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Discussion Starter #1
Folks, check this out. Attached is the final filtered response for my room and speakers. The top blue line is the combined response with both speakers, the red line is the left speaker only, and the green line is the right speaker only. Overall I am happy with the graph aside from the various nulls I have. For the most part, the combined graph shows where I have dips in either the Left or Right speakers. What really got me perplexed is the monster null at around 80hz which seems to show up out of no where. As you can see in either of my individual speaker graphs, there is no dip at 80hz.

Curious to know what the cause may be?

Total FINAL Response.jpg
 

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I'm no expert & I'm sure someone with more REW knowledge will chime in shortly, but if I had to guess...I'd say it is a phase issue between the two speakers. Just a guess.
 

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Try adding and subtracting a couple feet incrementally to the subwoofer distance and see if it changes. It looks like a phase issue between the subs and mains. Or try adjusting the phase control on the subwoofer. Re-sweep and compare results. Adjust accordingly. Disclaimer: also not an expert.


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Discussion Starter #4 (Edited)
Thanks guys...I think I may have found something. I was looking at an older graph of my right speaker taken about a week ago before I repositioned it. It shows the same massive, and steep null at 80hz. But I thought I remedied this by moving the speaker and by also closing the door to my listening room. As you can see from the graphs I included above taken just yesterday, the right speaker no longer has this null. I have taken many many measurements of my right speaker since I moved it and starting closing the door to my listening room and REW does not show this 80hz dip any more.

In this graph, the blue line is my current room response with both speakers, and the green line is just my right speaker from a few days ago.
80hz Null.jpg
 

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I'm no expert & I'm sure someone with more REW knowledge will chime in shortly, but if I had to guess...I'd say it is a phase issue between the two speakers. Just a guess.
I’d tend to agree. Low-frequency nulls are typically a function of the distance between the speaker and a boundary. However the signal also “sees” another low frequency source as a boundary. That would explain the appearance of a null when both speakers are played together.

Regards,
Wayne

 

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Discussion Starter #6 (Edited)
I’d tend to agree. Low-frequency nulls are typically a function of the distance between the speaker and a boundary. However the signal also “sees” another low frequency source as a boundary. That would explain the appearance of a null when both speakers are played together.

Regards,
Wayne
Did more testing....using REW signal generator and the SPL meter, I tested 70, 80, 90 hz signals for each speaker and combined. At 70 and 90hz, each speaker and the combined response of both speakers measures above 74db. As suspected by others in this thread, at 80hz, even though each speaker measures around 73db individually but when I measure both there is huge null. So, the question is...is there anything I do about this? Unfortunately, in my little room I do not have a lot of freedom to move things around.
 

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Adjust the phase of one of the speakers in small increments & measure the effect. Or both in different directions.
 

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Discussion Starter #8 (Edited)
deleted due to my stupidity

EDIT..ok maybe not so stupid. I think there is something wrong with either my mic or REW. I just did 2 measurements and the 80hz dip was gone. Took another one and it came back.
 

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Discussion Starter #9
Right now at 5:30pm I can measure both speakers outputting 80hz. REW SPL meter reads 74.3.
 

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Discussion Starter #11
No change to system, now at 7:40pm both speakers measuring null at 80hz again, verified with my own ears at listening position.

How is it possible that there is sometimes a null at 80hz and sometimes there isn't? For the past hours I've no problems getting 74db measured at 80hz. Now all of sudden there is a null??? I am losing my mind.
 

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Discussion Starter #12 (Edited)
UREKA!!! I finally started to figure out whats going on! Sort of.

Let me explain my setup, I use Jriver as the output device. I use Jriver's convolution so that I can test out my filters to see how they actually perform in my room instead of relying on the predicted curve in REW. In my room, the left speaker as a massive +10db peak at 77hz and the right speaker has a similar peak at 62hz. When I create a filter to flatten out these peaks so they match my target curve, I have to use a -10 gain with a BW60 of 8-10. The result is a very nice flat curve for the left and right speaker. However, when I measure with both speakers I get inconsistent results. Sometimes I get the massive nulls precisely at where I had created a filter to deal with the large peaks for the left and right speaker. Other times I get a smooth curve with no nulls but with a large trough between 60-80hz. When I do a measurement with convolution turned off in JRiver the nulls at 62hz and 77hz goes away.

So clearly Jriver's convolution engine is doing something strange with the filter's I created. Why would I get a flat curve when only measuring 1 speaker but get inconsistent results with both speakers? Also, when I do get measurements that result in a trough versus 2 nulls, the trough actually dips below the db level of each individual speaker. How is that possible?

Here are some pics that may help.

This are two different measurements of both speakers, with Jriver's convolution engine using the same filters. Notice in one measurement there are two nulls at 62 and 77hz but with a good response in between. In the other measurement, there is a wide trough.
Both speakers filters applied.jpg

Here is a measurement of both speakers with JRivers convolution engine turned off (no filters applied).
No filters both speakers.jpg

Here are measurements of each speaker and combined. Notice how the trough in measurement for both speakers actually extends below the db level of each individual speaker in the 60-80hz range.
Each speaker and both.jpg
 

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Discussion Starter #13
More testing....when I use the Trace Arithmetic feature in REW and sum the filtered Left and Right response, I get the combined response with the two dips at 62 and 77hz (BTW I am amazed at how closely the predicted sum matches one of the actual measured response). So my question is why do the big dips occur precisely at the center of the large negative gain I applied to the Left and Right filters but these dips do not show up in the measurement of each individual speaker? Since my Left and Right speakers do not show the big dips at where I applied the filters, are these dips showing up in the combined response real or just some glitch in REW?

Trace Arithmetic Sum of Left and Right.jpg
 

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Tonto was on point (see Post 2).
Room modes and reflections impact the phase of the sound at various frequencies of each speaker. Nulls will occur at frequencies related to major room modes at certain locations in the room. The 2 speakers are impacted differently in all but the most symmetrical room setups. When both speakers are active with a different phase response, the phase interaction of the 2 results in a different net response. This is very common and can impacted by significant changes to the location of the speakers, the LP, or significant changes to the room.
 

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Discussion Starter #15
Tonto was on point (see Post 2).
Room modes and reflections impact the phase of the sound at various frequencies of each speaker. Nulls will occur at frequencies related to major room modes at certain locations in the room. The 2 speakers are impacted differently in all but the most symmetrical room setups. When both speakers are active with a different phase response, the phase interaction of the 2 results in a different net response. This is very common and can impacted by significant changes to the location of the speakers, the LP, or significant changes to the room.
Thank you and I totally understand the phase explanation. What I am not understanding is why I am getting different measurements even though nothing has changed?
 

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Discussion Starter #16
More testing...I manually entered the REW filter settings into JRMC parametric eq and reran the measurements. The combined graphs show the double dip at 62 and 77hz (precisely where I have my negative gain filter on the R and L speaker respectively). The measurements for each speaker is also very similar to when I use JRMC convolution engine. Therefore, I have to believe the inconsistent results of the combined measurement has to do with the JRMC convolution engine.

Regarding the phase...I noticed that when I have all filters off and just measure individual frequencies in the range of 60-80hz, the measured combined output is about 4-5db less than the individual levels. For example, at 70hz the left and right speakers measure 77db but the combined output is 73db. So does this mean I have a slight phase issue in this range? If so, is there a way I can fix it?
 

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In general using IIR EQ filters; the smoother the SPL, the smoother the phase will be. Room modes and strong reflections are special cases and nulls due to those cannot be effectively addressed with EQ.

I understood the original question to be;
Q; why is there is a SPL difference when both speakers are active?
A; Added speaker interaction; Phase differences. You are now good on that one.

Q; Why different results when the EQ is applied?
A; EQ changes the SPL / Phase so an SPL / phase /interaction change is expected.

Q; Why different results when nothing is changed.
A; Something has changed, otherwise the results would be the same. EQ is a change, but also the change may be in the room setup, the measuring system or possible the REW settings used for the analysis.

Q; Regarding the phase...I noticed that when I have all filters off and just measure individual frequencies in the range of 60-80hz, the measured combined output is about 4-5db less than the individual levels. For example, at 70hz the left and right speakers measure 77db but the combined output is 73db. So does this mean I have a slight phase issue in this range?
A; Yes, a phase issue.

Q; If so, is there a way I can fix it?
A; Possibly, but not likely with EQ, as this is usually the result of room modes or strong reflections.

Sharp nulls normally do not sound as bad as the charts look. Most of us live with some of them as we are not willing to take the more difficult steps needed to avoid them.
 

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Discussion Starter #18
In general using IIR EQ filters; the smoother the SPL, the smoother the phase will be. Room modes and strong reflections are special cases and nulls due to those cannot be effectively addressed with EQ.
What do you mean "smoother" SPL? You mean smooth response curve? That is what I've been trying to do! :)


Q; Why different results when nothing is changed.
A; Something has changed, otherwise the results would be the same. EQ is a change, but also the change may be in the room setup, the measuring system or possible the REW settings used for the analysis.
I am telling you, nothing changed. I made a measurement, left for about 30 mins, came back, took another measurement and the response was dramatically different. I have to believe something is wonky with JRMC convolution.

Q; If so, is there a way I can fix it?
A; Possibly, but not likely with EQ, as this is usually the result of room modes or strong reflections.
This is actually good news. I was hoping I could attempt to remedy the problem with some more room treatments.

Sharp nulls normally do not sound as bad as the charts look. Most of us live with some of them as we are not willing to take the more difficult steps needed to avoid them.
100% agree here. Deep, narrow nulls do not bother me as much, so long as I do not have a lot of them. In my case, with the nulls at 62 and 77hz, these are caused by the large negative gains I had to apply at peaks. When i remove those particular filters, the two deep nulls go away but of course that leaves two massive peaks. Hopefully I can reduce the peaks a bit with some room treatments.
 

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jtalden; Sharp nulls normally do not sound as bad as the charts look. Most of us live with some of them as we are not willing to take the more difficult steps needed to avoid them.[/QUOTE said:
Agreed. They usually don't cover a wide enough frequency range to make a huge difference. Everyone's mileage may vary of course.




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Diffusion will help as long as it is not the speaker to speaker distance that is causing the null. You might also consider a resonant panel for the ceiling. How much toe in are you using?
 
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