I’m curious about measurements, largely beyond frequency response. I’m a data scientist / cognitive scientist and would like to get an overview of what measurements reflect which quality in a headphone. I’m also interested in the limits of how much measurements can really tell us. For example, with frequency response, you look at the graph and you get a good idea of the timbre of a headphone, as well as potential problem areas. But timbre is only one aspect of the sound. I’m going with the assumption that our ears and brains are nothing but measurement tools, and thus every difference that can be heard can in theory also be measured. I will however accept that limitations in the measurement equipment we use and our knowledge about the mapping between measurements and perception may limit how much we can learn about a headphone from graphs alone. This is precisely what I’d like to explore a little bit.

(1) To get an overview, let’s list perceived qualities of headphones (and I’m trying to keep orthogonal qualities separate): [A] Timbre, [B] soundstage (size, location), [C] imaging (precision of positioning, separability of instruments), [D] cleanliness and [E] dynamics / response speed / punchiness. I’m leaving out detail retrieval on purpose as I believe it to be a combination of amount of treble, cleanliness and dynamics. I’m not sure about “naturalness” of sound and whether that is a separate quality from the others. I’m also not quite sure if cleanliness is truly separate from dynamics. Am I forgetting any perceivable qualities, or am I confounding orthogonal (separate) qualities?

Now to how we measure these...

(2) [A] Timbre: Maps largely to frequency response (with a decent amount of smoothing applied).

(3) [B] Soundstage: If I understand this right, RTINGS compares the frequency response in the 1-15 kHz region to some target response, but I’m not quite sure how this works exactly. Can someone explain?

(4) [C] Imaging: RTINGS measures this as a combination of amplitude, phase, and frequency mismatches between the left and right side, as well as group delay.

(5) [D] Cleanliness: What I mean here may map to harmonic distortion. RTINGS performs a frequency-weighted HD rating.

(6) [E] Dynamics: This is not part of the RTINGS tests, but based on my experience it is a distinct quality. What I mean here could be described as speed / transient response / punch. How much fast transients (e.g. snares / click sounds) stand out, how quickly the headphone goes all silent once the signal is back down at 0, and difference in amplitude between the input signal and headphone output (i.e., compression factor or something like that -- I hope you understand what I mean). I feel like these are different aspects, but can be summarized in the "dynamics" term, just like imaging combines multiple facets in one term.

Bonus questions:

(7) The FR graphs we’re used to looking at are all smoothed by a certain amount. This can mask thin spikes in either direction in the graph. A headphone with completely smooth FR might look to have the same FR after smoothing as a different headphone with a lot of narrow and tall spikes. I’m assuming the differences between these two hypothetical headphones would be audible. Or more generally, a headphone with a lot of tiny spikes might sound a little weird? Why are FR graphs commonly smoothed quite a lot?

(8) Why is the only graph people ever seem to talk about here frequency response, and everything else is conveyed in fairly vague language?

(9) Some people strongly oppose EQing headphones. What unwanted side effects can come up and which measurements would they affect?

Enlighten me! :)

Happy new year!

edit: To be clear, I'm not asking what we can learn from frequency responses. I'm asking mostly about all the other measurements that can be made and how they correspond to the perceived qualities of headphones precisely because frequency response obviously isn't everything.