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Help me understand my REW measurements

Sparky

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Hi all, I hope you are all well?

So I bought some new speaker stands for my Genelec 8341A's which required me to carry out a new GLM calibration and REW sweep for my subs and crossover.
I ran the GLM software which, as some of you may know, is a fairly painless procedure and I was over the moon with the individual response of both my 8341A's especially considering my left speaker is close to a side wall.
I decided to run the calibration as individual speakers as my room is not a perfect rectangle and they're not positioned in an ideal way.
Anyway, after storing my new settings to the speakers, I then fired up REW and measured my mains individually to confirm the GLM calibration which was spot on so I then measured both mains at the same time and that's when the fun began........

The frequency response of both mains playing together was absolutely awful with huge cancellations throughout the entire range!
There was an enormous -20dB suck out at 70hz and it didn't get much better from there.
Is this normal? Did I do something wrong?
Can anyone explain this to me.
 

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  • Mains and Subs Full Range.zip
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ppataki

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I have exactly the same issue. I also have an irregular shaped room (six corners, asymmetric).
If I correct the speakers individually they are OK but then when I re-measure L+R together there is a huge dip around 50Hz and around 90Hz
I have tried correcting that and it worked, I got a totally flat response but then ringing became intolerable so now I just live with the dip....
 
OP
Sparky

Sparky

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I have exactly the same issue. I also have an irregular shaped room (six corners, asymmetric).
If I correct the speakers individually they are OK but then when I re-measure L+R together there is a huge dip around 50Hz and around 90Hz
I have tried correcting that and it worked, I got a totally flat response but then ringing became intolerable so now I just live with the dip....
Its funny isn't it. Is it cancellations of the same frequency played at the same time? Does it even matter?
Do you have subwoofers and if so, how are you selecting the correct crossover frequency or do you go off summation of all speakers playing combined?
 

ppataki

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I am 100% sure that it is the room causing this
As far as I remember there are other forum members here with exactly the same issue

Those huge dips appear when the same signal is played from both the speakers at the same time
If you start to rotate the phase of one channel (crossover slope is a good example) you will see that the dips will change
Try playing around with the slopes (12dB/octave, 18, 24, 48, etc) until the dips become more benign (if ever...)
This is what I used to do when I had subs

Now I don't have subs anymore but this:

No need for subs anymore
 

HarmonicTHD

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Hi all, I hope you are all well?

So I bought some new speaker stands for my Genelec 8341A's which required me to carry out a new GLM calibration and REW sweep for my subs and crossover.
I ran the GLM software which, as some of you may know, is a fairly painless procedure and I was over the moon with the individual response of both my 8341A's especially considering my left speaker is close to a side wall.
I decided to run the calibration as individual speakers as my room is not a perfect rectangle and they're not positioned in an ideal way.
Anyway, after storing my new settings to the speakers, I then fired up REW and measured my mains individually to confirm the GLM calibration which was spot on so I then measured both mains at the same time and that's when the fun began........

The frequency response of both mains playing together was absolutely awful with huge cancellations throughout the entire range!
There was an enormous -20dB suck out at 70hz and it didn't get much better from there.
Is this normal? Did I do something wrong?
Can anyone explain this to me.
I run the same system, but with Neumanns ;-)

Anyway, nulls are common in any room for multiple speakers (room modes), just physics. What you can do is play with the sub location to compensate the nulls created by the main speakers. Sub locations to test can be corners, 25%, 50% from either side wall or front wall. To test switch off GLM, run Rew and see where you get the smoothest response, than run GLM measurements. Also the Rew room simulator can give first hints on which positions to test. For additional info see Floyd Tool’s book on Sound Reproduction.

In my living room (Kef) the sub cancels nulls best when placed about 25% along the side wall. In my study (Neumann) it is best about 25% along the front wall.
 

SIY

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Those huge dips appear when the same signal is played from both the speakers at the same time
There's your clue. Do the measurement, then move the mike a couple inches. That interference pattern will change significantly. It's a consequence of mike position. It's devilishly hard to get exact positioning so that arrival times are exactly equal, and in a reflective room, you'll be chasing that forever. So... make your measurements an average of multiple measurements, either with an array of mike positions or using something like the moving mike method.
 

theREALdotnet

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The frequency response of both mains playing together was absolutely awful with huge cancellations throughout the entire range!
There was an enormous -20dB suck out at 70hz and it didn't get much better from there.
Is this normal? Did I do something wrong?
Can anyone explain this to me.

I cannot see any 70Hz suck-out in the measurements you attached. If anything, the two mains both have a very even response and play well together.

The one thing that doesn’t seem to be set up well are the subs. They don’t fill much at all where they would be needed (~ 35Hz and below). They should be crossed over at ca. 35Hz and have their output increased by at least 10dB.

1650109674397.png

(I changed the graph colours around a bit, they were hard to distinguish on my system)
 
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Sparky

Sparky

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I am 100% sure that it is the room causing this
As far as I remember there are other forum members here with exactly the same issue

Those huge dips appear when the same signal is played from both the speakers at the same time
If you start to rotate the phase of one channel (crossover slope is a good example) you will see that the dips will change
Try playing around with the slopes (12dB/octave, 18, 24, 48, etc) until the dips become more benign (if ever...)
This is what I used to do when I had subs

Now I don't have subs anymore but this:

No need for subs anymore
I think you're right, don't think there is much we can do about it really. It's not like we listen to music in pure sinewave form anyhow.

A line array!! wow!! :) I bet it sounds amazing!!!!!
 
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Sparky

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I run the same system, but with Neumanns ;-)

Anyway, nulls are common in any room for multiple speakers (room modes), just physics. What you can do is play with the sub location to compensate the nulls created by the main speakers. Sub locations to test can be corners, 25%, 50% from either side wall or front wall. To test switch off GLM, run Rew and see where you get the smoothest response, than run GLM measurements. Also the Rew room simulator can give first hints on which positions to test. For additional info see Floyd Tool’s book on Sound Reproduction.

In my living room (Kef) the sub cancels nulls best when placed about 25% along the side wall. In my study (Neumann) it is best about 25% along the front wall.
Hehehe. The old Genelec/Neumann rivalry.... :)

The problem I have is that my speakers and subs are set up in a living room (lounge) environment so subwoofer placement is incredibly limited as is speaker placement. I have to work with what I have available to me I'm afraid.
My room is ppataki's in that it is not a rectangular so the room mode calculator won't help in my scenario.
 

ernestcarl

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I cannot see any 70Hz suck-out in the measurements you attached. If anything, the two mains both have a very even response and play well together.

The one thing that doesn’t seem to be set up well are the subs. They don’t fill much at all where they would be needed (~ 35Hz and below). They should be crossed over at ca. 35Hz and have their output increased by at least 10dB.

View attachment 200214
(I changed the graph colours around a bit, they were hard to distinguish on my system)


1650116674912.png


Subs and LR mains were time-aligned and subsequently vector averaged. One can compensate for the cancellation to some reasonable degree by overlapping with the sub(s). Moving away, however, from the MLP or where you align the monitors will change the response / location of the dips.
 
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Sparky

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There's your clue. Do the measurement, then move the mike a couple inches. That interference pattern will change significantly. It's a consequence of mike position. It's devilishly hard to get exact positioning so that arrival times are exactly equal, and in a reflective room, you'll be chasing that forever. So... make your measurements an average of multiple measurements, either with an array of mike positions or using something like the moving mike method.
I think doing an average is a great idea as my head isn't in a vice when I'm listening so why would I measure to just one location!
I'll definitely try this next time! I just have to now learn how to align two subs in a multiple measurement situation. :D
 
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Sparky

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I cannot see any 70Hz suck-out in the measurements you attached. If anything, the two mains both have a very even response and play well together.

The one thing that doesn’t seem to be set up well are the subs. They don’t fill much at all where they would be needed (~ 35Hz and below). They should be crossed over at ca. 35Hz and have their output increased by at least 10dB.

View attachment 200214
(I changed the graph colours around a bit, they were hard to distinguish on my system)
Hmmm. There was definitely as large suck out at around 70hz when I measured then directly. I wonder what you're seeing as my subs combined play to around 17hz? I like what you've done though, I never use Psychoacoustic filtering but I may start doing that to get a better idea of how I actually hear the sound......
 

ernestcarl

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I think doing an average is a great idea as my head isn't in a vice when I'm listening so why would I measure to just one location!
I'll definitely try this next time! I just have to now learn how to align two subs in a multiple measurement situation. :D

I first would study the IR and/or step responses and phase plots -- are both the same polarity and relative start position in time? -- overlay windows can help here. Or simply use REW's auto 'Align IR start' button in the All SPL tab. Another way to more easily visualize both magnitude and phase at the same time is through the 'Alignment tool' window.

1650117571579.png
 
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Sparky

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View attachment 200220

Subs and LR mains were time-aligned and subsequently vector averaged. One can compensate for the cancellation to some reasonable degree by overlapping with the sub(s). Moving away, however, from the MLP or where you align the monitors will change the response / location of the dips.
Wow. Can I ask what alignment settings brought you to the above response? That looks real good!!

I clearly have a LOT to learn here.
 

ernestcarl

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Wow. Can I ask what alignment settings brought you to the above response? That looks real good!!

I clearly have a LOT to learn here.

You just select the responses whose start time you want aligned in the All SPL tab and click on the 'Align IR start' button in the options tab for that window.
 
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Sparky

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I first would study the IR and/or step responses and phase plots -- are both the same polarity and relative start position in time? -- overlay windows can help here. Or simply use REW's auto 'Align IR start' button in the All SPL tab. Another way to more easily visualize both magnitude and phase at the same time is through the 'Alignment tool' window.

View attachment 200230
I have no idea at how to do any of this I'm afraid. :(

There's so much information out there that it's difficult to know which is useful to my situation and which isn't so I end up watching numerous YouTube videos and not actually getting anywhere! :D
 
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Sparky

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You just select the responses whose start time you want aligned in the All SPL tab and click on the 'Align IR start' button in the options tab for that window.
Ok, I'll have a play around with it to see if I can learn something. Thank you!
 

ernestcarl

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I have no idea at how to do any of this I'm afraid. :(

There's so much information out there that it's difficult to know which is useful to my situation and which isn't so I end up watching numerous YouTube videos and not actually getting anywhere! :D

I believe some of the alignment video tutorials in youtube do not even bother with the phase response. Much of what I know was kind of learned also from pro audio folks using smaart. Phase and IR time are always part of the picture when setting up a system, and not just in the background sideline.
 
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Sparky

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Quick question for you guys:

I've seen multiple ways of measuring speakers using a UMIK-1 and REW but which is the preferred way?
So normally, I don't change any of the REW settings and allow it to continue in it's default form at -12 dBFS and level match my mains and subs with the volume knob of my minidsp SHD and the SPL meter built into REW.
I've seen some videos where people have set the Level to -20dBFS but why would you need to do that?
Also, I saw a guy who set his AVR to 0dB and set REW level to -30dBFS.

Which method is the optimal or does it really not matter?
 
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Sparky

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Currently reading this:


Hopefully, I'll get a grip on this! :)
 
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