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Need help with left-biased soundstage (early reflection?)

edechamps

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Hello,

I have the following near-field setup: (click to enlarge)

PXL_20210109_174450503.jpg PXL_20210109_174505799.jpg

The speakers are Genelec 8030As. The large white and black panels are bass traps; specifically, RealTraps MondoTraps.

My problem is that the soundstage is not properly centered. If I watch a movie, the vocals seem to come from the left of the screen, near the left speaker. This is true regardless of room correction EQ, and even though the measured frequency response seem aligned between both speakers. I tried to isolate problematic frequency ranges using bandpass filters but was unable to (they all seem to lean left).

Experiments show that I need to add about 4 dB gain to the right channel for vocals to sound centered. This seems like a hack though, and even with that in place it still feels like there's more ambience coming from the left.

Now, what strikes me as the most likely cause is asymmetric reflections off the left wall, as there is no wall on the right.

So I figured a simple fix could be to simply put up an additional panel on the left wall:

PXL_20210109_174542751.jpg

To my surprise, this didn't make any significant difference. At best, it might have reduced the imbalance from 4 dB to about 3.5 dB (and that's assuming it's not placebo). I find this odd, given that these panels are supposed to have lots of broadband absorption (absorption coefficient close to 1), especially in localizable frequencies. I don't understand this result.

So now I'm out of ideas. Changing the layout to get rid of the left wall is not an option. I'm wondering if I could fix it using a different type of panel, but I don't really see how, given the one I'm using is already very absorptive. (Maybe the absorption data is misleading with regard to incidence angle?)

In case it's useful, here are (ungated) measurements of the right channel with and without the panel:

panels.png


And both channels with the panel, which shows that the channels are reasonably aligned as far as measurements are concerned:

flfr.png


Any suggestions would be appreciated.
 
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Χ Ξ Σ

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You can use DSP to delay the left channel and see if you like the result. If increasing the inner channel time difference is not enough, then increase the amplitude difference.
 
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Absolute

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Put on some headphones and check if your ears are out of balance. Usually that's the case for most of us, at least over part of the spectrum.
 

RayDunzl

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You can use DSP to delay the left channel and see you like the result.

Or just pull the right speaker closer to you, an inch or two at a time.

Easy "timing" experiment.
 

pozz

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You're getting shortly spaced one bounce (left wall) and two bounce (desk + left wall) reflections.

I assume your listening position is somewhat back from the desk and when you're listening there's no wall to the left of your head. First thing to note is that the white panel (if that's the one you're referring to) doesn't cover all of the left wall. That alone, plus the fact that fabric covered panels are somewhat reflective at HF anyway, and the corner gain up to the low mids, is enough to cause the left bias.

If you lean your head all the way forward, almost touching your computer monitor, how does the left bias change?

Most practical thing to do is to move your desk as much as you can and apply that hack you mentioned.
 

Kvalsvoll

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Or just pull the right speaker closer to you, an inch or two at a time.

Easy "timing" experiment.

This may actually work, at least it is very easy to try.
 

Kvalsvoll

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The absorption is not sufficient to remove enough of the early reflected energy.

If you have dsp in the system, you can fix it by adding some delay to the left speaker, start with 1ms and increase in 1 ms increments. Adding gain to right channel will not work so well, the level difference will be quite high before you manage to shift the image in place, and this will also introduce other problems.
 
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edechamps

edechamps

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Thanks for the many suggestions :)

Naive question, but did you try to swap the speakers L<>R?

I think I did that some time ago. I'll double-check. It would be a huge coincidence if one speaker happens to be lower in level but the FR measurements are still aligned…

Can you add a wall on the right side?

Not really.

You can use DSP to delay the left channel and see if you like the result. If increasing the inner channel time difference is not enough, then increase the amplitude difference.
If you have dsp in the system, you can fix it by adding some delay to the left speaker, start with 1ms and increase in 1 ms increments.

Tried that. +0.25ms on the left channel makes things a bit more centered. More and the soundstage breaks down, as one would expect. Just like level adjustments, this feels like a hack and not fixing the root cause.

Put on some headphones and check if your ears are out of balance. Usually that's the case for most of us, at least over part of the spectrum.

Yeah, I can relate, I've had issues with uneven earwax :( Not the case here though, I checked. Everything feels centered except when played over this setup.

You're getting shortly spaced one bounce (left wall) and two bounce (desk + left wall) reflections.

Good point, I didn't realize that the left reflection would also bounce on the desk too. That would indeed make it even worse.

I assume your listening position is somewhat back from the desk and when you're listening there's no wall to the left of your head.

I listen in my office chair in the exact position it appears in the pictures.

First thing to note is that the white panel (if that's the one you're referring to) doesn't cover all of the left wall.

It does in the last picture. The picture might be a bit misleading visually; the part of the wall that's still visible is actually past a right angle.

That alone, plus the fact that fabric covered panels are somewhat reflective at HF anyway
The absorption is not sufficient to remove enough of the early reflected energy.

RealTraps states these panels have an absorption coefficient close to 1 at HF. Am I being misled? Is it an angle of incidence thing?

and the corner gain up to the low mids, is enough to cause the left bias.

In actual usage I compensate that corner gain via EQ, such that left and right are actually aligned in that part of the spectrum. Still leans to the left even with EQ.

Measure the ETC of each speaker with and without the Mondo traps. They may not attenuate the reflections well enough.

Interesting idea. The ETC with and without the panel is intriguing:

etc.png


According to my reflection path physical distance measurements, the reflection from the left wall (without panel) should be delayed by approx. 4.8 ms relative to the direct sound, and that roughly checks out in the ETC. I get the impression that the panel doesn't remove the reflection - instead it is moved forward by about 1.1 ms. That corresponds to about 35 centimeters, which seems roughly consistent with the hypothesis that the reflection path is shorter simply because the surface of the panel is closer than the wall. This would lend credence to the idea that the panel simply isn't effective at removing the reflection.

So I guess my next question would be: is there a different kind of absorber I could use that would be more effective at removing this reflection?
 
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gvl

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I think I did that some time ago. I'll double-check. It would be a huge coincidence if one speaker happens to be lower in level but the FR measurements are still aligned…

Yeah, unlikely, but checking basics first is a good start. As a variant of building a wall suggested above perhaps add a solid bookshelf or the like on the right with its back towards the desk to make the layout symmetrical.
 
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Bjorn

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Thanks for the many suggestions :)



I think I did that some time ago. I'll double-check. It would be a huge coincidence if one speaker happens to be lower in level but the FR measurements are still aligned…



Not really.




Tried that. +0.25ms on the left channel makes things a bit more centered. More and the soundstage breaks down, as one would expect. Just like level adjustments, this feels like a hack and not fixing the root cause.



Yeah, I can relate, I've had issues with uneven earwax :( Not the case here though, I checked. Everything feels centered except when played over this setup.



Good point, I didn't realize that the left reflection would also bounce on the desk too. That would indeed make it even worse.



I listen in my office chair in the exact position it appears in the pictures.



It does in the last picture. The picture might be a bit misleading visually; the part of the wall that's still visible is actually past a right angle.




RealTraps states these panels have an absorption coefficient close to 1 at HF. Am I being misled? Is it an angle of incidence thing?



In actual usage I compensate that corner gain via EQ, such that left and right are actually aligned in that part of the spectrum. Still leans to the left even with EQ.



Interesting idea. The ETC with and without the panel is intriguing:

View attachment 104901

According to my reflection path physical distance measurements, the reflection from the left wall (without panel) should be delayed by approx. 4.8 ms relative to the direct sound, and that roughly checks out in the ETC. I get the impression that the panel doesn't remove the reflection - instead it is moved forward by about 1.1 ms. That corresponds to about 35 centimeters, which seems roughly consistent with the hypothesis that the reflection path is shorter simply because the surface of the panel is closer than the wall. This would lend credence to the idea that the panel simply isn't effective at removing the reflection.

So I guess my next question would be: is there a different kind of absorber I could use that would be more effective at removing this reflection?
You need to measure first the speakers invidually both with and without the panels. And then compare left and right, both with and without panels. That's two different comparisons. I'm not sure what you're showing.
You can't measure speakers simultaneously FIY.
 

Haflermichi

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I have the same problem. Only a half wall on the right but a full wall on the left. It's extremely aggravating.
And each of my floor standing speakers are at least 3 feet out from the sides and 2 feet from the back wall.
I'm afraid that is what balance controls are for! But I know that's not the quick and easy answer you want.

Unless you build a "wall" on the right, which could be as simple as foamcore board (at least for experimenting) up from the desk to the ceiling, you're going to need tons of absorption on the left to equal the "nothingness" of the right side. The path lengths are multiples different.
 
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edechamps

edechamps

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You need to measure first the speakers invidually both with and without the panels. And then compare left and right, both with and without panels. That's two different comparisons. I'm not sure what you're showing.

All the measurements I've shown so far have always been single-speaker measurements. (FR means "Front Right" - sorry that wasn't clear) My last ETC shows the right speaker response only, with the panel and without the panel.

Here are Left/Right ETC comparisons:

frflwo.png

flfrwith.png


What I believe is the left wall reflection sticks out like a sore thumb in the FR (Front Right) ETC, at ~4.75 ms without panel and ~3.75 ms with the panel (presumably because of the surface of the panel being closer). As expected, it does not appear in the FL (Front Left) ETC.
 
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Haflermichi

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You seem to have several panels at your disposal. I see 3?
Instead of processing, delaying, absorbing etc. why not put a panel on the right side mirrored as close as possible to the left side?
Then, for at least the depth of the absorbers, both sides will be physically similar. At least for near-field.
 
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edechamps

edechamps

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You seem to have several panels at your disposal. I see 3?
Instead of processing, delaying, absorbing etc. why not put a panel on the right side mirrored as close as possible to the left side?
Then, for at least the depth of the absorbers, both sides will be physically similar. At least for near-field.

Yeah, I considering doing something like this. The problem is, having a panel (or any other kind of partition) on the right side of the desk would be aesthetically problematic because it would block the view towards a window. But I might end up doing this if I don't have any other option.
 
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edechamps

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I remembered I have some cheap, basic acoustic foam lying around. So let's try panel + foam. It's actually very easy to put up because the foam sticks naturally to the panel fabric:

foam.jpg

It has a very notable effect on the ETC, bringing down the problematic peak by about 10 dB:

foametc.png

foametcflfr.png


So it does really looks like the MondoTrap by itself does not provide good absorption at higher frequencies.

In listening, that didn't solve the problem, but it did seem to attenuate it somewhat. The imbalance now seems to be about 2 dB with panel + foam, compared to 3.5 dB with just the panel, and 4 dB without the panel. Maybe I can improve on that further by using a proper acoustic surface that is designed for that use case, if such a thing exists?
 

suttondesign

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I have the same issue, and I finally ran it down when I was about 20 years old: I have a hearing imbalance of 4.5db, favoring my left ear. Headphones don't reveal it well, only a properly-set-up speaker system. Switch the speakers and see if the bias doesn't switch sides.
 

Kvalsvoll

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RealTraps states these panels have an absorption coefficient close to 1 at HF. Am I being misled? Is it an angle of incidence thing?

Absolutely not, no reason to believe you are mislead, I believe those are really good products that works very well according to specifications given. An acoustics expert that has actually tested this product, is welcome to object, of course.

The problem is that it still does not absorb enough of the sound to make the acoustic surroundings similar to R speaker. You can see this in you measurements, try to look at phase and GD, perhaps you can see the reflection there.

Covering more of the wall surface may help, this will make the absorption work further down in frequency. Then you soon end up with something that is not practically possible.

I suggested trying some delay because from you description, you start with a system where you at least have some sort of image. In many situations, with completely untreated rooms, they don't even have that.

The loudspeakers also make a huge difference in how sensitive they are to this sort of problems with different acoustic surroundings for L-R. Currently, there are unfortunately extremely few speakers that small - small enough to be placed like that - that will make a significant improvement, none that know of.
 
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