I’ve often seen space criticized for having audio foley effects since there’s no sound in space due to the lack of atmosphere to carry sound waves. But while watching the final battle in Serenity the other night, which takes place high above a planet, I got to wondering how high up does one need to go before the lack of an atmosphere means the battle would be silent to an outside observer?
Surprisingly high. Space is generally considered to start at a height of 100 kilometres, but apparently the point at which there is no longer sound transmission is at about 160 kilometres (Anacoustic zone).
Personally, I think it would break the drama a lot to have a space battle in silence.
Bear in mind that even in a regular dogfight in the earth’s atmosphere, the distances between combatants are usually vast, the planes may be moving at supersonic speeds, and there is no position you could be where you would hear all of the weapons and explosions clearly (let alone what’s being spoken by the pilots). Why suddenly the accuracy when it comes to space battles?
Having said that, it seems like it would be interesting to model some of the sound changes of a battle high in the earth’s atmosphere, as pressure, temperature and density changes have a number of complex effects on how sound is transmitted (link).
If there were really pilots dogfighting in space, I imagine that their headsets would have computer-generated 3D audio of the action outside the ship. For example, the computer could create a back-to-front woooooSSSHHHH as an external ship was approaching from the rear. If it sees an explosion in space, it could approximate the sound it would have made in the atmosphere. Movies do this with surround sound. Computer games do the same 3D audio today to make the game sound effects have the proper position. So in sci-fi movies, I assume I’m hearing the same 3D computer generated audio that the pilots are hearing in their ship.
In current aerial battles the only thing the pilot hears are his computers, engine(s), radio traffic and his own grunting and breathing. Missiles make a very short Whoosh as they leave, bombs make a loud Clunk, and the other aircraft whirling about are totally silent.
Bombs exploding on impact are also silent. A SAM victim or dogfight loser probably won’t live long enough to hear the explosion that kills him. Maybe just barely. The winner certainly won’t hear it.
And that’s at sea level up to 30,000ish feet. Going higher it’s only going to get quieter, not that that’s going to affect perceptions much.
To be sure, aerial combat at middle altitudes is noisy as heard by somebody on the ground not in a noisy buttoned-up tank. As are bomb explosions.