REW + multisub - my journey begins - Page 4 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 08-29-09, 07:00 PM
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Re: REW + multisub - my journey begins

LOL – brucek, I don’t think we every got on the same page here! Sure, two signals of equal amplitude will combine for a 6 dB increase in SPL, and that 6 dB total gain reduces proportionally if one of the signals is at a lesser amplitude. I never had an issue with that.

But when you introduced the normalizing here, I got the impression you were saying that combined response of two unequal signals would be lower than the louder signal, even though it should be obvious that this will probably never be the case. Perhaps you can see where I got that impression:
Quote:
brucek wrote: View Post
So for a 67dB and a 79dB signal mixed = 20LOG(10^(67/20)+10^(79/20)) = 80.95dB

Remember that we turned the signal down after the two were combined by 6.02dB to normalize them.

If we do this with the 80.95dB signal, it becomes 74.83dB result.

All I was ever saying is that combined (summed) response of two signals that each singularly have their own peaks and valleys will track the combined peaks and ignore the dips, plain and simple. Which is pretty much the same thing you've concluded (only stated a different way):
Quote:
So, I conclude that if a sub has a dip and another has a peak at the same frequency, that the second sub will indeed help that dip....

Here’s the three-trace left/right/combined graph once again, this time without the 3 dB offset of the combined trace. As you can see, and in keeping with your calculations, it tracks highest where the signals are identical, and less so where one is greater than the other (primarily at ~80 Hz and ~>100 Hz. But the combined signal never drops below any peak. Which has been my point all along.

Greg, not trying to be rude by not responding to your post, but it appears the whole thing was the result of confusion caused by the graph I presented with the shifted combined trace. I only shifted the trace to better show that it was precisely tacking the peaks in the 80 Hz and 100 Hz regions where the signal levels were unequal. I thought (foolishly perhaps) that that might be less than obvious if the trace was floating some "distance" above the L/R traces. Sorry 'bout that...

Regards,
Wayne

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Old 08-29-09, 09:51 PM Thread Starter
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paulspencer

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Re: REW + multisub - my journey begins

Out of curiosity I decided to trace Wayne's measurement to test the idea that the peak curves determine the curve. I traced whichever peak was the highest, then offset to match the combined measurement, so that we can see how close the two are. In looking at it I could see this seemed true in some places but not others.

From 20 - 70Hz it proves true almost exactly. Above it deviates a little and below quite a lot. I wonder if phase enters the equation at these points.
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Old 08-30-09, 07:28 PM
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Greg

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Re: REW + multisub - my journey begins

Quote:
Wayne A. Pflughaupt wrote: View Post
Greg, not trying to be rude by not responding to your post, but it appears the whole thing was the result of confusion caused by the graph I presented with the shifted combined trace. I only shifted the trace to better show that it was precisely tacking the peaks in the 80 Hz and 100 Hz regions where the signal levels were unequal. I thought (foolishly perhaps) that that might be less than obvious if the trace was floating some "distance" above the L/R traces. Sorry 'bout that...
No problem. I worded my post poorly... it was certainly not my intention to imply that yours was misleading intentionally... I'm not usually that rude...
Quote:
paulspencer wrote: View Post
Out of curiosity I decided to trace Wayne's measurement to test the idea that the peak curves determine the curve. I traced whichever peak was the highest, then offset to match the combined measurement, so that we can see how close the two are. In looking at it I could see this seemed true in some places but not others.

From 20 - 70Hz it proves true almost exactly. Above it deviates a little and below quite a lot. I wonder if phase enters the equation at these points.
This is enlightening indeed... it occurs to me that at ~92Hz, where it seems to track the two theories the least, that there's an original dip in BOTH signals... I believe this would be consistent with a room mode... If so, would that explain why the combined signal there is so, um, wack?
Then there's the vy low end response... Is that real response down there or is that below the useful range of your (Wayne's) equipment and just a trick of the calibration files?
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Old 08-30-09, 09:11 PM
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Re: REW + multisub - my journey begins

Quote:
Is that real response down there or is that below the useful range of your (Wayne's) equipment and just a trick of the calibration files?
The SPL meter's calibration file, I think. brucek knows for sure, I forget...

Regards,
Wayne

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Old 08-31-09, 06:29 AM
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brucek

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Re: REW + multisub - my journey begins

Quote:
Is that real response down there or is that below the useful range of your (Wayne's) equipment and just a trick of the calibration files?
Yeah, as the response drops into the noise, the calibration files increasing offset causes an upturn in the graph that looks like usable response, but it is not... It's just noise.

brucek
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