Wait. Does that EL8 Titanium measurements on dyiaudioheaven are yours? There Is no perfect headphone, but i could be an ideal headphone for each person, if only he could find out what type of freq response he could like..
Yes,
@solderdude is Diyaudioheaven
Re: your question about the ideal headphone being for a particular listener being the one with a frequency response they like, this is kind of correct, but is different from the question of transparency.
A transparent device is, like
@Blumlein 88 and
@solderdude have explained, a device that makes no audible difference when it is added to the signal chain. But a person could prefer a device that
does make an audible difference over one that does not.
Harman's headphone testing, for example, showed that younger listeners tended to prefer headphones with elevated levels of bass compared to older/more experienced listeners. This does not mean that these headphones are transparent; it means rather that they are preferred by some listeners. There are extra complications when it comes to headphones because it is very difficult to determine what the frequency response of the headphones are on a particular listener's head. With speakers, we have similar (but arguably not quite as complex) problems, because the electronic signal does not contain information about the polar response of the speaker or the room, and every speaker/room is different in this regard. So, from a theoretical perspective at least, two speakers could have different polar responses and/or be set up differently/in different rooms, and still be equally true to the source. So it doesn't quite make sense to talk of a headphone or speaker as transparent - the performance of the device cannot be fully separated from the effects of the room or the listener's head. Harman has done a lot of research, however, on what most listeners prefer, and this shows some clear trends which are useful for speaker/headphone designers and for listeners wanting to use measurements to make informed choices about which devices to audition or buy.
When it comes to an active electronic component or a cable, it is easier (but still difficult) to determine whether something is transparent. The question is, can the listener reliably discern under controlled conditions whether the device is or isn't in the signal chain.
We can also look at the results of controlled studies in which, for example, distortion is digitally added to signals which are used to test listeners. For example, there have been studies that have showed that some listeners, under some conditions, have been able to reliably discern some music signals with 0.01% added nonlinear distortion from the same music signals
without added nonlinear distortion.
From this we can conclude that, if a device (e.g. a DAC or amp) produces 0.01% nonlinear distortion, it is unlikely to be transparent to all listeners with all signals.
However, I'm not aware of any study, for example, in which any listener has been able to reliably discern a music signal with 0.001% added nonlinear from the same signal with no added nonlinear distortion. From this (and from other studies into
masking), we can conclude that a device that produces 0.001% or less nonlinear distortion is likely to be transparent to most listeners with most signals.