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How to look for room modes in REW measurements

CleanSound

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Hi All,

I've played around with REW many years ago, but I am now just getting serious into REW after spending tons on a decent mic and audio interface (I know I don't need to, but I was already buying it for something else).

I must say that this software is very sophisticated and it's not for the faint of hearts and what makes it even more challenging to master is all the things you have to learn about acoustics.

Anyways, I suspect that I have some nasty room modes and I'm trying to figure it out from the graphs. I've must have watched 5 or 6 YouTube videos but I am not getting the very specific information I need and was hoping some of you can give me some pointers by either giving me feedback on what you see in the below graphs or linking me to a detail video or article that explains how to look for room modes/standing waves.

Having said that, my understanding is that you need the waterfall or spectrogram to look for room modes. So below are mine. [All measurements are measured with a calibrated soundcard and calibrated microphone at listening position. The room is treated with general room acoustic guidance that I have read online.]

Looking at the graphs, my interpretation is that I have two nasty room modes at 53hz and 76hz, but when both speakers are playing, everything just gets nasty below 100hz. Would love hear your feedback on what you think.

EDIT: User error with the original graphs posted, please see updated graphs here.
 
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Purité Audio

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Often left and tight channels display peaks at the exact same frequencies , you can check with an on line room mode calculator and the frequency generator in REW.
Keith
 

Keith_W

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Beware that not all "fingers" on a waterfall plot are due to standing waves. That 50Hz "finger" which does not drop in amplitude could be a ground loop or mains noise, for example. And I know of someone who went nuts trying to diagnose a 1kHz "resonance" which turned out to be a computer fan. That 50Hz peak does not seem to decay even at 1000ms, which makes me suspect that it is not a standing wave. Most likely it is mains noise.

For further reading about room modes, I refer you to Chapter 8 of Toole. If you buy the Kindle version, it is quite inexpensive and really good value. It is written in an approachable style which should be easily understood by anybody with a high school education.

It is a bit much to summarize his entire chapter into one ASR post. Very briefly, if you take a single dimension of your room, it will support a standing wavelength of twice the room dimension. This is a first order room mode. If we assume that your 50Hz peak is real, we input that into an online sound frequency-wavelength calculator, and we find the wavelength to be 6.86m. This may correspond to a room dimension of 3.43m, 6.86m, 10.29m, and so on. A room that supports 1st order room modes will also support 2nd, 3rd, 4th and so on - so if it supports 50Hz, it will also support 100Hz, 150Hz, 200Hz, etc. so you look for peaks at those frequencies as well (which I do not see on your graph, btw).

You could also use the room simulator in REW to predict where the modes will be. Compare that to your measurements, and it should tell you what is responsible for what.

Trying to understand all this is a challenging, but laudable task.
 
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CleanSound

CleanSound

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That 50Hz peak does not seem to decay even at 1000ms, which makes me suspect that it is not a standing wave. Most likely it is mains noise.
I could do a sweep starting at 60 hz and see if I pick up anything below 60.

Curious, any idea how would a main noise make it's way into of my amp and at this level when I don't hear it in my music?

This is going to be months of learning and playing around with REW to master it enough to feel confident in my measurements before I take tangible action to correct things.
 

KSTR

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Anyways, I suspect that I have some nasty room modes and I'm trying to figure it out from the graphs. I've must have watched 5 or 6 YouTube videos but I am not getting the very specific information I need and was hoping some of you can give me some pointers by either giving me feedback on what you see in the below graphs or linking me to a detail video or article that explains how to look for room modes/standing waves.
Why not start with the obvious and listen for room modes specifically?
 
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CleanSound

CleanSound

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That 50Hz "finger" which does not drop in amplitude
Here is a close up with a different angle of that 53hz, it does look like it's decaying.

close up.jpg
 
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CleanSound

CleanSound

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Why not start with the obvious and listen for room modes specifically?
Let me give this bad boy a read.

Like I mentioned earlier, I'm just starting to get deep into room acoustics and REW. This is probably one of the harder topics in this hobby.
 

KSTR

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@CleanSound ,

Please take all the time you need, and listen to the test signals several times, getting used to the patterns (also, if possible, with headphones). You will have "booms" and suckouts at various frequencies -- also with extreme sensitivity on actual listening position -- and once you know these frequencies, it is much easier to interpret what you see in REW plots. Note that these plots can change shape dramatically depending on the parameters used.
 

olieb

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Here is a close up with a different angle of that 53hz, it does look like it's decaying.

View attachment 358248

I do not think this is hum as 53 Hz just does not fit. But this plethora of modes with very slow decay looks weird. I would not trust this waterfall too much, as @KSTR mentioned it is complicated and can be misleading in too many ways.
How does the FR look like?
And if your room happens to be rectangular I found "Room sim" in REW a very good tool to inspect bass response.
 

olieb

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Left speaker FR (1/6 smoothing)
That is already a lot of smoothing. What window setting do you use?
The 53Hz mode produces more magnitude here than in the waterfall. The ≈78Hz mode is there too. But the magnitudes do not look too extreme, but that can be a result of smoothing and windowing.
What is weird in the waterfall is the super high Q of the mode and at the same time only moderate boost. But with a microphone position close to a node of the mode that might happen, or something else is going on. Complicated.
 

Ron Texas

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I have an 8 Db peak at 60 hz and 5 Db at 128 hz. Those look like room modes to me.
 

GaryY

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Could it help if you swap L and R channel to see if peak is from room or amp-speaker_
 

ZolaIII

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This is exel table for virtual bass area optimisation in FIR filters it works pretty much the same as bass trap or opposite phase firing sub's with moderate results of course. Table is useful to show you where their should be for first three peeks and two deeps so you can easily identify them in measurements. It does calculate averaging for a first one so correcting it lowers the impact it other peeks and deaps behind it. Needless to say it's determined the most by the length of the room. REW simulator also does deacent job showing you where and what to expect.
 

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Purité Audio

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I am sure you didn’t but the microphone wasn’t moved in between measurements?
Keith
 
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CleanSound

CleanSound

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I am sure you didn’t but the microphone wasn’t moved in between measurements?
Keith
And the room is far from a "normal" quadrilateral. It is a shared space between my home office and listening room.
 
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