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I am curious about the properties of sound at low atmospheric pressure. Not the speed of sound, I want to know how lower pressure will affect the distance sound will carry and the frequency range. For example, I know that sound does exist on mars at around 600pa pressure, but travels only a very short distance. Are there any mathematical models for this?

I ask because I have constructed a vacuum chamber for various experiments with a pump rated down to 5pa. At maximum vacuum(I have no way to tell how close to 5pa it is except that water at 0C boils, so below 600pa) sound can be heard through the chamber but only lower frequencies. Above 7khz it is silent, while I can hear 16khz with no vacuum. I wonder if the low pressure is filtering out the high frequency, or is all direct sound transmission lost and the low frequencies are being physically transmitted through the chamber floor?

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    $\begingroup$ You didn't show your setup or how you mounted noise-generating device (e.g., bell, audio speaker) in your vacuum chamber, but I would suspect that the low frequencies are being coupled through parts of the vacuum chamber rather than going through the rarefied air in the vacuum chamber. I say this because I don't believe that there is any reason why there should be a strong frequency dependence to the transmission of sound through rarefied air. $\endgroup$ – Samuel Weir Dec 18 '15 at 21:14
  • $\begingroup$ Thanks for your answer. The chamber is still a diy work in progress, so I am using sheetmetal for the floor of the chamber through which the vacuum fitting attaches. A jar sits on top of the sheet with wires to a speaker running under the edge, inside the chamber. I am currently using a thick putty to temporarily seal the edge of the jar to the floor. I think the metal floor is conducting and amplifying the sound, I can feel the vibration of the bass in the sheetmetal. $\endgroup$ – platatomi Dec 19 '15 at 5:36
  • $\begingroup$ It's probably vibrations getting through whatever solid mounts are holding the speaker. You might give some thought as to how to minimize that as much as possible by using thin wires, sound dampers, etc. $\endgroup$ – Samuel Weir Dec 19 '15 at 20:36
  • $\begingroup$ I did try suspending the speaker by wire and wrapping it in foam to isolate the vibrations, but I can still hear the lower frequencies, albeit very faintly. Somewhere I read that the theoretical pressure limit for sound transmission is when the mean free path exceeds the wavelength of the sound. I have no idea if that is true or not. That would explain why only the higher frequencies drop out. But you believe that at pressures possibly under 100pa that no discernable sound propagation should occur? $\endgroup$ – platatomi Dec 19 '15 at 21:04
  • $\begingroup$ Hard to say without hearing what your system sounds like as it is being pumped down. There's a system similar to your setup at the Exploratorium (exs.exploratorium.edu/exhibits/no-sound-through-empty-space) I recall that the sound intensity dropped off dramatically as it was pumped down to roughing-pump type vacuums, which seems similar to your vacuum. $\endgroup$ – Samuel Weir Dec 19 '15 at 22:57

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