Those are some interesting graphs! It seems to me that the Harmon in room graph from 25 years ago is closer to what most people are shooting for in their house curves. Both have a steep roll off above about 15kHz, but I'm not sure how much that matters to most listeners. I'm 36 years old and my hearing maxes out at about 16 to 17kHz. What happens above that won't be audible to me. Both curves appear to have about a 10dB boost in bass above 75Hz with a roll off from there to about 200Hz to baseline. The newer graph has more of a boost around 3kHz, which is interesting, because from what I understand that is where the ear is most sensitive. Audyssey even gives users the ability to decrease the output in that range. It seem that the jury is still out on preference curves, but most would agree a 10dB boost from baseline in bass is preferable.If you really believe this, TurtlePaul, then perhaps you'd better take another look at some of the Toole videos and links posted above. Some new data has been accumulated though in the last 25 years on the in-room and dispersive characteristics of loudspeakers, thanks in part to websites like this one.
I'm going to turn my thoughts back to the subject of headphone measurements though (at least for the moment). And some of the other potential relationships they could have with the measurements of loudspeakers imho.
These are the same two average sound power loudspeaker curves shown in one of my previous posts above. But with one minor change or addition...
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Both of the curves have now been combined with the diffuse field HRTF response of HBK's new 5128 HATS measurement rig.
The diffuse field response of the 5128 is represented by the dashed purple curve on this ASR graph of the Sennheiser HD-650 headphone...
It's the curve with the brightest tilt or response on the graph. And is noticeably brighter in the treble than both Amir's recalibrated version of the Harman curve for the 5128, and also the measured in-ear response of the HD-650 (which is considered by some audiophiles to be a fairly neutral-sounding headphone in the midrange and treble).
To a casual observer, the 5128 DF curve looks way too bright to really be of much use for analyzing the in-ear responses of headphones (or anything else for that matter). What the purple DF curve represents though is the HBK 5128's HRTF response to a spectrally flat sound field that is essentially equal (or diffused) in all directions.
When you're listening to speakers in a room in your home, sound is also coming at your ears from all different directions, like in a diffuse sound field. The volume and timbre (and arrival time) of the sound is not the same though from all directions. Because both the directivity or dispersion characteristics of the speakers, and the acoustics of the room reflecting the sound back to your ears will change the timbral balance or composition of the speakers so they have a darker overall tilt than they would if you were listening to just the speaker's direct/on-axis response in a more heavily damped or non-reflective (aka echo-free or anechoic) space. This is because most speakers disperse their sound more broadly at lower frequencies than at the higher frequencies. So the room tends to reflect more LF back to the listener than HF. And the net result is an in-room response that's tilted more in favor of the bass, and less in favor of the treble (like my homey TurtlePaul just mentioned above).
So what does this have to do with my headphones, or the price of beans in Chile?... Well, it turns out that if you take something like a HATS measurement rig's, or any person's in-ear response to a spectrally flat diffuse sound field, and then combine that with a speaker's spectrally-tilted sound power (which is just a summary of it's diffuse response in all directions in a room), what you seem to get is something that looks awfully close to the speaker's probable in-ear response in an average domestic listening space.
It kinda makes sense when you think about it, because what the normal DF response of a measurement rig is generally missing is that darker "tilt" that you get when listening to speakers in a semi-reflective room. And the sound power of a neutral loudspeaker seems to fill in that missing timbral information quite nicely. So combining the two should give you something fairly close to an approximation of a speaker's in-ear response, if you were listening to them in a typical semi-reflective room (which is one of the models Harman uses in its spinorama calculations).
How that all works is still a bit of a mystery to me, since I'm not much of a math or engineering genius. But it seems to be borne out, at least implicitly, by much of Harman's data and research. And also the independent measurements I've looked at on both headphones and speakers, including the graphs in Pierre's spinorama database, and also raw and compensated headphone measurements by ASR and other headphone graphers/reviewers (such as Oratory1990, Crin, Resolve, Inner Fidelity, Rtings, the SoundGuys, and so forth).
This is not something which can really be proven though until there are more actual in-ear measurements of speakers to compare with sound power + DF HRTF results, like the ones that I've posted above.
These are the two average sound power curves modified with the 5128 DF curve, overlaid for a little easier comparison btw...
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Note the slightly more pronounced dip in the mids on the pink curve (which represents the V-shaped speakers with some cross-over/directivity issues).
Although these curves have a fairly "Harman-ish" look to them, there are a few differences between them and the over-ear headphone target Harman developed for it's own GRAS and KEMAR-based measurements, particularly in the treble. So these curves are mostly useful for comparison just with the other HBK 5128 measurements made by ASR, or by Jude at Head-Fi. And by one or two other sites that are starting to use HBK 5128 systems for their reviews (like some of the recent 5128 headphone plots by the Sound Guys).
I'd like to see more information about how lower listening levels or deviation from reference levels affects the preference curve. Audyssey have a dynamic EQ feature that supposedly causes mids and highs to decrease at a faster rate than bass frequencies when decreasing the volume. From what they're saying the lower the listening level the more of a difference between the bass vs mids and highs there needs to be to maintain psychoacoustic balance. I'd like to see what the preference curve looks like at 55db vs 85db.