That will give you an opportunity to measure that speaker's response. You can correct the output of that HATS so it measures the same as the speaker with normal mic. (I assume gated ?).
This basically is how HATs are calibrated but then in anechoic or specified conditions.
When you have calibrated your DIY HATS under your room conditions doesn't mean it will measure the same in nearfield, diffuse field and anechoic conditions. It will give a correction for only that HATS in that room.
A headphone couples completely different than speakers in any room, so that calibration is of little use but are kinds of calibration one can obtain.
This is where Harman research differs.
Yes, differences between BK5128 and 45CA are there. Amir has measurements of the same headphone done on 5128 and 45CA so you can compare. Also there are others that have compared the rigs. With headphones also seal comes into play for low frequencies. because of shape differences between HATS and 45CA fixture this too can lead to differences.
measuring transducers in all kinds of circumstances is a bitch. It pays to have standards but not all standards are equally good and relevant.
We have a starting point for well balanced sounding speakers that I assume to be accurate and good enough: a target curve that is flat up to 1 kHz and then slopes down linearly to -6 dB @20 kHz, as measured according to psychoacoustics principles (so yes, gated, of course) by a standard measurement microphone like the ECM8000.
What type of pressure do speakers that measure in such way impart on the listener's eardrums?
We need a binaural microphone to find out. One that is geometrically as close as possible to the boundaries that the listener's eardrums see in reality when listening to speakers (canal, pinna, head), since those boundaries affect the pressure at the eardrum itself.
This is the reason why I think the 5128 is better than the 45CA with pinnae: it has an anthropomorphic head that would allow for a more precise measurement of that pressure, and therefore finding a target curve that is more meaningful.
But for 40 grand I would expect this work to be done by B&K already, not having to do it myself.
Since both speakers and headphones during listening are pretty much at a standard configuration in respect to the listener's head (give or take some variance in positioning), all we need to do is put the binaural microphone at the sweet spot of the speakers, measure (once again according to those psychoacoustics principles) and get our target response like that.
We don't need to be concerned about free vs diffuse field or anything like that.
However, measuring speakers in quasi-anechoic conditions would be strongly advisable for more precise measurements independent of the room.
Only if one wants to make binaural recordings would they need to equalize the binaural microphone. For headphones measurements there's no need for that.
Leaks aside, in the bass region, for rigs that are similar to each other from the point of view of the pressure produced at their capsules by headphones, such as the 2 rigs in question (as opposed to pressure produced by speakers, for which the shape of the entire head makes a difference), there will be a difference in the lows for the same headphone pair that will be the same for other headphone pairs.
Actually, capsule response difference aside (and assuming proper seal), for these two specific rigs in question I don't see any reason to expect much of a difference in measurements at all, as far as the bass response goes.
The main difference will be in the highs, and it's there that I expect the 5128 to have the edge... if only they provided a target curve measured as explained above or similarly.
And of course above 10 kHz there are all the other problems due to headphone placement that make it very tricky to get meaningful measurements without doing a lot of measurements and trying to find the variance in response due to the headphones and extract it out of the variance due to the different placement.
But below 8-10 kHz the measurements would be as good as they can possibly be, keeping in mind that an averaged canal geometry is definitely not 100% accurate for everybody but only the next best thing, short of an individual measurement of the pressure at the listener's eardrums (which is not possible because inserting a mic probe in the canal alters the geometric boundaries too much anyway).