In my experience, getting a good seal has more to do with the shape and size of your head than with wearing glasses.
I am tempted to think that a pair of headphones that isn't properly designed to fit a wide range of heads is poorly designed
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Most headphones are just poorly designed in my opinion in very non-sensible, occasionally downright stupid ways (lack of resistance-less cup swivel, insufficient range of motion for example, improperly located yoke to cup attachment, etc... the list is endless).
I'll go one further and posit that a pair of headphones that is excessively sensitive to leakage also is poorly designed, particularly in 2022 when we have plenty of reasonable options to reduce that issue.
Could it be that temperature has something to do with it? (sitting on a human head makes the temperature increase and also actually playing audio increases temp.)
What can happen as far as temperature is concerned is that the pads warm up. Some pads' foam will then compress a little bit over a few minutes as it gets increasingly more supple with heat.
Using in-ear mics, I've seen that effect most pronounced on my pairs of Austrian Audio Hi-X65. They have very slim, memory foam pads. When measured after wearing them for a dozen minutes there are small differences vs. cold that roughly correspond to how the FR varies with pad compression. While for me these headphones seal very consistently in all situations, even when the pads are cold, since they behave in a very non-linear way under pad compression, this alters the sound colouration a little bit (so it isn't just a question of leakage).
Conversely on a lot of other headphones I have not measured any difference whatsoever.
No idea how DCA headphones behave in that regard.
Otherwise I would certainly hope that headphones are designed to minimise the effect of humidity / temperature across a sensible range of possible, real-life values. Just like the leakage question above, I'd consider them poorly designed if that isn't the case.
Beyerdynamic seemed to make explicit mention of that issue in their promotional videos for the driver of their more recent DT 900 / 700 Pro X.
Did some more digging and found these comments from Sean Olive (Senior Fellow at Harman International) and Dan Clark. Followed by a research report from Todd Welti (Distinguished Engineer at Harman International), and some figures of his results, showcasing how big the differences can be in the bass region with certain headphones due to leakage.
Rtings has for a long while measured the bass response on five different real humans, and merged it above a few hundred Hz with their ear simulator measurements :
Frequency Response Consistency describes the variations in a headphones' frequency response due to their fit on your head.
www.rtings.com
By now this might be the most extensive database of bass response on real humans available ?
I don't think that leakage or sample variation are the only source of individual to individual or ear simulator to individual variations / deviations though
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