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Discussion Starter #1
My center channel sits on the top shelf of an A/V rack beneath my display about 2.5 feet from the front wall.

Running full system sweeps per pg.100 of the REW Step by Step Guide, I generated the graphs below for the center only, and for center+sub. There's a large dip centered around 100Hz that I think is due to SBIR; [ fc = c/4dwall = 112.5Hz ], so that would need to be addressed first.

Any help to understand or solve these issues are greatly appreciated!
  1. What are the reasons for the dips close to 200Hz and 400Hz?
  2. Why does the FR have a rising trend? Room gain?
  3. Does sub phase need to be adjusted to provide good SPL support through the xover region for both the mains AND the center simultaneously? IOW, if it's good for the mains, is it good for the center?
  4. Would an external time delay (my AVR disables them when using Dirac) make my sub play nicely with both mains and center?
Layout 1 FR - C Only (-12dB In).jpg Layout 1 FR -  CenterSub.jpg
Layout 1 FR - LR Only.jpg Layout 1 FR - LRSub.jpg
 

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Sub level looks a little low. Where are you crossing mains to the sub? If it's 90-100 then the dip there could absolutely be the culprit.

200 and 400 could be SBIR from front wall, side walls, ceiling, etc.
 

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Discussion Starter #3
Sub level looks a little low.
Thanks for your quick response, Bryan! The sub level for the Center+Subs measurement is low because I needed to reduce REW's input level to avoid clipping.
Where are you crossing mains to the sub?
Good question! The measurements were taken with auto-EQ disabled, so I thought bass management was as well. Now I'm unsure how my processor handles crossovers. I do know that its Dirac Live LE feature uses non-adjustable 80Hz crossovers - yes, that's a peculiarity of Dirac Live for Emotiva (LE). So I looked it up on pg.53 of the manual, and found that when certain playback modes are active, so is bass management. This simple functionality didn't occur to me before, so I have no idea at what point the crossovers were set! I will need to look them up in the processor's menu system when I get home.
 

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Thanks for your quick response, Bryan! The sub level for the Center+Subs measurement is low because I needed to reduce REW's input level to avoid clipping.

Good question! The measurements were taken with auto-EQ disabled, so I thought bass management was as well. Now I'm unsure how my processor handles crossovers. I do know that its Dirac Live LE feature uses non-adjustable 80Hz crossovers - yes, that's a peculiarity of Dirac Live for Emotiva (LE). So I looked it up on pg.53 of the manual, and found that when certain playback modes are active, so is bass management. This simple functionality didn't occur to me before, so I have no idea at what point the crossovers were set! I will need to look them up in the processor's menu system when I get home.
I think he meant the sub level itself is low.
 

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Yes - exactly. The sub isn't balanced to the mains.
 

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Just look at the general level compared to 80 and above.
 

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That's a mystery to me, and I believe you, but how can you tell without an mdat or a sub-only graph?


This was kinda my question in the other thread. You mentioned reducing the sub level for REW , and it looked like a rise up to 70ish. Curious, why not just lower the MV?
 

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Discussion Starter #10
This was kinda my question in the other thread. You mentioned reducing the sub level for REW , and it looked like a rise up to 70ish. Curious, why not just lower the MV?
Either will work.
I'm getting old... my memory fails me. I thought I had reduced the soundcard's input level to avoid clipping, not the sub's output, but I can't find the post now. I'm making such a big deal about it because I'm thinking I should have taken all measurements over again, not just CENTER+SUB. Then the relative levels between graphs would synch up.

But you're not talking about relative levels between graphs, are you? I think I finally get it - you're saying the sub's level should be brought up to match the mains, correct? I was not aware of this system tuning step, so I always left it up to auto-EQ. Yet it makes sense to correct as much as possible before auto-EQ to reduce audible artifacts resulting from too many filters.
 

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Discussion Starter #14 (Edited)
Where are you crossing mains to the sub? If it's 90-100 then the dip there could absolutely be the culprit.
200 and 400 could be SBIR from front wall, side walls, ceiling, etc.
I checked the processor's settings. All speakers are crossed at 80Hz and are set to "small." To these novice eyes, it doesn't appear that subwoofer phase is the issue. Crunching some more numbers for center channel half-wave center frequency:
  • 2.5ft = Distance to front wall --> fc = c/2dpath = 1125/2(5) = 113Hz
  • 3.3ft = Distance to side wall --> fc = 85Hz
  • 1.8ft = Distance to floor --> fc = 156Hz
Layout 1 FR - C Only (-12dB In).jpg Layout 1 WF - C Only.jpg

Hmmm... doesn't seem to be any correlation between the graph and center channel HWCF. Maybe I should be looking at the glass as half full. IOW, pay attention to the peaks instead of dips? My limited knowledge of acoustics leaves me grasping at straws. Given that the center can only move backward about half a foot, what would you do if this were your room? Are there any GIK products that could help? Am I obsessing over center channel performance to the exclusion of the rest of the system?
 

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Discussion Starter #15
AudiocRaver made me aware of the half-wave cancellation frequency in another thread. I found a formal definition here and repeated it for convenience below.

Speaker boundary interference does not only apply to your front wall. Bass waves also reflect off your ceiling, side walls and rear wall, causing comb filtering when they combine with the direct sound from your speakers. In this case we need a more general formula to calculate the cancellation frequencies.

Half wavelength cancellation frequency:

fc = c / 2 x ( dreflect - ddirect )

Where:
fc is the center frequency of the cancellation notch.
c is the speed of sound (343 m/s, or 1125 f/s).
dreflect is the distance of the reflected path from your speaker to the listening position.
ddirect is the distance of the direct path from your speaker to the listening position.
 

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Correct. SBIR can be from any of those surfaces. The sub level still looks low to me. I would raise it 4-5 db and re-evaluate.
 

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Discussion Starter #18 (Edited)
Correct. SBIR can be from any of those surfaces. The sub level still looks low to me. I would raise it 4-5 db and re-evaluate.
These measurements were taken with the utmost of care. That still doesn't mean they're right, though! Much different from my original measurements in post #1. I'm not sure what to think now that a large dip's appeared from about 22-45Hz. Should I maybe try again for repeatability? That can't sound good with half of the low bass missing in action! It doesn't even look like the same speaker to me!

Frequency Response: Center Only (Compare 0 Traps with 2 Traps)
Layout 4 FR (0T2T5T) - C Only.jpg

Frequency Response: Center+Sub (Compare 0 Traps with 2 Traps)
Layout 4 FR (0T2T5T) - C S12.jpg

Resonance: Center+Sub (Compare 0 Traps with 2 Traps)
Layout 4 WF (0T2T) - C S12.jpg

Resonance: Center+Sub (Compare 0 Traps with 5 Traps)
Layout 4 WF (0T5T) - C S12.jpg
 

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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. 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.
 

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Bryan, sorry to interject but I'm curious why you suggest raising output in the middle of the dip. Assuming it worked, would raising the sub level as you suggested and then EQing down the peak starting around 45hz be as good or a better alternative? Was there something else you were trying to learn by seeing if the dip raises? Also, is the waterfall showing the peak at 45hz pointing to a room mode?
 
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