The reason I ask relates to the relatively large physical spacing between some of your drivers, which based on pictures I've seen I suspect is around 3 feet between the lowest of the four woofers and the tweeter. On my speakers, also, the two woofers are a significant distance (about 15 inches) above and below the two tweeters, which in turn are about at listening height.
The reason I started thinking about that is it occurs to me that the greater the physical separation between drivers, the greater the distance should be between the speakers and the measurement microphone, which in turn (assuming the speakers are not measured outdoors) will necessitate shortening the duration of the measurement window (prior to arrival of the first reflections), which in turn will raise the minimum frequency that should be corrected and/or reduce the accuracy of the corrections.
The reason I'm envisioning for that is not related to off-axis dispersion of the drivers, since the mic is placed at the level of the drivers which presumably have the narrowest dispersion (i.e., the tweeters). What I'm envisioning is that with the mic placed at tweeter level, the closer it is to the speaker the greater the difference will be between the distance from mic to tweeter and from mic to other drivers. And if the drivers are widely spaced, the amount of that path length difference will be significantly different than the difference between those path lengths as they exist at the listening position, due to the shallower angle between those drivers as viewed from the listening position.
In other words, it seems to me that if drivers are spaced relatively widely, and the mic is not moved correspondingly further away from the speakers during the speaker calibration process (with the downside of shortening the "window," and hence the accuracy and/or low frequency extension of the corrections), the speakers may be corrected for a timing error that won't exist at the listening position.
I've done some geometric calculations for the 15 inch distance between the woofers and the tweeters on my speakers. At a 4 foot measurement distance the path length differential between the distances of the mic to the tweeters and the woofers is 0.18 feet. At my 11.5 foot listening distance that differential is only 0.06 feet. The difference between those differences is 0.12 feet, corresponding to a propagation delay at the speed of sound of about 0.11 ms (milliseconds). Which would seem to mean that the DEQX will correct for a 0.11 ms timing error that won't exist at the listening position, if my speakers are measured at a distance of 4 feet, and a somewhat larger error than that in the case of your speakers.
The planes of the baffles on my speakers, btw, are such that the woofers are mounted a little forward of the tweeters and mid-ranges, presumably to help with time alignment. But that is unrelated to the point I am describing.
Also, to provide a bit of perspective on a 0.11 ms timing error, that would be readily perceivable on the step response graphs JA provides with his speaker measurements in Stereophile, those graphs having a time scale of 1 ms per major division. One of the purposes of those graphs being to provide some idea of the time coherence or lack thereof of the speaker.
I understand what you are getting at, Almarg.
These exact considerations are important when we are setting up a time-coherent loudspeaker in a listening room - the distance of the listening position relative to the driver plane is important such that the drivers integrate at the listening position to avoid a separate-tweeter-separate-woofer effect. I know that Green Mtn Audio had a heck of a time with the review magazines who never did understand the concept & almost always put the mic at the tweeter level & to their self-created dismay found that the drivers did not integrate & that the time-coherent speaker was not what the manuf advertised!! I understand that it took an enormous amt of effort on Green Mtn Audio's part to educate the reviewer. That's why if you look at the measurements in Stereophile of any Green Mtn Audio measurements they look terrible - they were mostly all done incorrectly! But the effect was devasting to the business, as you can imagine.
So, I agree that you would need to push the mic further away based on the driver vertical separation BUT you run the risk of measuring reflected sound as well. I suppose that's why the manual recommends mattresses/cushions/blankets in between the mic & speaker. Maybe what's better is using some room reflection treatment material like the Owen Corning 703/705 sheets? A royal PITA but maybe worth the effort esp. if outdoor measurements are a no-go for you?
I suppose you are shortening the measurement window to avoid catching the reflected sound?
It's a trade-off (like all of engineering!! ;-))
In your calc - the error is 3:1 - 0.18' at the measuring distance & 0.06' at your listening position. that's a pretty big error looking at it in absolute terms but...
Maybe that's not much of an issue? If I understand this correctly, the human ear cannot tell an echo (reflected sound) if the reflected sound is less than 1/15 of a second (& 0.11mS is much less than that) but, as Drewan77 stated, you could end up with a "hollow" sound if you measure indoors due to partial reflections. I also suppose that measuring outdoors is better because you have a perfect absorption environment - no echoes....
