Watched/listened to all of the above HBK conference videos. The first roughly 2/3 of Dr. Olive's talk on
Modeling And Predicting Listeners’ Headphone Preference Ratings was mostly material that I'd heard before, which basically summarized alot of the research that led to the development of the Harman headphone target. So the more recent study involving the HBK 5128 system doesn't really begin until around 28:30.
The points in the video regarding the custom modified GRAS pinna, and its improved modeling of leakage effects were fairly important imo. And helped me to understand why Harman felt that some comparisons between the two systems would be valuable.
At ~34:25, Dr. Olive commented on some other discrepancies though between the two systems, which seemed to be somewhat specific to the DCA Stealth measurements, pointing out the fairly sizable dip between the main resonances at ~3k and 8k on Jude's 5128 plot of same. This is something Amir also commented on at the beginning of this topic, and circled for us in the graph below...
The dip on the 5128 Stealth plot looks a bit pronounced. But it is not that unusual to see dips like this in the headphone plots of some other measurement rigs than the GRAS. There are similarly recessed areas between the first two ear resonances in several of Harman's other 5128 measurements shown in the above video. (Though they aren't generally quite as obvious or pronounced as on Jude's Stealth plot.) The graph of the Sennheiser HD650, which is the third graph in the top row at around 36:15 in the above video, is one of the better examples of this. The blue curve on that graph is the 5128 measurement.
You can see the same type of feature on some the SoundGuys 5128 measurements as well, especially on their Audeze Mobius and Senn HD 6XX graphs.
Audeze Mobius
Sennheiser HD 6XX
The same type of dip can also be seen in the pre-Harman target Olive-Welti in-ear response curve, based on the sound of speakers in a room...
Which I believe came from this 2013 AES white paper...
There is little consensus among headphone manufacturers on the preferred headphone target frequency response required to produce optimal sound quality for reproduction of stereo recordings. To explore this topic further we conducted two double-blind listening tests in which trained listeners...
www.aes.org
Several years ago (around 2015?), I also computed the average response for some of the better-sounding headphones which were available at that time, based on raw FR meausrements on a now defunct Korean headphone review/measurement site, called GoldenEars. Which I believe was using a Head Acoustics system similar to the one that Tyll was also using at Inner Fidelity. And you can see a similar dip and series of resonances in the upper frequencies in the average response of those 21 headphone measurements as well.
It's a bit hard to see, but the FR ranges on these plots are the usual 20-20000 Hz. And the amplitude range is 50 dB...
The peaks and valleys formed by this average response curve also appeared to me to follow a general pattern of concentric arcs.
Peaks:
Valleys:
Pattern of concentric arcs:
And this became one of the features that I increasingly began to look for in the headphone measurements on these types of rigs.
Most of the 21 headphones in the above sampling were audiophile type headphones, btw, rather than consumer headphones. So as a group they were probably a bit on the brighter side.
A somewhat similar patterns of peaks and valleys could also be seen in many of the Rtings raw response graphs. Especially on plots of some of the better headphones. So I tried computing some averages from that data as well. I was still trying to follow the Harman target model at this time (more or less), so the resulting average curve had a similar rise in the upper midrange at around 1.5 to 2 kHz as that target response curve. (Which is something I've since decided that I'm probably not such a huge fan of.)
Here is one of those averaged Rtings curves compared to the 2013 Olive-Welti curve (top) and 2018 Harman over-ear target (bottom)...
Although this is only vaguely hinted at by the three rather tantalizing peaks at around 3, 8 and 16 kHz in the 5128's diffuse field compensation curve, the same type of dip and resonant structures in the treble can also be seen fairly plainly (imho) in Amir's raw 5128 graph of the Sennheiser HD650...
Based on the DCA Stealth and some of the other 5128 plots by the SoundGuys and Amir above, it appears as though at least
some of the better headphone measurements on the 5128 system could end up following a similar pattern of dips and resonances in the higher frequencies, maybe putting it a bit closer to the response of the Head Acoustics system in that area than the GRAS 45CA.
I think this difference between GRAS and 5128 resonances was also briefly touched on in the recent
Headphone Show livestream with Dr. Olive as well. (Though I can't remember if it was Mad_Economist, Resolve, Dr. Olive who initially brought this up.)
The resonances on the Head Acoustics and similar systems (which appears to maybe also include the HBK 5128) seems to be fairly consistent with the resonant frequencies of the ear canal show below...
And the somewhat sizable dip between the first two resonances also seems to coincide with what alot of audiophiles refer to as the "sibilant range", which generally falls in or around the region between the first two resonances in the ear canal shown above. Substantial peaks in this area between the two ear canal resonances are generally disliked for their sibilant characteristics.
This same sibilant frequency range also appears to be described by the ITU-R 486 noise weighting curve. Which is the black curve on the graph below that peaks in the low treble at around 6 kHz. Roughly in between the resonant peaks of the ear canal and many of the measurements made on the Head Acoustics system (and it appears now also at least some of the measurements on the 5128 system)...