It is generally known that a new pair of headphones must go through a "warming up(aka break-in)" phase for certain period of time before it opens up its true sonic potential.
According to Akihiko Hosaka, the senior manager at Samsung electronics Inc., who developed Samsung YA-EF310,
"The diaphragm of a headphone available in the current market is mostly made out of a polymer film. In this case, since this type of a headphone doesn't need a separate damper mechanism (such as a spider&suspension of a loudspeaker) implemented, I am quite certain breaking it in would have no effect at all.
However, manufacturers sometimes utilize a separate damper, which is made out of either rubber/urethane-related materials, to the driver of their high-end models. As this separate damper works, just like with a loudspeaker, above the glass transition temperature, its physical property might change slightly as a result of breaking-in. Should a break-in occur, it must be related to something happening at the resonant frequency."
Additionally, AKG's opinion on this break-in issue:
"...we cannot confirm that there is a burn in effect of the transducers taking place. Normally the sound of headphones changes only over many years and then mainly caused by the ear pads (less low end since the ear pads get more densely by sweat etc.). However, during the first hours of use of headphones, the ear pads - in the beginning a little stiff – start to accommodate to the users ears and head and the sealing becomes better, as a result the bass can be increased a little, on the other hand the distance between the headphones and the ear may become closer..."
from http://www.akg.com/forum/index.php/topic,1736.msg5270.html#msg5270
As far as I know, internationally, there have been quite a few attempts to verify the effect of a headphone break-in, 1 2 3 4 5 6, over the years. And it seems there's definitely a physical property change occuring, but its audibility still on debate.
Test methods
A brand-new SONY MDR-EX1000 is to be broken-in for 100 hours straight with XLO's break-in sample(100 dB SPL @ peak), driven with SONY NW-S639f with all of the tone controls turned off. Once the headphone is inserted in EURI's ear canals , its physical placement must remain untouched for the next 100 hours in order to prevent any placement-related deviations. Each 10 times averaged Pre-break-in & post-break-in measurement data are to be compared, and should any type of change occur, they are to be reproduced back utilizing a binaural recording method to be ABX-compared.
Test results
Impedance
Frequency response
Left channel:
Right channel:
Less than 1dB increment in the frequency response, and it is not under my system's measurement deviation.
Time-domain characteristics
The waterfall plots are actually two different responses of each channel superimposed on each other.
Spectrogram
Harmonic distortion
Subjective assessment
ATTENTION: In order to accurately reproduce these binaural recordings, listeners must use a flat diffuse-field equalized headphone, such as Etymotic Research ER-4B and STAX Lambda PRO with ED-1. The accuracy of the reproduced result can not be guaranteed otherwise.
Reference sample:
A brand-new MDR-EX1000:
A post 100hr broken-in MDR-EX1000
High quality sample: http://www.4shared.com/music/l1iijmBW/post_The_Lion_King_-_01_-_Char.html
Conclusion
As previously confirmed by others, 1 2 3 4 5 6, the physical effect of break-in is evident. Moreover, the psychoacoustic aspect of SONY MDR-EX1000's break-in has been demonstrated for ABX comparison so that listeners could verify the audibility of 100 hours of what-so-called 'warming-up'. Still, a radical sonic metamorphosing quality, of which audiophiles usually refer to, is nowhere to be seen.
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