After the first positive listening impression, I was honestly a little disappointed when I took measurements and the lower frequency range looked almost unchanged at first glance. I thought my ears had deceived me and I had invested 1000 euros for nothing. Fortunately, the spectrogram view eventually confirmed what I had heard.
Yeah this can be quite a common experience. Sometimes, things can even measure worse in the frequency graphs after spending loads of money, because as one problem gets fixed, another thing reveals itself more. For instance, there could be one frequency being boosted by one part of the room and cancelled by something else. The frequency domain might look flat, but in reality it just the sum of various things going on in the room that are giving a kind of "false positive".
The main thing is to look at the time domain and work out what is happening in the room, and to follow any improvements in the time domain. The frequency domain can show us a few things but to make sense of anything you need to consider what is causing the graph to look like that (speaker positioning, treatment, time domain etc). An ideal room is never going to measure completely flat, even in the absolute best designed rooms there'll be some unavoidable dips. As you fix certain time based issues, the boundary ones that you can't do much about will still be there.
It can also be frustrating because we want a couple of panels to make a drastic improvement, and in reality, we need SO MUCH absorption to have a significant effect, combined with the fact that as you add more treatment, they'll have less and less effect.
With your tall ceilings, it should be fairly easy to hang quite a big thick cloud (or several). If you have the means to do DIY, they are VERY easy and inexpensive to build (like a fraction of GIK prices), and you have the benefit of choosing your own sizes and materials. Once you go over 20cm or so, it's best to use less dense insulation. With your dimensions, I'd probably consider something like a 40cm thickness but with low density fibreglass. Ceiling clouds also don't take up any space that you'd use for something else so you can have pretty significant gains from it without losing much.
