Till what altitude above earth sounds can be heard?

Question

In the new video of SpaceX SAOCOM 1B launch and landing RCS thrusters and other sounds can be heard during boostback burn. Falcon 9 boostback happens at nearly 100 Km altitude. Does the air density at that altitude allow normal propagation of sound or was the footage audio enhanced?

Answer 1

The title of the question asks Till what altitude above earth sounds can be heard? @uhoh gave a detailed answer to that question. I'll instead speculatively answer an implied question in the body of the OP, What is the sound in the linked video?

The OP implicitly assumes the sounds in the video were transmitted through the air to a microphone. (Many commenters at a Reddit thread make a similar assumption.)

A couple of things to note from the linked video:

• The speedup is not constant. The linked video is 2 minutes and 19 seconds long. Stage 1 landed at about 8 minutes and 7 seconds into the flight. If the speedup was constant, that would have meant MECO should have occurred about 40 seconds into the video. MECO instead occurs about 12 seconds into the video.

• The sound remains more or less the same for the first 12 seconds of the linked video. There is little change at 5 seconds into the video, which is when the vehicle went supersonic.

Back when the Concorde flew, passengers oftentimes remarked how quiet the aircraft suddenly became on going supersonic. While the passengers could still feel the rumble of the jet's engines, they could no longer hear the massive sounds emitted by the engines' exhaust. The sound of the exhaust would only have been audible behind the aircraft. The aircraft left the sound behind.

That the sound did not suddenly drop 5 seconds into the video suggests that the sounds were not recorded by microphones in the air. They sound instead most likely comes from vibration sensors such as accelerometers designed to be sensitive to vibrations or microphones "listening" to the launch vehicle itself.

In addition to sounding cool, the recorded vibration data would be very useful to SpaceX engineers. Engineers perform stability and controllability analyses with regard to a launch vehicle's control system with regard to the control system itself and with regard to how flexing of the vehicle interacts with the control system. These analyses also need to address sloshing of liquids in the tanks for those vehicles that use liquid propellants.

The key problems are that excessive flexing or sloshing can make the control system behave in a very bad way if there are overlaps between the flex, slosh, and control frequencies, and that the control system can similarly excite excessive flexing or sloshing in a very bad way if such overlaps occur. Flex and slosh can excite one another if their frequency responses overlap. Vehicle flex can be reduced / changed in frequency by adding stiffeners, and tank slosh can be reduced / changed in frequency by adding baffles to the tanks. But if these are unneeded, the stiffeners and baffles are just excess weight that reduces payload mass.

Engineers use multiple models to estimate flex and slosh modes, but in the end, these are just models. "All models are wrong, but some are useful." Having actual measurements of vehicle vibration during launch would be very beneficial toward validating and refining these models.

Answer 2

tl;dr: There's certainly some propagation of sound waves possible at 100 km altitude. With a density a million times lower than at the surface the mean free path of individual molecules will approach a millimeter, so ultrasonics might be impacted, but for Human or GoPro frequencies it will be much quieter, but still there.

Till what altitude above earth sounds can be heard?

There is no single altitude at which sounds can suddenly not be heard. There is a steady drop-off in sound pressure with atmospheric pressure, and the drop-off accelerates when the mean free path approaches the wavelength of a particular sound, but these are smooth transitions.

Does the air density at that altitude allow normal propagation of sound or was the footage audio enhanced?

I'm not sure what "normal propagation" means.

The volume of transmitted sound steadily decreases as density decreases, the same way that it gets even louder under water (+61 dB!) But at some point when the mean free path (only several microns for a standard atmosphere) begins to approach the wavelength of the sound, then the drop off changes to exponential as propagation becomes evanescent.

This is explained in great detail in @ honeste_vivere's excellent answer to At what altitude would the air be too thin to carry a sound wave? I'll quote the last bit here:

Answer 2

The model only went to 100 km but even so, our source would become difficult to hear if we moved a little more than ~100 m from it. Given that the density decreases exponentially with an e-folding distance of only ~8.5 km (pressure does so similarly as well), if we extrapolate our estimates for $L_{i,src}\left( h \right)$ then the value drops to ~10 dB by ~177 km.

So by ~200 km a human probably could not hear a source ~1 m away that produced a 100 dB, 1000 Hz intensity level at sea level.

See also:

• NASA JPL's LDSD: Supersonic Test Flight (HD) which includes a GoPro camera's microphone recording small solid rockets at different altitudes, as linked in in How did the tangential thrusters for the 2014 LDSD test spin-up then spin-down so nicely?

Also check answers to

• Where does the noise on spacewalks come from?
• What would sounds on Mars be like?
• Can astronauts hear sounds during space walks for repairs?

From the last one:

According to WIRED Magazine's article and video Watch Astronauts Answer Your Burning Questions about Space (also viewable on YouTube):

Sounds exist in space, but humans can not hear them.