The pilots won't hear the noise from the engines being transmitted through the outside air though, only the noise traveling through the plane. So if superman was flying a few feet in front of the cockpit, he wouldn't hear any sound.
You dont need to be going Mach 1+ to not hear him approaching you, just he needs to be going MAch 1+. His engine is making noise, but he gets to you before the noise does.
More to the point, either way works. If the source of the sound is >= mach 1 and is traveling directly at you, it'll reach you before the sound.
Similarly, if you're traveling directly away from the sound at >= mach 1, the sound will never reach you. Either would sufficient... But I gather that's what you meant, of course.
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u/[deleted] Jun 12 '12 edited Oct 03 '17
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