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-   -   Trying to Fix Center+Sub FR (https://www.hometheatershack.com/forums/home-audio-acoustics/166274-trying-fix-center-sub-fr.html)

bpape 03-08-17 08:06 AM

Re: Trying to Fix Center+Sub FR
 

Yes - that's correct.

willis7469 03-08-17 09:28 AM

Re: Trying to Fix Center+Sub FR
 

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

Lumen 03-09-17 06:21 AM

Re: Trying to Fix Center+Sub FR
 

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

Lumen 03-09-17 01:15 PM

Re: Trying to Fix Center+Sub FR
 

2 Attachment(s)
Quote:

bpape wrote: (Post 1561938)
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

Attachment 138265 Attachment 138273

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?

Lumen 03-10-17 09:44 AM

Re: Trying to Fix Center+Sub FR
 

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.

bpape 03-10-17 09:51 AM

Re: Trying to Fix Center+Sub FR
 

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.

Lumen 03-14-17 10:59 AM

Re: Trying to Fix Center+Sub FR
 

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

Lumen 03-21-17 01:59 PM

Re: Trying to Fix Center+Sub FR
 

4 Attachment(s)
Quote:

bpape wrote: (Post 1563481)
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)
Attachment 139658

Frequency Response: Center+Sub (Compare 0 Traps with 2 Traps)
Attachment 139554

Resonance: Center+Sub (Compare 0 Traps with 2 Traps)
Attachment 139562

Resonance: Center+Sub (Compare 0 Traps with 5 Traps)
Attachment 139570

bpape 03-21-17 02:11 PM

Re: Trying to Fix Center+Sub FR
 

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.

John Stewart 03-21-17 08:29 PM

Re: Trying to Fix Center+Sub FR
 

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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