Spires rear distance placement with room treatments...

[MPS]

While the principles are always valid, real world acoustics has so many variables that generalization is pretty hard to do

We could say that direct sound must have much higher amplitude than reflected sound which need to be enough delayed and reduced to make optimal environment for (stereophonic) music listening. How to reach these goals depends on each individual room and speaker acoustic properties.

 

[Rich]

My only comment IMO is that while the back wave focuses the sound (opposite the front) the angle is slight, so the focus point would likely be much farther than most rooms' front wall to the speaker panel (10+ feet?). So if anything, being close to the front wall allows a diffuser behind it to diffuse the vast majority of the wave since it's too close for it to become a single vertical line.

I have to disagree with this. The panels on my Summit have a pretty strong curve. I just did some ballpark measurements and I would guess the rear wave converges to a vertical line somewhere between one and two feet behind the panel, probably around 15 inches would be my guess. Any of you mathematicians want to figure it out exactly, it shouldn't be too hard. At any rate, I would guess that if 15" is correct, then it would take another 15" for the wave to expand back to as wide as the summit panel. Thus, at 3ft from the front wall (very common setup with ML's) the back wave is still pretty tightly focused and a well diffuser behind it is going to be less effective. At five feet, however, I would expect the wave to be at least as wide as a standard diffuser panel. So you get two things benefitting you at five feet vs. three. You get a greater time delay between primary and secondary wavefronts, and you get greater efficiency out of your well diffuser panels.

I have been testing this lately in my own room. With absorption, I had the speakers three feet from the front wall with no issues (other than smaller soundstage). With the well diffusers behind the Summits, I notice a huge difference in the quality of the sound when going from three to four and then to five feet. For whatever reason 60" seems to be the magic number.

 

[Gordon Gray]

I notice a huge difference in the quality of the sound when going from three to four and then to five feet. For whatever reason 60" seems to be the magic number.

Agreed my friend.

Distance I used for some 20 years plus with four different ML models.