Originally Posted by derbigpr
Lower quality plastics, not as stiff, not as good acoustically used, as well as different mesh design on the back of earcups, much lower quality ear pad material, etc. All of those things play a big role in how the headphones will sound, maybe even bigger than drivers themselves. Just imagine strapping those same drivers into a ear cup made of cheap card board, it would sound horrible. But them into a very solid, well engineered cup like the HD558 and especially 598's have, and you transform the sound to something completely different. Despite having same drivers, HD518's and 598's dont sound like they come from the same planet. The difference between the two is as big as the difference between 598's and 650's.
Interesting point, but here's a wrinkle you may not have considered: what makes a material "lower quality"? The acoustic properties of a lower-cost polymer might actually be beneficial for a desired sonic characteristic. You could make the inverse argument: Imagine that the HD518s were built out of solid platinum. Do they now sound better or worse?
This "lower quality" plastic may flex in such a way that amplifies lower frequencies, hence the perception of deeper bass. To a bass head, "cheaper plastic" may be more desirable.
The only real way to know is to run the experiment, which the OP did when he showed waveforms of the 518, 558, and 598 that showed very slight variation. That slight variation is exactly the difference in enclosure -- no more, no less. We were wondering what the difference an enclosure makes and the data tells us right there. Now, if you measure the "area under the curve" of the net deviation from one curve to another I can already tell you just by eyeballing that the difference is going to be less than a fraction of 1%. The frequency response of these headphones are 99% similar.
Of course, the microphone used to generate this waveform data won't tell the whole story -- psychoacoustics are not modeled in that data. And it's up to you how much stake you put into the subjective experience.
I read an interesting scientific paper recently that demonstrated that people who drink hot chocolate out of an orange mug report that it tastes better than in any other color mug. The molecules that run across the tongue haven't changed, yet for reasons unknown, the sight of orange as you drink chocolate enhances the subjective experience.
I'm not claiming to know either way, but it's not outside the realm of possibility that the cream & oak color of the HD 598 is entirely responsible for it's perceived difference, despite fact that the air molecules are vibrating in a way that is 99% similar to the HD 518. The hot chocolate didn't change, but the taste did, so to speak