I think that if the ipod has any Achilles heel, it's the middling battery life it has. Other things that are problems (easily scratched, poor stock earbuds) are relatively easily fixed. However, in reading through posted information about headphones, audiophiles talk about how one set "drives" a portable player better than another set.

From my limited understanding, this has to do with a headphone's characteristics, particularly its impedence (and also maybe the sensitivity?). What I've gathered is that higher impedence--> harder for a portable player to "drive" unless connected to an amplifier.

My question is, how much does this all affect battery life? Does anyone have a way to figure out how much difference in impedence affects the battery life (if it does), either formally (through some sort of math calculation) or informally (a gut-sense).

This would help in looking at headphones-- for example, I could look at the impedence between two comparable sets and factor that into the decision about picking one vs. the other.