the-lowest-density-necessary-for-sound-to-propagte-through/590836/4 "2011-07-30T16:47:38Z")
On a microscopic level, sound waves are the coordinated motion of skillions of gas molecules/atoms. If there are more in one particular region of space than in another particular region, then the molecules will “want” to jostle around and move from the denser region to the rarified region. Set up your rarifications and compressions just right and you’ve got yourself a sound wave. Where this breaks down is when the gas gets so rarified (on average) that the frequency of the “jostlings” between the gas molecules is smaller than or comparable to the frequency of the wave you’re trying to put in. If the gas molecules aren’t bumping into each other often enough, then they can’t even out their energies quickly enough before the next peak or trough of the wave is supposed to come by. In other words, the gas’s pressure can’t change quickly enough to support a wave at that particular frequency. This frequency of molecular collisions depends on the pressure and temperature of the gas you’re trying to put a wave through. You can use [this calculator](http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/frecol.html) to estimate the collision frequency; however, as noted by **John Mace** , there’s not a hard-and-fast limit on the upper frequency. My gut feeling is that waves whose frequency was 1000 times less than the collision frequency would probably propagate just fine, 100 times less might be OK too, and 10 times less probably not. But really, I’m just spitballin’ with those numbers. ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [July 30, 2011, 4:51pm UTC](https://boards.straightdope.com/t/what-is-the-lowest-density-necessary-for-sound-to-propagte-through/590836/5 "2011-07-30T16:51:22Z")
> [@Whack-a-Mole](#): > > At 40 atoms per cubic meter I am hard pressed to see how a pressure wave will manage to knock one atom into another and then again and again to keep a pressure wave going. Note that ultra-low frequency waves will also have tremendously long wavelengths. Forty atoms per cubic meter is pretty rarified, but if your wavelength is on the order of thousands of kilometers (say), then your atoms are still going to bump into each other before they can travel a distance comparable to the wavelength. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [August 1, 2011, 12:20pm UTC](https://boards.straightdope.com/t/what-is-the-lowest-density-necessary-for-sound-to-propagte-through/590836/6 "2011-08-01T12:20:09Z")
> [@MikeS](#): > > Note that ultra-low frequency waves will also have tremendously long wavelengths. Forty atoms per cubic meter is pretty rarified, but if your wavelength is on the order of thousands of kilometers (say), then your atoms are still going to bump into each other before they can travel a distance comparable to the wavelength. How about a wavelength of [30,000 light-years?](http://www.timesonline.co.uk/tol/news/world/article1157687.ece)