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method to identify room modes
Hi all...been reading & researching for a year & a half now, and just recently finished the basement so I've started calibrating my subs...i'm in the process of trying to identify my room modes so that I can place the subwoofers in best locations for minimal eq necessary with the BFD and to eliminate nulls at the seating positions...
this is my room layout (home theater/media room in lower left, or south west corner):
![Green Technology]()
The media room is closed off from the rest of the basement, with framed drywall walls all around. the south and west walls have concrete block behind the framed walls, the walls are all filled with insulation. concrete floor with carpet.
I've run the spreadsheets to identify what room modes I theoretically should encounter, and had some ideas to combat those modes with 2 svs 25-31 pci's...but what I'm finding is that some modes are showing up, and some aren't, and some are coming out of nowhere...
These are the predicted room axial modes:
![Text Line Font Parallel Design]()
And this is the process I've followed so far, please correct me if I'm misunderstanding something...
1. Put 1 subwoofer in southeast corner, put spl meter close to floor in north east corner (near 2 closed doors)
2. Ran 15-200 hz sweep with REW
I see easily recognizable 19, 38, 57, and 77 hz modes/peaks for sure…but the puzzling thing is that there’s a 31-32 hz NULL in the corner…and it seems to remain in other measurements also when I move the mike out to seating positions too…
I will try to post REW graphs soon, but for now wanted to see if I’m doing something wrong, or if I’m misguided…how can you get a null in a corner? That’s my main question right now…
this is my room layout (home theater/media room in lower left, or south west corner):
The media room is closed off from the rest of the basement, with framed drywall walls all around. the south and west walls have concrete block behind the framed walls, the walls are all filled with insulation. concrete floor with carpet.
I've run the spreadsheets to identify what room modes I theoretically should encounter, and had some ideas to combat those modes with 2 svs 25-31 pci's...but what I'm finding is that some modes are showing up, and some aren't, and some are coming out of nowhere...
These are the predicted room axial modes:
And this is the process I've followed so far, please correct me if I'm misunderstanding something...
1. Put 1 subwoofer in southeast corner, put spl meter close to floor in north east corner (near 2 closed doors)
2. Ran 15-200 hz sweep with REW
I see easily recognizable 19, 38, 57, and 77 hz modes/peaks for sure…but the puzzling thing is that there’s a 31-32 hz NULL in the corner…and it seems to remain in other measurements also when I move the mike out to seating positions too…
I will try to post REW graphs soon, but for now wanted to see if I’m doing something wrong, or if I’m misguided…how can you get a null in a corner? That’s my main question right now…
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Here is my graph of the corners measurement...
![Text Line Plot Purple Font]()
I ran this with REW & the sub calibrated to 80 hz baseline, using the REW pink noise output.
to me, it sure looks like the axial length 19, 38 & 57 hz modes are all showing, along with the axial width 44 hz mode. But what are the sharp valleys in there - they sure look like nulls to me, around 56, 77 (shouldn't that be a peak instead, from the axial length modes?!).
Is it possible to get nulls in a corner? Do you think that's what those are, those sharp valleys, or are those just what it's supposed to be at, and the modes are so pronounced that the parts in between appear to be valleys...
Based on these mode measurements, I had wanted to put the subs at the 1/3 & 2/3 positions along the length wall, based on a suggestion from Ed Mullen, but when I did that and mic'ed the corner again, it didn't really eliminate the 19 hz bulge, and I still got a 31-32hz null, coming out of nowhere...will have to redo that measurement and post the graph to show you.
I ran this with REW & the sub calibrated to 80 hz baseline, using the REW pink noise output.
to me, it sure looks like the axial length 19, 38 & 57 hz modes are all showing, along with the axial width 44 hz mode. But what are the sharp valleys in there - they sure look like nulls to me, around 56, 77 (shouldn't that be a peak instead, from the axial length modes?!).
Is it possible to get nulls in a corner? Do you think that's what those are, those sharp valleys, or are those just what it's supposed to be at, and the modes are so pronounced that the parts in between appear to be valleys...
Based on these mode measurements, I had wanted to put the subs at the 1/3 & 2/3 positions along the length wall, based on a suggestion from Ed Mullen, but when I did that and mic'ed the corner again, it didn't really eliminate the 19 hz bulge, and I still got a 31-32hz null, coming out of nowhere...will have to redo that measurement and post the graph to show you.
The accuracy of the predicted room modes is based on rectangular room dimensions, once the room deviates from a rectangular shape out goes the accuracy! It takes allot more math for oddly shaped rooms, which is why I suggested going straight for the seating location 
Is that with both subs in the two corners? That’s some pretty weird-looking response. Almost looks like some kind of comb filter happening. Have you tried taking a reading of each separately. It would help for comparison’s sake. As brucek eloquently put it in a recent thread, the only real reason to separate subs would be to achieve a combined response that you can’t get from each separately.Here is my graph of the corners measurement...
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Regards,
Wayne
Well I have found 2 locations that seem to give pretty good response over the 8 seats. I'm putting one sub along the bottom wall, in the middle. The other sub is going along the east wall, about 38" off the corner.
I have measured, and remeasured...I loved the response I was getting with both subs by themselves, but when I added in the main speakers, it threw things off a little...so I adjusted them slightly...
This is what I'm getting now, pre BFD. The charts will start at the leftmost seat on the couch, and go right. 8 seats total...
Left 4th seat out:
![Text Purple Line Plot Pink]()
left 3rd seat out:
![Text Blue Line Plot Diagram]()
left 2nd seat out:
![Text White Line Blue Red]()
left middle seat:
![Green Text White Blue Line]()
right middle seat:
![Green Text White Line Diagram]()
right 2nd seat out:
![Text Pink Line Plot Purple]()
right 3rd seat out:
![Text White Line Blue Plot]()
right 4th seat out:
![Text Blue White Line Plot]()
Can you guess which seat I'll be sitting in?
Next I'll guess I'll measure each sub separately at all seating locations, try my hand at some filters on each sub, and then remeasure together.
Oh, another thing. The eastmost sub is set at 130 degrees phase while the one in the middle is set at 0 degrees phase. the couch is pulled 13" off the wall.
I have measured, and remeasured...I loved the response I was getting with both subs by themselves, but when I added in the main speakers, it threw things off a little...so I adjusted them slightly...
This is what I'm getting now, pre BFD. The charts will start at the leftmost seat on the couch, and go right. 8 seats total...
Left 4th seat out:
left 3rd seat out:
left 2nd seat out:
left middle seat:
right middle seat:
right 2nd seat out:
right 3rd seat out:
right 4th seat out:
Can you guess which seat I'll be sitting in?
Next I'll guess I'll measure each sub separately at all seating locations, try my hand at some filters on each sub, and then remeasure together.
Oh, another thing. The eastmost sub is set at 130 degrees phase while the one in the middle is set at 0 degrees phase. the couch is pulled 13" off the wall.
here's a graph showing specific null frequencies I found when I had just 1 sub in the corner, measuring along the length of the room (the longest dimension)...they occur at specific locations that just happened to be in bad seating locations.
![Text Line Plot Design Diagram]()
here are the height nulls I found, when the sub was on the floor, and the mic was at 51, 57, and 58" from the ceiling.
![Text Line Plot Design Slope]()
the width nulls didn't really line up with the theoretical predicted nulls at locations expected, these occured at 96 & 99" from the south wall, along the east wall. the sub was at the southeast corner.
![Green Text Line Plot Diagram]()
I tried putting the subs at the 1/3 & 2/3 locations along the length at first, it didn't seem to help much. Then I tried a sub in the corner and one in the middle of the length to combat the huge 19 hz hump. Then I tried one in the middle of the east wall, and the other middle of the bottom...didn't like that.
I might also try 1 sub at 96/99" along the east wall, and the other in the middle of the length to see if that gives even flatter response...I had to pull the east sub away from the corner, and the couch away from the wall or there was a major hump around 44 hz...
It's really hard to explain all the different combinations that I've measured, but I thought I'd try to explain a little in case others were wondering...
here are the height nulls I found, when the sub was on the floor, and the mic was at 51, 57, and 58" from the ceiling.
the width nulls didn't really line up with the theoretical predicted nulls at locations expected, these occured at 96 & 99" from the south wall, along the east wall. the sub was at the southeast corner.
I tried putting the subs at the 1/3 & 2/3 locations along the length at first, it didn't seem to help much. Then I tried a sub in the corner and one in the middle of the length to combat the huge 19 hz hump. Then I tried one in the middle of the east wall, and the other middle of the bottom...didn't like that.
I might also try 1 sub at 96/99" along the east wall, and the other in the middle of the length to see if that gives even flatter response...I had to pull the east sub away from the corner, and the couch away from the wall or there was a major hump around 44 hz...
It's really hard to explain all the different combinations that I've measured, but I thought I'd try to explain a little in case others were wondering...
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