Finding "neutral" - my HD600 EQ profile

For those wondering, I spent the last couple of days with hours of measuring, listening, comparing and fine tuning the HD600 to achieve a really "neutral" kind of target.

It took me several approaches to get a spectrum which I would regard as natural and smooth enough being comparable to pink noise coming from a frontally placed speaker. 

First I measured a speaker placed around 30° off-axis for both my left and right ear, and I also measured the HD600 trying to match both responses above 2khz manually as close as possible separately for left and right. In the following measurement you can see again the speaker response in grey, the HD600 in green and the equalized response in blue which I tried to match up to around 14khz:

I did this for my both ears separately and the result although interesting didn't sound quite right to me. It appeared way too sharp, as if an EQ was applied twice. Although the spectrum improved it was simply too extreme. Then I tried to reduce all values manually, a pity that there is no "strength" adjustment of a particular filter set, please tell me if I am wrong, but it could be a nice feature within EqualizerAPO being able to adjust the amount of a filter from 0-100% which could help finding an average more quickly. The result was still not satisfying that's why I started to equalize the HD600 manually from scratch, using Oratory1990's OptimumHifi profile as base with my own adjustments on top. The Optimum Hifi target appears pretty valid to me up to around 1k, above that I start to deviate way more, but after several days of repeated listening sessions and adjustments I finally have an EQ for the HD600 which I think gives me a way smoother and more natural spectrum, sounding more similar to what I perceive from a frontally placed speaker. On top of Oratory1990's 10 filters, I had to add another 17 filters for fine tuning. When finalized I will try to simplify the filters down to 10 again hoping that the final response will still be smooth enough, this will also be my "reference" tuning for the HD600 to be used for my binaural recordings.

Below you can see the HD600 measured with my closed microphones and averaged for left and right ear, in grey raw without EQ, green with Oratory1990's OptimumHifi EQ and in black my current EQ, below in blue you can see the delta between Oratory1990's and my EQ:

I am not sure how much sense it would make to use this target also for measurements, as it won't really be able to include all those fine nuances because it's an average over both my ears and derived from several measurements done at different positions as an average again. Here would be left and right for the equalized HD600 compensated with the same EQ as target which shows that it won't measure perfectly flat as just a single position is captured which may deviate from the target that is smoothened at the same time, besides the average over left and right channel will of course measure flatter than a single channel:

Nevertheless I tried doing some headphone measurements using this EQ with 1/6th octave smoothing as compensation target and this would be the results for HD600 (red) and Focal Elex (blue) (both channels averaged):

And here HD800S (purple) and HD560S (green):

I am aware that my EQ may not sound the same to everyone, due to different HRTFs, besides there are still some spikes which are noticeable from the spectrum but I asked both my son and my wife to have a listen and tell me which sounded more natural to them and they both picked my EQ over Oratory1990's original OptimumHifi EQ, thus I hope that despite not offering perfect results to everyone, it will still improve the response of the HD600 for a more natural and smoother sound than any EQ I tried so far. 

I would like to ask any HD600 owner to give my EQ a try and let me know your opinion. In my case sweeping through the range of 1K-16K (which is my own personal upper limit in the meantime) results in a pretty constant amplitute with only minimal variations remaining. Please find the EqualizerAPO configuration file attached.

A big thanks to Mika for sharing his binaural pink noise file that he created, which I would also like to post here for others. His pink noise sounds way more natural when listened over headphones as it's a mix of correlated pink noise for low frequencies, blending over into uncorrelated pink noise for the upper frequencies with some additional crosstalk applied. It's not suitable for speakers and really ment for listening over headphones, it sounds more realistic to me than both correlated or uncorrelated pink noise. 

There is still lots of work to do, but finding my own neutral target will improve all my future work including the quality of my binaural recordings as they will be equalized with minimal colorations remaining.

Apart from that I can use my target for any future tunings, equalizings of other headphones etc, given that my target gives similarly decent results to others. Of course this also means that I need to rework my "neutral" IEM tuning as I want it to sound exactly the same as both the headphone and similar to the speaker. Although I managed to improve my "newtral" tuning for the Free Pro 2 in the meantime, I need to tweak it even more to get it closer to my HD600 tuning, as I think this one sounds definitely more "correct" to me.

Thanks to everyone for your continuous support, I hope I can manage to incorporate my latest findings onto any upcoming future projects.

Update: I haven't updated this EQ for a long time here, although I continuously tried to improve it over time. Unfortunately I am still missing a perfectly neutral reference which I had planned to get with the Neumann KH80, which turned out to be a fraud, thus please keep this as a work in progress state. I still think that my later version V06 which I achieved some time ago, is even smoother than my original V01 EQ I posted before. Feel free to try it out.