# SPL Drops when I add Mains (LR) to my Sub



## Philm63 (Jan 1, 2015)

Why does the SPL drop when I measure my mains with the sub versus just the sub? Below is a graph showing this effect:









Same mic location (MLP), same settings in the AVR, same volume, same everything. What gives?


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## tonyvdb (Sep 5, 2007)

I believe that a phase mismatch will cause this.


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## Philm63 (Jan 1, 2015)

Doesn't Audyssey account for this when calibrating? These measurements were post-Audyssey, as is my usual protocol. 

...or does Audyssey not phase-match mains to subs...?


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## willis7469 (Jan 31, 2014)

I wonder if it could just be the overlay spacing. This is adjustable. If you look at the sweeps separately is the spl the same? I also considered phase, but it seems too consistently separated. 
:don't know:


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## Philm63 (Jan 1, 2015)

Naw, each sweep, when looked at independently, shows the same dB spread as the two combined on one graph - clearly a 4 to 5 dB drop. I suppose it's not really a big deal as I've got plenty of headroom - I can just turn it up!

The engineer in me just wants to know the mechanism behind the difference, that's all.


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## willis7469 (Jan 31, 2014)

Agreed. I would want to know too.


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## primetimeguy (Jun 3, 2006)

As mentioned, probably phase alignment issue. Try increasing and decreasing the sub distance in the AVR 1ft at a time and measure each to find the distance that gives the best response and most output


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## Philm63 (Jan 1, 2015)

Being still new to all this, perhaps my expectations of what Audyssey is supposed to do are set too high. Wanting to give it the benefit of the doubt, I am of the assumption Audyssey is phase-aligning the system correctly, but this assumption appears to be flawed; I imagine there are more variables that I'm not thinking about yet. There is much to learn, still.

What I'm starting to see is room for improvement over what room correction can offer - with ears and graphs required to go that last step toward perfection, or at least as close as one cares to take it.

I'll experiment with the timing on the sub - it has a delay function built-in - and I'll also toy with the distance setting in the AVR and run more graphs to see if I can get better agreement between the "with mains" and the "without mains" SPL.


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## tesseract (Aug 9, 2010)

You got a leg up on most Audyssey users by checking it with REW, Philm63. Good job! Audyssey is a wonderful tool, but rarely gets it right the first time.


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## kingnoob (Mar 12, 2013)

tesseract said:


> You got a leg up on most Audyssey users by checking it with REW, Philm63. Good job! Audyssey is a wonderful tool, but rarely gets it right the first time.


Yeah I had to run Xt32 twice to get the sound I wanted, too bad new onkyos no longer have Xt32. Accidentally had the ceiling fan on first time it ruined mic readings.


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## tesseract (Aug 9, 2010)

kingnoob said:


> Yeah I had to run Xt32 twice to get the sound I wanted, too bad new onkyos no longer have Xt32. Accidentally had the ceiling fan on first time it ruined mic readings.


I often forget to turn my central air off. Of course, even if it hasn't come in an hour or so, it reminds me by turning in the middle of running the DSP.


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## Philm63 (Jan 1, 2015)

Ok, so I ran some more sweeps this morning, and got some unexpected results. 

We all thought it would likely be just a phase mismatch between the sub and the mains causing the SPL to differ by 4 dB between measurements with just the sub, and measurements with the sub and mains.

So I changed the sub distance in the AVR, 1 foot at a time, 4 feet in either direction, and learned something new. The only place that was affected by these changes was the crossover region - as I virtually moved the sub away from or closer to the MLP, the FR just in the crossover region got further and further away from the initial measurement that Audyssey set for me.

This seems to be consistent with what is now my new understanding of phase relationships between subs and mains. This relationship appears to be important primarily at the crossover region and has little effect anywhere else. 

For giggles, I am attaching a couple of this morning's graphs with and without smoothing (1/6 smoothing for clarity). Each graph shows 3 measurements; sub only, mains only, and sub with mains.

















This still does not explain why there should be a drop in measured SPL when the mains are added to the sub measurement. Of course I'll keep digging.


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## boober (May 12, 2010)

Why not just let the subs do all the work?

It's been awhile since I've looked into this sort of topic before, but odds are your subs are going to be much better at handling the low stuff then your mains are. Unless your mains are actually better than your subs are. Then maybe you just might want to get rid of your subs? 

Anyone else?


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## gdstupak (Jul 13, 2010)

How about adjusting the phase control knob on the sub amp, if it has a control knob?

As a side note... I also use my mains full range with my subs because I like to get as much bass head room as possible. But...using my phase control knob never gets all of the sub frequencies to match with the mains, a separate equalizer helps to get the stray frequencies in line with each other.


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## Philm63 (Jan 1, 2015)

Well, even though Audy sets my Chane A5rx-c's at 40 - I still run them back up to 80 and let my one sub do the heavy lifting - it's quite capable, me thinks. 

What throws me for a loop is why, when I measure just the sub, and then measure the sub with the mains, the graphs come in around 4 or 5 dB's apart. I'd expect them to be pretty much the same, provided my phase is matched between the sub and mains - which I've confirmed - it's spot on.

I have to believe it has something to do with how the sound interacts between the sub and the mains in my large open space - perhaps with the mains in the measurement, there is some slight interference that cancels some of the energy... or...? Strange. I could see this happening in the crossover region, but below 20? My mains don't go there, not with any authority anyway, so why would the graph be affected?


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## willis7469 (Jan 31, 2014)

I still think it's in rew more than an actual measurement. Not sure how though lol. Maybe john or Wayne could answer that. I know those guys know rew inside and out. Maybe run the 2 different sweeps and watch with a handheld SPL meter. I would think it'd show any discrepancies. Just a long shot. It just seems too consistently different to be phase to me.


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## Chromejob (Feb 19, 2015)

I don't see a problem, frankly. The curves look good with the sub involved, and the 2-3 dB variance would be hard to hear. I'd chalk it up to the measurement and move on.


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## Lumen (May 17, 2014)

FWIW, I find that amplitude differences like that can attributed to slightly different reference levels from one run to the next. Once REW levels are set with "Check Levels," all volume controls are in calibrated position and should not be changed.


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