Been trying to fiddle around with this issue and have been reading up on the existing solutions and standards. Of course, this issue has been mitigated with the release of GRAS new RA0402 but I want something that people like me can use without dropping thousands on a small (though admittedly very high quality) hunk of metal. The background for this of course stems from the fact that most couplers suffer from resonance (even the old IEC711/318-4 which was why GRAS made the RA0402) so certain peaks, while appearing on the measurements, does not appear on subjective listening because our human ears do not have the same resonance points. And so, we need some sort of way to see if certain peaks are because of system resonance or are inherently there based on transducer output and not the measuring rig itself. This presentation by Knowles provides great insight to this (starting at the 22:45 mark) so give that a watch if you got the time. Hearing aid companies already use procedures based on IEC TS 62886:2016 to measure the higher frequencies of drivers, which is a system that essentially has a resonant point that's so high up that it doesn't interfere with the important data range (in this case, being 20Hz to 20,000Hz). If you would take a look at my existing IEC60318-4 measurements, you'd see that there's always a familiar peak at around 8-10k that shifts with insert depth. That would be something that should be ignored, but I need something more that just "because I say so". As a supplementary measurement, I decided to try to use the IMM-6 microphone with a much shorter coupler to sort of replicate the 0.4cc coupler used in the high frequency measurements based on 62886. Here's a small test sample for the "double coupler" procedure. These are IEMs that I have on hand with me right now so I apologise in advance if they are unfamiliar models to some of you. Red represented measurements using the IEC60318-4 while grey represents measurements using a IMM6 and small volume coupler. First, the TF10. An undoubtedly warm IEM with an odd treble peak at 7k on the IEC rig that I do not hear on subjective listening. Testing on a small volume couple (outlined in grey) shows that the peak does not exist. Here is the iBasso IT01, a Chinese dynamic driver IEM. The main issue on the IEC graph is the large peak at ~8k. On the small volume coupler, there is no peak whatsoever. 8k doesn't sound emphasised to my ears either. Here is something that does show up on the small-volume coupler: the TFZ King Pro. Also a Chinese dynamic driver IEM. On the IEC graphs, there is a "twin peak" of sorts, a sharp ~8k peak and a smaller one at ~9k. However, on the small volume coupler measurements, only the 9k peak is shown, which suggests that a 9k emphasis would show up on subjective listening still. Do take note that this 9k peak did not show up on the TF10 or IT01 small-volume coupler measurements either so it's not likely to be a function of resonance. Here is another interesting IEM with a "twin peak" in the higher frequencies: the Zero Audio Carbo Tenore. However, the peak placement is odd in the small-volume coupler measurement, being at ~7.5k as opposed to the IEC's peaks at ~6.5k and ~9k respectively. I am inclined to say that the lower peak is the "real" one, but I'm not too sure. I'll look into this further when I got the free time. For now, do give me your thoughts.