AmericanSpirit
Member of the Trade: Night Oblivion
Best informative piece of observation that I read on headfi for entire 2021!Some Brief Thoughts on Individual HRTF and IEM Preference
Hey everyone, I wanted to share some comments on the relationship between individual HRTFs and IEM preferences. I'm not an expert or anything of the sort; these are just some of my thoughts on the subject. Disclaimers aside, I see a lot of talk about how "everyone's ears are different and YMMV", to the point of which it often comes off as sort of a platitude. Well...at the meet-up yesterday, we took measurements of the Everest. This was done using an in-ear microphone with the headphones over it and playing a sine sweep through REW. The interesting thing about this is that this measurement procedure is dependent on the bodily anatomy of a listener unlike an external coupler with IEMs. Most industry headphone measurement rigs also use a set ear for consistency. Anyways, all four of us took turns taking measurements with the Everest on our respective heads.
(Green - Juan, Dark Blue - MRS, Light Blue - Me, Red - Evan)
I can't show the rest of the measurement as the project is still under wraps, but pictured above is the pinna compensation region. For those who might not be familiar with the term "pinna compensation," your body has natural resonance peaks that a headphone or IEM must account for, as these transducers bypass many of them. That in mind, the use of "pinna" in the term is not technically correct; it's just become somewhat colloquial.
You'll notice something interesting with the measurement above, though: the pinna compensation region is different for all of us. There were no broken seals here, as all of us measured identically up until around 1.5kHz. The above (disregarding measurement variation, placement, and other confounding factors) basically shows that all four of us have inherently different pinna compensations. What follows is my totally unscientific theory.
Juan (green) and MRS (dark blue) have noticeably more elevated pinna compensations, which implies that their perception of neutrality is based on a higher degree of pinna gain. We know that IEMs bypass the pinna; therefore, IEM manufacturers have to effectively "guess" at what the ideal pinna compensation is for a given listener. This might explain why MRS and Juan enjoy more "shouty" IEMs. They adore the Moondrop SSR, for example, because the SSR's high pinna compensation gets them closer to what they normally hear in real-life. These types of IEMs might not even sound shouty to them! On the other hand, we can see that my pinna compensation (light blue) is lower than both of theirs. All the IEMs that I regularly use - the 64 Audio U12t, CFA Andro 2020, and Sennheiser IE900 - have what would qualify as more relaxed pinna compensation. I find these IEMs more pleasing, and closer to my neutral, likely because my requirement for pinna compensation is inherently lower. Evan's measurement is more of a wild card, as his favorite IEM is the Shure KSE1200. The KSE1200 does have quite the dip after 2kHz, though, which is reflected in his measurement (he has the greatest dip after the pinna, moving into 4kHz).
I think this illustrates why an IEM that might sound "shouty" to one person might not to another, and why a more "muted" IEM might not sound so for one person but will for another. Now, I know, I know. This probably isn't anything groundbreaking (or really even new), as it mostly serves to corroborate what we already know. This also isn't an excuse for why some stuff sounds and objectively measures like trash. That said, it's stuff like this which might help to explain some broader listener preferences, and it's what really gets my head ticking because I want to learn more about this stuff.
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I had personal guess for main drivers of HRTF are the resonance peaks pinna/concha and maybe higher harmonics, and this piece of experiment is very valuable finding of supporting that! I generally simply put “peak spot hitter” or “conservative” under my cross referencing notes which specifically referred to gain spot approach of each IEMs to my individual HRTF.
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