Extreme stuntman Felix Baumgartner plans to free-fall from more than 120,000 feet next week, becoming the first skydiver to break the sound barrier. If you’re free-falling at supersonic speeds and you shout, can you hear your own voice?
Sort of. Baumgartner will wear a helmet during his dive, creating a pocket of air around his mouth and ears. Since the air and his body will be moving at the same speed, he will be able to speak and hear normally. In effect, it will be just like riding the old Concorde transatlantic jet, on which passengers easily held casual conversations.
What if Baumgartner were to jump without a helmet? Hearing his own voice in this hypothetical situation would run into some serious practical complications. First, the rush of air past Baumgartner’s ears would be so loud that it would drown out all other sounds. In addition, Baumgartner probably couldn’t open his mouth without causing severe physical injury. However, if he were to engage his vocal chords without opening his lips, and we could somehow eliminate the noise of the rushing air, and his ears weren’t ahead of his vocal chords in his direction of motion, some small fraction of the noise emanating from his throat would likely bounce off of his eardrums. It’s an admittedly messy answer, lacking the elegant beauty of Einstein’s “What would happen if I were riding on a beam of light, and I looked in a mirror?”
One thing Baumgartner certainly won’t hear, no matter what he wears, is his own sonic boom. When an object surpasses the speed of sound, the sound waves that had previously moved outward from the object in all directions are concentrated into a Mach cone that creates an enormous thump of noise. Since that cone could never reach a supersonic skydiver’s ear, he would be oblivious to the boom.
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Explainer thanks Victor Sparrow of the Pennsylvania State University.