# Trying to Fix Center+Sub FR



## Lumen (May 17, 2014)

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!
What are the reasons for the dips close to 200Hz and 400Hz?
Why does the FR have a rising trend? Room gain?
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?
Would an external time delay (my AVR disables them when using Dirac) make my sub play nicely with both mains and center?


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## bpape (Sep 14, 2006)

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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## Lumen (May 17, 2014)

bpape said:


> 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.


bpape said:


> 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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## Phillips (Aug 12, 2011)

Lumen said:


> 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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## bpape (Sep 14, 2006)

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


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## Lumen (May 17, 2014)

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


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## bpape (Sep 14, 2006)

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


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## willis7469 (Jan 31, 2014)

Lumen said:


> 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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## bpape (Sep 14, 2006)

Either will work.


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## Lumen (May 17, 2014)

willis7469 said:


> 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?





bpape said:


> 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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## bpape (Sep 14, 2006)

Yes - that's correct.


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## willis7469 (Jan 31, 2014)

Yep. Exactly. It looks like the sub was calibrated 10db lower than the center. There's our mystery rise.


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## Lumen (May 17, 2014)

Thanks for helping me iron that out, guys! Next I'll be on the trail of that dip.


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## Lumen (May 17, 2014)

bpape said:


> 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















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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## Lumen (May 17, 2014)

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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## bpape (Sep 14, 2006)

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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## Lumen (May 17, 2014)

Will do, Bryan. Appreciate the feedback. Have to take care of a few things before I can get back to this.


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## Lumen (May 17, 2014)

bpape said:


> 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)*








*Frequency Response: Center+Sub (Compare 0 Traps with 2 Traps)*








*Resonance: Center+Sub (Compare 0 Traps with 2 Traps)*








*Resonance: Center+Sub (Compare 0 Traps with 5 Traps)*


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## bpape (Sep 14, 2006)

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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## John Stewart (Jul 5, 2014)

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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## Lumen (May 17, 2014)

bpape said:


> 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.



bpape said:


> 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.


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## Lumen (May 17, 2014)

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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## bpape (Sep 14, 2006)

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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## Lumen (May 17, 2014)

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.


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## willis7469 (Jan 31, 2014)

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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## Lumen (May 17, 2014)

Not ignoring you guys... just a little behind schedule!


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## willis7469 (Jan 31, 2014)

It's fine Lou. I'm just over here eating worms.....
?


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## Lumen (May 17, 2014)

A can of which I've opened, no doubt (lol).


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## willis7469 (Jan 31, 2014)

Rofl!!! Yes...yes you did. And a very colorful one at that.


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## willis7469 (Jan 31, 2014)

Gummi worms. My favorite.


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## Lumen (May 17, 2014)

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.


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## Lumen (May 17, 2014)

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.
















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.









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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