Thanks for all that, Matthew. I agree 100% with everything you said. Unfortunately people are becoming even more difficult to convince of this--mainly because "Room EQ is Fun!" They don't want to believe it.
And wouldn't it be nice if we could do that? People often mis-state that Floyd Toole is "against EQ" or against it at high frequencies. That's not true:
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What he's against, is trying to do that with steady-state in-room responses. It just can't be done correctly as you have noted.
Kevin Voecks said that pretty much any passive speaker, including their top of the line Revels, can be improved with high resolution EQ at high frequencies--but based upon anechoic data, not in-room response. That's the basis for their "Anechoic EQ" in the SDP-75. It uses anechoic data to "fix the speaker" with high resolution EQ, and then the "room EQ" portion is only used for low resolution, smooth tonal balance changes to the user's preference.
I'm working on a method for advanced users to use Audyssey XT32 as a "poor man's" (very poor man's) version of that which you may or may not approve of.
It's such a shame to have thousands of FIR filters at your fingertips and leave them unused by limiting correction to the transition frequency when they could actually improve your speakers if you just told them what to do. And some speakers need a lot of improving! But even good speakers generally leave quite a bit of room for improvement. Telling all those filters what to do, that's the tricky part....
Most people won't have good anechoic data of their speakers. I'm putting forth that people can get good enough quasi-anechoic data with REW or Omnimic to improve their speakers at
high frequencies with the right measuring techniques. Most people aren't going to be able to drag their speakers (and AVR) outside to do proper measurements free of reflections and that will limit how low they can get useful data (the lower frequencies require more reflection-free space). But since at very low frequencies you want to EQ the speaker and the room together (the way Audyssey does it) anyway, it sort of all works out (with a slight grey area in the middle).
By measuring several speakers from various locations in the room, I've convinced myself that if you use gated measurements of around 4 ms at 1 meter, one can get consistent, repeatable results down to around 800 Hz or so without room reflections interfering. Here's a measurement of a speaker in the center of my room and one with it pushed up against a wall:
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(That's using omnimic in "blended" mode set at 4 ms, no smoothing at all.)
As you can see, above about 800 Hz, the results are basically identical (I was actually surprised the first couple times I did it that in that I could set the mic up that consistently in different locations multiple times). I think with careful technique, some may get useful data even a little below that--depending upon their speakers, how big their room is, etc. With these particular speakers (large MF and HF horns with a decent distance between the drivers) I don't really trust the data much below that at 1m (I don't even know if I can believe that dip at 950 Hz, I think it may disappear with a more far-field measurement).
So, in my opinion there's still an "area of uncertainty" from about 500-1000 Hz where speaker type and measuring technique can affect the validity of the results. I'm still trying to figure out best practices for this area.
But if we forget about that zone for a minute and just focus on the higher frequencies where the measurements should be reliable, we come to the point of all this. If one uses my (rather tedious but doable) technique to fool Audyssey into doing what you want, the capability of XT32 to fix things at high frequencies is astounding, as I was alluding to above.
Here's a 1m gated measurement of the speaker "fixed" above 1000 Hz or so, with
no smoothing:
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As you can see, it's +/- 1/2 db from 1050 Hz to 18K+. That's a very JBL M2-like high frequency response. While XT32 generally doesn't do high resolution changes at high frequencies (as well it shouldn't--as you, Toole, ect have pointed out it doesn't have enough information from steady state room measurements to do it properly) it certainly has the capability to do so if you tell it to. This is what I found most astounding, I had no idea XT32 would actually be able to make such fine corrections. That's a boatload of DSP horsepower so many people already own--11 channels of it in a box they already have integrated into their system.
That EQ results in the in-room measurements I posted above. Basically "room-EQing" at low frequencies, the above quasi-anechoic EQ at high frequencies and letting the "room curve" take the shape it naturally takes.
Of course many won't be left with such a nice, flat, smoothly declining room curve even if they get the high frequencies perfectly flat at 1m--I happen to have pretty good speakers with excellent directivity control. If my speakers, for example, had a directivity mismatch at the crossover as is so common, I'd expect to see a dip in the room response there--and I'd leave it there. Many speakers have other directivity issues as well that will cause uneven in-room measurements and as you and others have noted, that can't be fixed with EQ.
But I think in most cases if one gets the frequency response smoothed and flattened quasi-anechoically, the speaker will be improved and sound as good as its basic design is going to allow. Most of the time this will be better than flattening/smoothing the room curve and screwing up the direct sound in the process.
Anyway, that's the path I've been headed down in an effort to put all that Audyssey DSP power to good use--the right way. I'm planning to write up a procedure but I'm still experimenting around to try and figure out best practices to give the average guy who might not live and breath this stuff the best results. But I'd be happy to describe the process if anybody else wants to experiment with it.