Howdy all - prompted by a Twitter dialogue, I’m posting the HRTFs we’re presently using for compensation of headphone measurements, and their provenance.
@taronlissimore will probably tell me I’m silly for my formatting later, and that’s fair, but the only way I could figure out was linking google sheets.
4128 DFHRTF - this one is the kludgiest data we have, and it shows in the smoothing. This DFHRTF was derived from the free field data that Brüel & Kjær make available for the 4128C with the straight canal pinnae. As you can probably see, it’s been smoothed a bit - variably by frequency - because the “free field” that B&K used for the 4128 wasn’t all that free, and there were meaningful modal resonances in the response. In the long term, this is going to be the most likely next target for replacement as a result, so watch this space.
5128 DFHRTF - this one also comes from the official data from B&K, which thankfully is a lot more actually free field this time. This is the HRTF we’re happiest with at the moment, and while we’d of course jump at the opportunity to get even better data down the line, this is the least likely to be revised in the near future. Both the 5128 and 4128 averaging math was done by the eternally lovely Oratory1990 in correspondence with me, so big shout out to him here!
KEMAR DFHRTF (NOTE: uses older KB0065 pinnae!) - this is the one we’re actively working on replacing, but for the moment this is what GRAS measurements are compensated using. It comes directly from GRAS, and unlike the other data was measured in a reverb chamber rather than synthesized from anechoic data. As a result, it’s quite noisy, and like the 4128, we had to get creative with the data processing to get an HRTF that wasn’t adding more noise than it removed. Stay tuned, because below is a sneak peak of how we’ll be replacing this one very soon…
Of course if anyone has any questions, commentary, or anything of the like, this is a fine place to ask too!
Just one thing to note - these are not target curves the way you might see with preference elements from say Harman or any kind of tilt applied. They contain no preference data.
Yeah, EQing to straight DF…well, there’s probably like 5% of people who would prefer that, but odds are that you, the reader, are not one of them. DFHRTFs are just that: the head related transfer functions of these mannequins in a diffuse field. Olive 2013 showed that DF is roughly equivalent to the harman in-room baseline measurement, so summing these with the shelf filters used in Harman, or applying a constant slope, or really anything else you’d think would be suitable for a speaker “room curve” is the play here.
We’ll be making a tool available in the future that should take some of the work out of doing just that for folks, in fact…
I think that this should be mentioned more often. People are used to seeing measurements compensated to Harman where flat might be good. I tried EQing to DF-1dB/octave when I first saw 5128 measurements of the headphones I own because I didn’t know any better. I used the preset for some time before I found out I don’t really like it. This makes me afraid that people thinking that flat is good will start a trend of manufacturers making headphones tuned to DF-1dB/octave to have “well measuring” headphones people want.
I personally prefer less ear gain compared to DF and the elevation to be shifted by a little bit to higher frequencies. I don’t mind another elevation after 5K. Moreover, I like warmer headphones so I prefer a small elevation in the midrange.
Yeah and this is where preference windows come in. For people who really want to dial it in, you’d want to apply whatever tilt you prefer. So for example, if you like a warmer sound signature, go with 12 or 14dB to start. These are just the compensations for each measurement rig, which has no preferences.
It should be noted that linear slopes aren’t the only way - again, it’s best to view DF as a baseline for applying a preferential EQ, which might well include notch or peak filters in addition to shelves.
Thanks, Mad_Economist. Looking forward to the KEMAR DFHRTF with KB500X pinnae! Are the loudspeakers linearity, room, and methodology compatible with the ones used for the 5128 DFHRTF or it doesn’t really matter?
Indeed, and that’s also MUCH easier to do when compensated.
Just to expand a bit here on what @Resolve said, this is actually about 40% of why I’m ardent that DF is the way. A diffuse field is a standardized sound field (where sound is non-incident/power is evenly distributed across space), just like a free field. A properly conducted free or diffuse field measurement is compensated for the response of the speaker(s) used, and has no room or other unique interactions. In premise, four labs on four continents, using four different speakers, should be able to get strongly matching results with the same head as long as each uses the correct methodology.