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Bass arriving out of phase at the listening position

edechamps

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So riddle me this:

flfr.png


The above graph shows the response at my listening position from my left speaker (red), right speaker (green) as well as the response of both playing at the same time (blue). All graphs use 1/12 octave smoothing for readability.

Ignore the large null on the right channel at 112 Hz - that's not what I want to talk about. (Yes, it's a problem, but not one I've been able to solve thus far.)

What's driving me nuts is the summation - or lack thereof - between the left and right speakers around 90 Hz.

Mathematically, this can only be explained by the sound arriving at the listening position out of phase between the left and right channels. And indeed:

flphase.png

frphase.png


There is a whopping 187° difference between the left and right channels at 90 Hz. (Note: these measurements were made using the REW acoustic timing reference feature to ensure this comparison is valid.)

Computing the vector vs. RMS average of the two channels yields a result that should come as no surprise at this point:

averages.png


And if we compute the difference between the two, we can see that this problem results in an eye-popping 6-12 dB destructive interference between the two channels across an entire octave:

vecrms.png


This leaves me somewhat flabbergasted. I've calibrated quite a few systems before, and I've never seen bass frequencies get out of phase like this. I just assumed there was some kind of physical law that somehow guaranteed that, as long as the speakers were at equal distance/delay (and in this case they are), in a typical room, bass frequencies would be in phase and would sum constructively. I guess that means my room is atypical in a particularly weird way…

This is a big problem because I'd expect most recordings to assume coherent bass summation, but that's not the case in this system.

I have no idea what's causing this in this particular room. I've tried various experiments (including covering an entire wall with bass traps) with no success. Sometimes I manage to change the responses of the left and right channels somewhat, but no matter what I do, they always stay out of phase.

The only solution I could find was to work around the issue using EQ allpass filters to restore proper phase - so that, after EQ, the summed response is consistent with the response of left and right channels. That works, but obviously a lot of finesse is required in designing the EQ filters. Someone who only has access to "basic" PEQ or doesn't know how to manipulate phase using allpass filters would have no chance of getting this right. They would have to make a carnelian choice between having a correct response when individual channels are playing, or the correct response when both channels are playing. And that's assuming they even spot the problem in the first place. I really hope my case is some kind of isolated extreme scenario - if it's not, then I guess we'll have to revise standard calibration procedures!

I'm not quite sure what I'm hoping to get out of this thread, but I guess it could serve as a warning to watch out for this problem. Obviously, if anyone has any theories or possible acoustical solutions (besides manipulating phase via EQ which is what I'm already doing), please shout!

Additional information

Pictures of the audio system, room and listening position:
1a.jpg 1b.jpg 1c.jpg

The speakers are Genelec 8030As. The squares/rectangles on the walls are RealTraps Fat MondoTraps. (They were positioned for optimal frequency response as empirically determined through measurements.)

Before you ask:
  • Did you try removing/moving the bass traps or the desk? Yes. It doesn't make any difference. It changes the responses somewhat but they are still out of phase by a similar amount.
  • Are you sure one speaker is not wired out of phase? Yes. Also it would be extremely obvious in the above graphs if that was the case. The sound is only out of phase between 60-120 Hz, not elsewhere.
  • Are you sure it's not a problem with one of the speakers? Yes. I checked for that by swapping the front pair for the rear pair. The results are exactly the same.
  • Are you sure it's not the DAC/some DSP/etc.? Yes. I tried a different DAC and cable. The results are exactly the same.
  • Did you try moving the speakers or the listening position? No, because unfortunately I can't really do that. As you can see on the pictures it's a small room and the current layout is the only one that's practical.
  • Maybe you can work around the problem by crossing the sub above the problematic region? I tried that, but I have to cross the sub so high (~110 Hz) that it becomes audibly localizable and the sound stage gets pulled towards the sub, which is located in the front left corner to avoid room modes.
  • Maybe a cardoid bass speaker like the Dutch & Dutch 8C could help? Maybe. I'm sceptical because even the D&D 8C is still omnidirectional at 90 Hz. The Kii Three seems to fare a bit better, maybe it's worth a try, I don't know. Given how much these speakers cost it's quite the risky bet!
 
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ernestcarl

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I just assumed there was some kind of physical law that somehow guaranteed that, as long as the speakers were at equal distance/delay (and in this case they are), in a typical room, bass frequencies would be in phase and would sum constructively. I guess that means my room is atypical in a particularly weird way…

Assuming it's not a speaker fault/unit difference thing, I think it's likely just room-induced boundary interference phase (or some combo with modal stuff) issues.

If the room acoustics and arrangement is the best it can be/cannot be improved upon much further, then EQ can be quite an effective "band-aid" tool, if you will. Apart from HP-LP and all-pass filters, one could also use independent FIR phase EQ to gently nudge one or both channels' phase response to be more aligned with each other.

For example, one of my DSP presets for my desk monitoring setup uses 83Hz all-pass filters to align the front mains' phases with the subwoofer's (kinked phase response which is caused by the room and positioning). Alternatively, one could "correct" the phase of the subwoofer itself to avoid applying all-pass filters on the mains -- to avoid increased group delay.

*BTW, you might want to apply some frequency dependent windowing and/or the 'alignment tool' in your measurements when comparing phases and magnitudes just to make it more readable.
 

Duke

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I don't know what the exact distances are in your room, but it looks to me like the front-wall bounce, rear-wall bounce, AND ceiling bounce could all be arriving at about the same time, resulting in a fair amount of cancellation at the listening position. If your side-wall reflections are arriving in the same ballpark timeframe, they could contribute to the cancellation.

If the dip moves or goes away at significantly different microphone locations, then it's probably a room-interaction effect.

If you cannot move things around, you might try adding a bass source which covers that frequency region whose output arrives at a significantly different time, like perhaps a subwoofer along the front wall, and then adjust its level, low-pass frequency, and phase and/or delay to fill in the dip as best you can.
 
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edechamps

edechamps

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Thanks for the thoughtful answers.

Is that a floor to ceiling window on one side? Makes the room very asymmetric at low frequencies.

Yes it is. I'm curious, why are floor to ceiling windows special in that regard? Does glass behave differently from drywall at low frequencies? (The window was sealed shut during the measurements.)

I don't know what the exact distances are in your room, but it looks to me like the front-wall bounce, rear-wall bounce, AND ceiling bounce could all be arriving at about the same time, resulting in a fair amount of cancellation at the listening position. If your side-wall reflections are arriving in the same ballpark timeframe, they could contribute to the cancellation.

So if I understand correctly, what you're saying is that there might be a lot of reflected sound coming from one speaker, and by sheer misfortune all these reflections might interfere constructively with each other but (because they are delayed) interfere destructively with the direct sound of the speaker. As a result, at the listening position the sound field is dominated by this reflected sound, which is delayed and thus out of phase. Okay I guess that makes sense. I'm a bit surprised I never had this problem before but maybe I was just lucky…

If you cannot move things around, you might try adding a bass source which covers that frequency region whose output arrives at a significantly different time, like perhaps a subwoofer along the front wall, and then adjust its level, low-pass frequency, and phase and/or delay to fill in the dip as best you can.

Understood. One thing that is especially painful in my situation is that most of the problem is around that 80-100 Hz region. This frequency range is a nightmare to work with because (1) if I try to fix it with the sub then I end up crossing too high and the sub becomes localizable and (2) room treatment is ineffective at 100 Hz and below. I guess if I use multiple subs then localization might be less of an issue simply because there are several sources…
 
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edechamps

edechamps

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3 different stereo systems:

Assuming you used a proper timing reference in these measurements (otherwise the phase relationships and vector sums are invalid), it looks like your 3 systems sum correctly. Which is consistent with my past experience - I've calibrated multiple systems before this one and this is the first time I come across this phenomenon. I suspect it's rare, but I think it's important to keep an eye for it because it can result in an effective bass frequency response that is much worse than what individual channels would suggest.
 

RayDunzl

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The above graph shows the response at my listening position from my left speaker (red), right speaker (green) as well as the response of both playing at the same time (blue). All graphs use 1/12 octave smoothing for readability.

I have a 48Hz dip, the rear of the room is closed to the right and open to the left.

The standing waves become 180 degrees out.

I never noticed it until measuring.

When the bass signal is stereo it tends to disappear. The bass has different phase between the speakers in that case.

Maybe I "hear" the direct wave with music, to explain why I don't notice it.

Maybe those fundamentals aren't often played, or the harmonics trick me into "hearing" the fundamental.

1663459107074.png


You attached 1/12th smoothing - what does it look like unsmoothed?

Mine, unsmoothed, left, right, both:

1663458648707.png


187 degree difference at the measured point

1663458915968.png
 

DWPress

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My guess would be room issues as well, it looks pretty squarish and small so lots of possible reflection and summation.

In the past I used a small sub, a cheap 8" Yamaha, to fill in a void in a similar region. If it has a steep HP and LP filter on either side of your problem octave and you dial in the EQ and phase it should stay well controlled even at higher SPL. This worked for me to compliment an 18" sub before I figured out my room better and purchased the SVS.
 

Inner Space

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I'm curious, why are floor to ceiling windows special in that regard? Does glass behave differently from drywall at low frequencies? (The window was sealed shut during the measurements.)
Glass is different than drywall, yes. It's about as reflective as smooth concrete, which is bad at HF, but - depending on pane size - it's also tympanically resonant, which can be helpful at LF, as bass trapping. It's not entirely inconceivable that your problem is caused by a resonance in the glass that reaches Xmax and returns out of phase with the incoming pressure.
 

Duke

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Yes it is. I'm curious, why are floor to ceiling windows special in that regard? Does glass behave differently from drywall at low frequencies? (The window was sealed shut during the measurements.)

In addition to what @Inner Space said, windows can behave like "soft spots" in the room boundaries, such that at low frequencies the modal behavior is as if that room dimension is longer, to a first approximation.

So if I understand correctly, what you're saying is that there might be a lot of reflected sound coming from one speaker, and by sheer misfortune all these reflections might interfere constructively with each other but (because they are delayed) interfere destructively with the direct sound of the speaker. As a result, at the listening position the sound field is dominated by this reflected sound, which is delayed and thus out of phase. Okay I guess that makes sense. I'm a bit surprised I never had this problem before but maybe I was just lucky…

Try a significantly different microphone location. If the dip shifts, then it's a room interaction issue. And then buy a lottery ticket, just in case you're due for some good luck to balance things out.

One thing that is especially painful in my situation is that most of the problem is around that 80-100 Hz region. This frequency range is a nightmare to work with because (1) if I try to fix it with the sub then I end up crossing too high and the sub becomes localizable and (2) room treatment is ineffective at 100 Hz and below. I guess if I use multiple subs then localization might be less of an issue simply because there are several sources…

Along the front wall and approximately centered between the speakers, a single subwoofer should be relatively benign as far as the soundstage goes. Two subs, on the left and right walls, have the potential to increase the sense of spaciousness.
 

ernestcarl

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Along the front wall and approximately centered between the speakers, a single subwoofer should be relatively benign as far as the soundstage goes.

This is pretty much the reason why I’m able to get away with a relatively large xo overlap between mains and my sub. However, if the sub position is too similar to the mains, say, moving horizontally only in the middle front and floor by the looks of the physical constraints in the room pictures, then the sub is likely going to have very similar problem dips and peaks as the mains. Multiple subs (and optimized positioning) may be better able to offset the issues more in this regard…
 

Chromatischism

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Assuming you used a proper timing reference in these measurements (otherwise the phase relationships and vector sums are invalid), it looks like your 3 systems sum correctly. Which is consistent with my past experience - I've calibrated multiple systems before this one and this is the first time I come across this phenomenon. I suspect it's rare, but I think it's important to keep an eye for it because it can result in an effective bass frequency response that is much worse than what individual channels would suggest.
I think Duke is on the case due to your seating location. You need to input your room dimensions into a room mode calculator.
 

Burning Sounds

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I don't know what the exact distances are in your room, but it looks to me like the front-wall bounce, rear-wall bounce, AND ceiling bounce could all be arriving at about the same time, resulting in a fair amount of cancellation at the listening position. If your side-wall reflections are arriving in the same ballpark timeframe, they could contribute to the cancellation.

If the dip moves or goes away at significantly different microphone locations, then it's probably a room-interaction effect.

If you cannot move things around, you might try adding a bass source which covers that frequency region whose output arrives at a significantly different time, like perhaps a subwoofer along the front wall, and then adjust its level, low-pass frequency, and phase and/or delay to fill in the dip as best you can.

Yep, as a longtime dipole user this is very typical of front/rear wall cancellation.
 
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edechamps

edechamps

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I have a 48Hz dip, the rear of the room is closed to the right and open to the left.
The standing waves become 180 degrees out.
I never noticed it until measuring.
When the bass signal is stereo it tends to disappear. The bass has different phase between the speakers in that case.

Right, it's the same issue. Glad I'm not the only one. Though in your case you are lucky enough that it's below 80 Hz so it should be a relatively easy fix - all you need is a sub. I'm not so lucky.

Maybe I "hear" the direct wave with music, to explain why I don't notice it.

I doubt it. My understanding is that, in bass frequencies, we don't distinguish between direct and reflected sound. The (magnitude) frequency response is pretty much what you hear (which is why EQ works).

the harmonics trick me into "hearing" the fundamental.

Could be.

You attached 1/12th smoothing - what does it look like unsmoothed?

Here you go:

unsmoothed.png


I might do some more experiments with microphone/speaker locations. So far I can see two potential solutions:
  1. Using subwoofer(s) crossed high, placed strategically such that the direction of incoming sound is roughly consistent with the mains.
  2. Using a cardioid bass speaker like the Kii Three to avoid reflections. Might not do much given the Kii Three's 90° response is only down ~2 dB at 90 Hz. Very expensive proposition.

By the way I'm also attaching the full REW mdat (see attached zipfile) in case someone wants to have a closer look…
 

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abdo123

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@JohnPM Sorry to barge into this thread but are there any plans in REW to support variable FDW? like 1 cycle at 10KHz to 20KHz but 15 cycles at 20Hz to 40Hz with a smooth (hopefully user defined) slope in between?
 

JohnPM

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@JohnPM Sorry to barge into this thread but are there any plans in REW to support variable FDW? like 1 cycle at 10KHz to 20KHz but 15 cycles at 20Hz to 40Hz with a smooth (hopefully user defined) slope in between?
No plans for that. Please use the REW forum for such questions.
 
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