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I don't know the technicalities...it's the rule. Aviation is run by rules ...that's why it is so safe.
i once asked a stewardess when she asked me to remove my earplugs during take off (music keeps me calm so this was not helpful). she said that i wont be able to listen to safety announcement with them in. guess keith is right - its just one of those rules. sometimes they are better left unquestioned.
am flying the day after tomorrow (or not - depends on the weather) and wish i could just blast myself with music during take off but will instead do my breathing