Most of my audiophile friends believe that headphones (and speakers and electronics) sound better after the first 100 hours of use than they do when they're brand-new. When I'm doing high-end product reviews I leave the "burn-in, break-in" question up to the manufacturer. If the company's reps claim their product won't sound its best until it has a solid month of use, I'll request a unit with enough hours on it that I can start working on the review right away. If the manufacturer scoffs at the very idea of burn-in, I start my serious listening immediately.
Audiophiles don't agree on exactly how long headphone break-in should take, and opinions range from 10 hours to many hundreds of hours. AKG's K 701 full-size headphones are "notorious" for sounding lifeless straight out of the box. The word on the audiophile street is they need 300 hours of break-in.
I believe headphones' sound "matures" over time, and I recently had the chance to compare a brand-new set of Etymotic ER-4PT in-ear headphones with my 10-year-old ER-4Ps. I felt the older set was "slightly more 'relaxed' and more laid-back in its tonal balance." The two models have identical specifications, and yet they sounded different. So beyond the burn-in question, maybe headphones "wear" over time?
I called upon a local (Brooklyn, N.Y.) headphone manufacturer, Grado Labs' John Grado, to weigh in about burn-in, and he said, "All mechanical things need break-in." He did not recommend leaving headphones playing continuously for a few days to hasten the process. He recommends using new headphones as you normally would, and after 50 hours or so the sound will be all it can be.
I imagine some of you must be wondering if anyone has ever tested and measured the effects of headphone burn-in, and luckily enough Inner Fidelity's Tyll Hertsens has done just that. Better yet, he measured a brand-new AKG 701 (specifically, it was the Quincy Jones Q701), and that's the model that so many audiophiles cite as notorious for its need for burn-in. So Hertsens measured them, starting after they had 5 minutes of use; then 25 minutes; 1 hour; 2 hours; 5 hours; 10 hours; 20 hours; 40 hours; 65 hours; and finally at the 90-hour mark.
I'll cut to the chase: Hertsens definitely found small changes in the AKG headphones' measured frequency response, and you can see evidence of that in the many graphs in his article. Even so, when I talked with Hertsens after the article was published he still had major reservations about the burn-in question. He thinks that some aspects of burn-in can be attributed to owners getting used to the sound of their new headphones, and that makes sense to me. Measurements are, as always, open to interpretation. Hertsens measures headphones in his reviews, and I had a million questions about how he does that. There's a lot more to learn about Inner Fidelity's mission, and I'll cover that in greater detail in another post soon.