but here are the measurements after I placed the desk in the room and after I built one more big poly diffuser on the ceiling in the back of the room:
Cool! More data to play with!
OK, firstly, you need to smooth your data. You are looking at it unsmoothed, but that produces graphs that are way, way overboard in the mids and highs, with details that aren't really there, and which you would never hear even if they were. So, when you are looking at the very lowest end, yes, you can leave it unsmoothed, or maybe apply 1/48 octave smoothing. For the lowest part of the low mids, I normally use 1/48 or 1/24. For the mids, 1/24 or 1/12, and for the highs, 1/12 or 1/6. Those are realistic smoothing factors that fit how our ears and brains work with the PERCEPTION of sound: that's a field of acoustics called "psycho-acoustics": how we perceive sound, as humans, rather pure acoustics, which is about how sound really is. It's not the same.
So apply smoothing as needed, depending on what you are looking at. In general, for rough overview, apply 1/24 octave smoothing.
Right, next: More on psycho-acoustics: we are very good at perceiving narrow to broad peaks in both frequency and time, but not so good at perceiving narrow dips. Since dips are often phase related (reflections cancelling each other) they don't "ring" like modes or other forms or resonance do. We are good at hearing resonance, not so good at hearing cancellations, unless it is rather broad. And since musical notes played on instruments are almost never tight and narrow, it's unlikely that a tight, steep, high Q dip is going to be terribly audible.
So, first question: can you actually HEAR that 146 Hz dip? As REW scans up the spectrum, do you really notice that dropout? If you do notice it, then it's worth doing something about it. If not, then it may not be worthwhile chasing.
Now for the good news: your desk is effectively killing your floor bounce! You've had a reflection at around 4.6ms since the start, and your desk is stopping that. This is good!
And the not-so-good news; you have a prominent reflection at around 10.5 ms that has also been there for a while, but is now stronger: perhaps because it was being masked by other stuff.
Martin-REW--IR--reflection-10.5ms.png
It's a fairly strong reflection, although not huge. I would use the "string trick" to try to track it down. 10.5ms is about 361cm of sound wave travel, and since this is a reflection that comes AFTER the direct sound reaches your ear, it is 361cm PLUS the ear-to-speaker distance. So measure the exact distance from the tip of our mic capsule to the acoustic center of the speaker, add 361 cm, and cut a piece of string that is exactly that long, plus 10cm. The 10cm extra is so that you have 5cm on each end for attaching the string! Tape one end to the acoustic center of the speaker, and the other end to the tip of the mic. Or better still, to a mic stand that you set up right where the tip of the mic is, so you aren't messing with the mic. So now you have a very long piece of string laying on the floor, that represents the exact time that the sound wave is taking to get from the speaker to your ear. Now you need to gently stretch out that string into a triangular shape (the speaker at one end, the mic at the other, and your hand at some point in between) and try to find the places in the room where that string will just touch one of the surfaces while BOTH sides of the string are taught. There might be several places, but if you mark them on the wall/door/floor/ceiling and join the marks, you'll probably find that all of the marks are on an elliptical curve. That's good. It's the location of the place where the wave-front is hitting and bouncing back. Look for the spot where the string seems to be "bouncing symmetrically" to the surface, with the "incoming" string and the "outgoing" string making roughly the same angle with the surface. That's the center of the reflection. That's where you will need treatment. To test if that really is the correct spot, try putting a large piece of 703 that covers it, and test again with REW. There should be a reduction in that refection.
If that works, then let me know, and based on where the problem is located, we'll figure out what to do about it. Considering the long distance, my guess is that it is something at the rear of the room, probably your door or maybe some of the HVAC duct work.
You also have two other fairly large reflections that might be worth dealing with, of you really want to: 13.7ms (small) and 19.2ms (large). So if you are up to it, then do the same thing with a piece of string that is 662 cm longer than the speaker-mic distance (19.2ms), and 471 cm longer (13.7ms) Both of those are probably second order reflections: bouncing of two surfaces in succession. That's a LOT harder to find with the string, but if you are careful, you might get it. There's a couple of much earlier reflections (around 2ms) that we might also need to look for, but that can be later...
I don't think these are related to the desk, or to the 146 Hz dip. They might be, but there isn't much connection.
There's more in your REW that is worth looking at , but go for those issues first. And there's good news, too: your room can get better than it is, if you want!

PM me...
- Stuart -