Originally Posted by Bilavideo
I agree with both answers above and partly with the cynical dismissal crowd. If you have a diaphragm straight from the factory, there may well be something to the idea that the material is stiffer before it has been used for the first time and that a certain amount of use can loosen it up a little, enough to be noticed by nerds with notepads. Then again, the audio world is also soaking in hype and placebos.
Not all claims of burn-in have equal plausibility. Tell me that a stiff paper cone needs a little use to loosen up and I'm there. Expand that idea to tell me that I have to run so much juice through my wires to properly "burn them in," and I'm laughing till rootbeer comes out my nostrils.
If such a thing as burn-in occurs, most of it is psychological - but not because the burn-in believers are nuts. Far from it. Listening to music is not a purely objective experience. When I got my first pair of eyeglasses, I was surprised at how it took time for my brain to adjust. It was a reminder that perception isn't passive; it involves active participation on the part of the perceiver. An old friend of mine once went to a bluegrass festival. On day one, it all sounded the same; by day three, there was so much variety, she couldn't believe she'd ever missed it. We tune in.
It's a bit off the subject to argue the issues with headphone break-in in a discussion of IEMs with fixed-armature drivers. At the same time, it's hard to deny that there's a psychological element involved - and experience in using IEMs.
I thought I'd experienced break-in with a Westone IEM, only to realize that learning to properly insert the IEM is a critical part of the learning curve. The smallest variance from a proper fit would lead to changes in sound.
I'd even go so far, however, too say that smaller dynamic earphones and earbuds probably have insufficient mass to make much difference in break-in. People seem to start really noticing break-in with 50mm diaphragms in dynamic phones. And certainly, psychology plays something a role, too.