An asymmetry is an asymmetry.
The HMS II HATS Rtings use shows poor adherence to IEC and ITU standards in the treble compared to the GRAS KEMAR Oratory uses for most of his recent measurements:
Possibly but as I said my observation concerns GRAS rigs as well. All of them are under-representing the ear canal gain region vs. what I am personally getting from the APM (but as I said this is just my own experience).
The fact that you find similar discrepencies with the AKG K371 as the APM strongly suggests in both cases that as I said before this is just down to response variation on different heads (mirrored by small variation in clamping pressure on different rigs), as this is a known issue with the K371.
I probably didn't express myself well enough. The APM and K371 are similar to me insofar as they both have a response below 3kHz that I wouldn't feel much need to EQ, and a pretty uneven response above that's I'd like to EQ. That's where the similarities end (for a start the K371 is unEQable in practice for me because of FR instability when I move about).
Their on-head behaviour under compression is actually radically different. While I can't know what happens with these headphones on a hammerhead style fixture, the AKG K371 on my head, under compression, show an elevated SPL increase across the entire pressure chamber band (ie up to around 4-5kHz at least). The APM on the other hand (and I suspect quite a few ANC headphones), show a similar SPL throughout the ANC feedback range (below 800Hz or so) and a varying SPL above that. So on the K371 compression issues don't affect to quite the same extent the tonal balance of the FR throughout the pressure chamber band, but they do on the APM.
Red and orange traces : K371 with a good seal in both case, but with slightly more compression for the orange trace (emphasis on "slight")
Turquoise traces : APM with varying degree of compression.
All done with blocked ear canal entrance measurements (so don't read too much into either the absolute values or even the relative values between the two headphones between 2 and 4kHz, look only at the relative values for the same HP).
Measured after the pads were warmed up (particularly important for me for the K371).
Traces were NOT normalised.
I think that it's quite normal that the response of the APM (or most other ANC headphones with a robust feedback mechanism) doesn't vary below 800hz, as the feedback mechanism applies EQ in real time in that range to deliver the SPL the input tells it to produce. But above that it doesn't work and the usual compression issues apply.
Hence why I suspect that when measuring ANC headphones with a feedback mechanism particular attention should be given to make sure that the overall balance between the ANC feedback range and the ear canal gain region is a good mach for the average human.
And hence why different people may experience quite varying levels of ear canal gain vs. lower frequencies with such headphones.
Oratory mentioned that to facilitate the ANC performance a good pair of ANC headphones may require a low acoustic impedance design, making them more susceptible to variations in the front volume, but I have don't understand what that exactly means and have no idea whether or not this increased susceptibility also applies in the ear canal gain region .
But at least as far as I'm concerned from seatings to seatings this issue isn't that prevalent - or at least not anywhere near as significant in the ear canal range than what I've seen from Oratory's spatial averaging graph :
APM individual traces with careless positioning. Not the best I've seen, but not the worst either. I'm not getting much variation at 1kHz for example.
The LCD-X's crap stock response and probable non-minimum phase error in the bass (look at the huge group delay swing there, combined with the kink in frequency response)
That could be a HPs to rig interaction problem that you may not experience on your own head.
In the case of the HD58X for example :
https://www.audiosciencereview.com/...nheiser-hd58x-jubilee-review-headphone.25474/
When I measure them on my head :
Resolve measured the LCD-X on head as well and noted that his own head measurements didn't exhibit the same "kinks" as his rig's measurement :
So I'd take these things with a pinch of salt.
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