Exactly, it's so easy to EQ the Truthear to get likely closer to the Harman target, why not use that post-EQ response to judge the target rather than its stock response?
Its not that easy, and this is the issue you are ignoring. There is a strong possibility all you can do is take treble out, and this will leave it potentially dark, and lowering the rest of the spectrum takes you back to where you started. Comparisons with that alone are quite subtle tonality-wise, but here you will likely revert to firing up the afterburners on the goalposts and say my hearing is poor without knowing anything about me, or have ever provided a shred of evidence you have actually heard this IEM and tried what you proposed to correct it. The IEM does not seem too inclined to provide any deep extension due to the relatively diminutive drivers in it. Another thing being ignored here by you in your references to a pressure chamber is the fact that the IEM doesn't go into a GRAS fixture, which is a true pressure chamber as far as I know with the coupler at the end. Instead it goes into an ear canal with an eardrum at the end of it that is a flexible membrane. This will likely pose a complex and varying acoustical impedance to the IEM at low frequencies. As a potential exacerbating factor, the IEM, at very low frequencies, likely does not look like a low-Z source anymore if it has to do much beyond just pressurizing a fixed volume. If you have a moderate Z source driving a complex impedance, you have the issue that any variations in the load will be impressed on the overall response. To further complicate this, the actual motions of the eardrum and ossicular chain may not be that linear. As such, the actual resulting pressure at low frequencies can conceivably vary substantially based on the physiology of the listener, and I suspect this is a potential hypothesis also for why it doesn't do much in terms of sub-bass. If there is a crummy impedance match down there, then not all of the motion of the transducers will actually translate into perceived sound. This is vastly different to circumaural headphones where the internal cup volume, pads, and resulting seal pretty much act as the acoustic load for the transducer, and not the volume of the ear canal and the ear drum, and any tiny variations in the ear canal get swamped out in regards to the acoustic performance of the headphone. Again, this is just a hypothesis, but its something I have not seen discussed much in regards to IEMs and how they interact with the ear itself. Many just treat it as an ideal volume of air in a stiff cavity, and ignore the trampoline at the other end. One would also assume that any sort of occlusion effects are already taken into account in the target. Maybe our more seasoned IEM aficionados can provide more insight as I would genuinely like to know. You like flexin' the citations. Got anything? Its a good place to start.
But, you can't just say "it has too much treble" without talking about the bass response. We have to be able to conclude that without it the IEM can still sound reasonably balanced with the boost there in the bass region. Again from some listening the results are subtle, but the sub-bass with EQ does not sound right. It has a sort of mushy character when EQed up, sort of like the sound a cheap sub when it starts to unload. This may even not be the IEM, for that matter. As I stated in my hypothesis, the mismatch with the ear down there can be the sole source of it. But given that it does this at even fairly modest volumes I'm not inclined to push them. I use them fairly regularly when I'm out so I don't want to pop them for the sake of some damn argument. However, any discussions of the treble response will need to at least take into account the overall IEM tuning and not "but bruh
... There's liek a gap there in the treble." Again, this IEM sounds pretty darn good, and should be a good introduction to the Harman target, but not as nice as a full-size headphone since it can't dig as deep. I don't see any major issues other than you voicing objections without ever having listened to it. Headphone measurements are not like speaker measurements. Speaker measurements only have to contend with the speakers, and they are quite precise and can fully characterize how the speaker will sound if they are comprehensive. Headphones and IEMs are not because there is a big chunk of aluminum with silicone ears stuck to it to replicate the measurement conditions at Harman, which in turn were to try and get some meaningful analog of a bionic "listener" that can be used to correlate to data from blind testing. There will be error involved, and this should be taken into account and put in the context of the actual curve and what distribution can be expected.
All of this is nit-picking, namely because you are obsessing over one thing and ignoring all the other uncertainties in the target and the IEM in question. Again, I am not saying you are automatically wrong, but you only ever use the paper cites and the measurements as a cudgel to try and win arguments rather than try and have meaningful discussions and the whole thing devolves into retreaded argument using the same talking points.
Do you? I suggest reading this to educate yourself:
INTRO: The output Impedance of headphone sources is one of the most common reasons the same headphones can sound different depending on wha...
The only real concern would be phase since if its too extreme, the amp could oscillate or have other issues. The device is a smartphone if you want to go on for some more, but I have tested it with resistive loads and its "ok" in terms of distortion, but it will get unhappy if the load is too reactive. However, in the sub-bass region its not 10 ohms its substantially higher at nearly 30 ohms or more, which is the area that I was EQing. The odd tonality persists on the amp, so you can cross that off your list. Edit: This is in regards to the distortion, remember. Not the IEM's impedance influencing the frequency response. That is
a possibility, but it didn't sound dramatically different. I do remember looking into this at some point, but don't remember what the actual output impedance is.
Edit: Some more insight to be had from Amir's commentary:
Impedance shows evidence of some tuning in bass frequencies:
Be interesting to see what happens when it is EQed up there. I know I have seen other people try. For me it sounds off-putting, maybe due to rising distortion from the IEM or some other phenomenon. At any rate unless there is any new information outside the current scope of the treble response this horse has been thoroughly beaten as far as I'm concerned. Nobody disagrees there is some small treble lift, but I have not seen anything to indicate thus far that it subjectively impacts the IEM enough to disqualify it in terms of getting some overall sense of the Harman target.