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?