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Discussion Starter #21 (Edited)
20-35 you're not going to do much with treatment. Above that it's got to be THICK like our Soffit Traps at 17" and even then it has to be where the problem is happening.
It's not taking long to find that out! My pressure traps aren't making a dent that low, at least not in the front corners where convention says to place them. I'm anxious to track down the problem(s) and fix it(them) to get better bass.

To me sub level still looks about 5 dB low and I'd be worrying about the peak between 45 and 70ish.
Do you have the ability to EQ the sub? Don't do a ton, just hit the middle of that broad null and see if you can change it with EQ. Don't do more than maybe 3 dB.
Okay then, the next REW session will find me raising the sub level and experimenting with that null. It's my understanding that if it reacts to EQ, then it's not a room mode, correct?

EDIT: In reviewing my measurements I found that the dip succumbed to the sub's phase adjustment as shown below. So along with EQ experiments, I'll be sure to check phase settings when remeasuring the Center+Sub response.
Layout 4 PhaseAdj - LR S12.jpg
 

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Discussion Starter #22
Once again I'd like to thank everyone for their interest and help. And so as not to waste your time, I thought I'd give you a heads-up - I'm not really sure if I should even be concerned about Center+Sub issues. I don't recall others working on this speaker/sub combo. I'm just pursuing it because the REW 101 Guide lists the combo as an essential measurement.

Do you know why we look at L+R+Sub rather than L+R+Center+Sub?
 

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If you can EQ out the dip it's going to be easier to raise the broad null than using treatment or positioning. Nulls that wide are almost certainly multiple things going on at the same time and not just a single problem. The peak can certainly be trimmed down. The other option is turn the sub level up and EQ down the 20 and 45ish
 
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Discussion Starter #24
I like that bit of insight! Thank you! I may need to follow an REW EQ path rather than a Dirac one. It seems my system may need special attention using PEQ, something not available with my AVR's version of Dirac. In any case, I played with the sub level and found that it didn't help to fill-in the broad dip as shown below. Tomorrow I'll try experimenting with PEQ as you suggested to see if the dip reacts at all. If not, I should be able to bring down the peaks.

Layout 4 FR - C+Sub (Level Increase Comparison).jpg
 

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Louie!!! Sorry for being awol. Not that I'm terribly valuable lol.
I think if you pulled 43hz down about 7db with an arrow filter and raised 30 with a nice wide one from 25-35 you might be good. Maybe I should open both eyes!
 

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Discussion Starter #31
Well let's see. Where do I start? I guess I've made up my mind to pursue a DIRAC calibration over an REW/PEQ one, so manually manipulating the center's FR is not possible with my AVR. I did measure the effect of the center's LF contour control and expected to see significant LF/MF boost, but the differences only registered fractions of a dB. Lowering the crossover frequency to 60Hz likewise made only a slight difference in measured response. Dialogue clarity still varies with source material, but immersion seems to have collapsed toward the front wall. Hand-off between the front stage and the sub seems artificial and abrupt. Punch is gone again. I might try raising the center's crossover to 100 and remeasure.

Layout 4 FR (6T @60Hz) - C+Sub.jpg
 

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Discussion Starter #32
UPDATE:

After some deliberation, I decided to bring in another bass trap just to prove to myself that many more of the proper type would be needed. As it stands, I only have two 20" dia bass traps and four 16" traps. ASC rates their effective absorption down to 30Hz and 55Hz, respectively, but I think we know that the real-world cut-off is much higher (as evidenced by my LF measurements taken with each trap addition). Here are a couple of comparisons between 0 traps and 6 traps. Note that the crossover was lowered when the 6th trap was introduced, so not apples-to-apples, but it shows that little if any improvement was accomplished.

Compare Traps - C Only (no Dirac).jpg Compare Traps - C+Sub (no Dirac).jpg


So back to the drawing board. Taking Bryan's advice to turn bring up the sub's level worked fine during manual calibration, but was defeated by a Dirac calibration as shown below. It's interesting to note that although the level was reduced below crossover, the Dirac calibration smoothed the broad 25Hz to 40Hz dip. Unfortunately, the sub did not provide good SPL support through the 60Hz xover region - I had to compromise on a phase setting that gave best combined support for both C+Sub and L+R+Sub. Luck was not on my side, as the best SPL support for those speaker groups occurred at opposite ends of the dial.

Compare Dirac - C+Sub.jpg

I still plan to implement Bryan's other idea of trying to raise that broad dip. Until now, it's been a decision between using manual PEQ in combination with REW-generated filters, and using no manual PEQ with Diarc Live LE. But I think I found a way to sneak a manual PEQ setting past Dirac: my processor offers two PEQ tone control trims, one for HF and one for LF. I should be able to dial-in enough boost at the appropriate fc and Q to observe or rule-out an effect. More to come!
 
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