Why can't this be easier?

[Basic Channel]

I don't think that is a good description of the Haas Effect, starting with 20 ms actually being 30 ms, but I won't get too pedantic. Suffice to say that it is an 'equal loudness' threshold rather than a detection threshold.

The detection threshold is the bottom line on the following chart, whereas Haas effect is the top dash line. The 'image shift' threshold, which I think you mean, is the second-to-bottom line.
View attachment 370379

© Toole, Sound Reproduction, 2006​

Toole's work is generally relevant to small rooms, ie domestic spaces, and he has commented on non-rectangular rooms. For example in an L-shaped room, such as @ohnonotagain described, he noted that the following subwoofer layout (2&3) greatly assisted with smooth bass, both measured and perceived:-

© Toole, Loudspeakers and Rooms for Stereophonic Sound Reproduction, 1990​

Thank you for the pedantry and I stand corrected, you can see why I kinda see it as related. I have found (just as a hobbyist) that it depends on the sound you are trying to throw between the speakers. Sometimes you can get away with a longer time, sometimes not. Obviously I don't mean over 40ms.

So when Ethan Winer says roughly for reflection within 20ms that 20db attenuation is desired, he's in agreement with Toole? It's confusing because the above graph would seem to disagree with Toole saying that early reflections are fine. Too early and loud and they are not fine.

 

[Purité Audio]

What would you look at in REW measurements, the difference in frequency response between channels or the impulse/ETC? If it's not my massively inappropriate speaker positioning and room, then I suppose maybe it's age or a personal thing. I don't hear the speakers clearly without absorption. It's not a frequency thing but a clarity and stereo imaging difference.

Yes I would compare left and right channels and then adjust any really obvious imbalance, obviously the nearer you sit the more direct sound you hear, which generally sounds sharper/clearer than sitting further away.
Keith

 

[Basic Channel]

Yes I would compare left and right channels and then adjust any really obvious imbalance, obviously the nearer you sit the more direct sound you hear, which generally sounds sharper/clearer than sitting further away.
Keith

Thank you for putting up with my questions. I appreciate you've forgotten more about speakers than I will ever know, etc. Whereas I have never heard that many speakers, and in various homes I have been in, I am struggling to think of more than one that had them set up appropriately distanced from the side walls.

 

[Purité Audio]

I would never profess to be an expert at anything, often in domestic shared space there is no option but to place speakers close to walls.
An off-axis FR that does not mirror the on axis will colour the sound of the speaker.
Firstly you need to see the anechoic/klippel measurements of the speakers which is not always possible.
If the off-axis is ragged then some absorption might be a benefit, I would just ask what ( in terms of frequency) and why ( do I need absorption) the companies that make the stuff will. happily sell you product.
REW and a microphone is always the first step, actually REW’s ‘room simulator ‘ feature or Amroc online room mode calculators are probably the very first step.
Keith

 

[Basic Channel]

I wasn’t emptily trying to butter you up, just pointing out the truth between our respective experience (or lack of)

At least we’ve all answered the OPs question “why can’t this be easier”, just not as the op wished or intended

 

[goat76]

REW and a microphone is always the first step, actually REW’s ‘room simulator ‘ feature or Amroc online room mode calculators are probably the very first step.

Those room simulators can certainly help get us a fairly close estimation of the behavior of the room acoustics, but even if we make pretty accurate measurements of the physical size of the room, it can still be somewhat off compared to the actual acoustical space of the room depending on the material the walls are made of, windows and doors into the room, and what type of floor we have.

REW and a microphone will likely give us the most accurate data, and if we want to be certain of getting all things accurately done we probably still have to do most of these things manually and do the math. It would have been cool if a single program could do this, but by doing it ourselves we at least learn something in the process.

The Filter Impulse Response widow in REW is very useful for this task. It's fairly easy to measure the extra traveling distance from all of the room boundaries to the spot of the microphone at the listening position. 1ms = 34cm

This is how the reflections look like in my room for the left speaker:

 

[Purité Audio]

I agree but while you are waiting for the microphone to arrive, provided you have a tape measure you can begin to gather useful information.
Keith

 

[dasdoing]

a good old technique is to measure the delay of the reflection in the IR, convert it to distance...and then fix a string between LP and tweeter that is calculated amount longer than the space between the two. you can then take some part in the middle of the string and move it to the surfaces until it is fully stretched. that point is the (or one of them) point where it was reflected.
this could theoretically be done automatically in a digital 3D model. I think most of the time there will be only a single point, unless we are talking about a center speaker.