So a strange kink at about 150Hz is a feature?
If you're talking about this :
Then that's most likely a measurement artefact. Whether you call it "rocking modes" or "step function" you can occasionally see such an artefact when measuring IEMs either in a coupler's metal canal extension or in pinnae (711 or 5128). You'll see it move a little bit up and down with different reseats / insertion depths, or even go away completely in certain positions.
You can minimise that issue by coupling the IEM with the ear simulator with putty, a bit like this :
It's more annoying to do in a pinna but still doable.
When you do so the step function disappears. Here's an example with the Moondrop Chu, which tips are particularly susceptible to that issue :
In actual humans, so far I haven't seen a single measurement where that rocking mode / step function appears. Other than for one individual, Harman kept the in situ measurements they performed a few years ago, using a modified Momentum iei, to themselves, but here's a more recent comment from Sean Olive on the subject :
CSGlinux has organised
a tour of a modified IEM with which participants can also perform in situ measurements, so far same story : not happening in the test subjects.
The database for this project is here :
Coupler research project database
www.hypethesonics.com
(just be very careful about interpreting the measurements past 1-1,5kHz, and actually down to 500-600Hz if you're bothered by tolerances of +/- 1dB or so).
For what it's worth I also haven't seen that phenomenon occur when measuring IEMs in situ on my own as well.
It could still be that in a specific situation someone will experience such a phenomenon, but for now I prefer to put it in the category "not happening until I see otherwise".
Note that in a pinna the shape of these rocking modes can be a bit different than a simple "step" for some IEMs, but here for Amir's measurements, just visualise a line cutting through the kink (a bit like the Chu above) and you'll get closer to what is most likely happening in your ears if you get a good seal. Basically don't get too worked up with these kinks (same applies for 5128 measurements, that fixture seems particularly annoying to deal with in that regard).