Not necessarily inaudible just comparatively inconsequential, or where it is consequential we’re generally talking about edge cases.
Most of the time, when people think they’re hearing differences outside of the frequency domain, it really is in the frequency domain just not identified as such.
Yes, at the moment we’ve been doing this with two generally competent headphones (even if some of us really didn’t like one of them), and we have yet to do the convolution filter application for truly cursed / broken products. Headphones can be ‘objectively bad’ for different reasons, and when those reasons are its frequency response and not stuff like excursion limits, massive harmonic distortion or whatever, it genuinely can be EQ’d to sound great.
A common one folks ask me about is a headphone like the DT1990 Pro, and whether or not it can be EQ’d to sound smooth - obviously it has meaningful resonances in the treble. And the answer is 100% yes, because it’s not bad due to anything other than those specific frequency response related problems. What we’re saying is that when you look at in-situ DRP data, most headphones that have audible problems fall into this kind of category.
The other thing we’re noticing is that when doing this kind of in-situ FR matching, we’re finding it much easier to isolate the non sound aspects that are relevant to psychoacoustics. So like the overall perceptual experience is still different between two headphones, since there are going to be differences in terms of occlusion, clamp pressure, weight, all of that stuff. ‘Headphone feel’ when worn, seems to be a noticeable contributor to the perceptual experience of listening to music as well. It’s super interesting to do this exercise because you realize the extent of that contribution when in-situ FR is identical.