Originally Posted by mthomas47
That's interesting! When you moved the speakers, did they get further away from a boundary wall, either behind or beside them? I would have expected the speakers to have moved closer to side walls if they moved outside the subs. Of course, the cabinets of the PB16's would also work as boundary walls, for the purposes of boundary reinforcement for the woofers in your towers, so perhaps the towers moved further away from the PB16's.
In any event, what was happening before probably didn't involve your towers mutually-coupling at their previous position. As I understand it, bass transducers (either speakers or subwoofers) can mutually-couple at frequencies where they are within 1/4 wavelength of each other. A 70Hz wavelength is approximately 16' long, so in order to mutually couple at 70Hz (and above), two transducers would have to be within 4' of each other. And, as you correctly assumed, that would add +6dB to their combined response.
Of course, that general principle doesn't actually work quite as well with speakers, as it does with subwoofers, since speakers are EQed separately. There wouldn't be an easy way to anticipate the extent of any mutual-coupling with speakers, as there would be with subwoofers which are EQed together. That might depend a bit on the type of room EQ being used, though.
In any case, I doubt that the towers were nearly close enough together to reinforce each other at 70Hz. My guess is that the boundary reinforcement for those speakers changed, if you lost -6dB at 70Hz, when you moved them. Perhaps there is another explanation that we are missing. Room modes for those speakers might have exerted an influence on their volume at that specific frequency. I assume that you reran room EQ after moving things around?
Everything you've said is what I was suspecting as well. The speakers were very close, probably 4.5' apart as I had them set up right next to my 55" which is 4 feet long. Like the comments above I more or less did that because it looked better. Now, both speakers are about 10' from a wall (I'm almost in the middle of the room, thankfully my FR is ok!).
I changed the locations and ran YPAO, which has given fairly consistent results in my room. The only difference in EQ that I noticed was a 1dB boost at 78Hz with a Q of 1.260 on the towers. After YPAO I took a measurement to ensure my FR wasn't messed up. On first glance it looked the same. It wasn't until I overlaid my old measurement that I noticed the loss in SPL.
With the subs on the outside one set of the 8" woofers were firing toward each other (and I assume coupling ~4.5' apart) and the other set was firing toward the PB16 cabinets about 2' away perhaps giving a boundary effect.
I had some time to mess around with it today, and boosted the 1dB PEQ at 78Hz to 5dB. I also changed the subwoofer level to -7 (it usually sets -6.5, but this time it was set to -8). Now the REW graphs are overlapping about as well as I could hope
. So it seems like nothing noticeable was lost in the shape of my FR but somehow I lost some SPL around/after my crossover.
I felt the sound stage was too narrow for movies in my previous position but was a bit hesitant to move anything because I didn't want to mess up my FR for music.
I'm hoping I can get in a serious music/movie listening session tomorrow so I can evaluate the change properly.