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

[Kvalsvoll]

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.

Looked at your picture once more, obviously this is just what you already tried. Then what is left, is that the absorber simply does not absorb enough.

 

[ernestcarl]

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.

4dB is a lot. I don't have an imaging problem, per se, but I used to sometimes hear vocals sounding just a tad bit louder coming from the right side with and without EQ. Increasing the left side's volume by about a 1dB - 1.5dB was able to fix the issue. But, adding three cheap additional 4-inch foam wedges vertically along my light switch proved to be a better, permanent fix. The monitor is in the middle of the hallway so I certainly get more sidewall reflections even with the monitor's horn waveguide and use of exaggerated angling.

 

[abdo123]

Okay here is what i normally do, which gets me to ~80% accurate stereo center.

When you're calibrating the speakers' volume for the measurements (at 75dB or 85dB .etc), play the pink noise in the range of 1k to 5k (where our hearing is most sensitive to spatial cues) and then tweak the balance in real time (preferably in the analog domain) so that the volume you're reading from both speakers is the same.

From there do frequency sweeps and measurments, equalize all the dips and peaks to the 1k-5k volume level you get in your Frequency response.

From there, if it is not centered, a tiny nudge can push in the right direction (OS Volume mixer) without destroying the sound stage.

 

[gvl]

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.

Plexiglass to the rescue!

 

[pozz]

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

Most reverb chambers in acoustics labs that test treatment don't provide data past 5kHz or so because drivers tend to beam and you can't reliable numbers there. So you do indeed get a coefficient of 1 for the exact tested mounting and direct incidence, but real absorption is better expressed in sabins per frequency anyway, and there it's more relative. Plus the fabric weave and type will affect permeability for air, even for the pro fabrics (Guilford of Maine), since it's the fibreglass underneath that's doing the absorbing. Normally not a problem but it matters in tight spots like yours.

Foams don't require covers and as long as the cells aren't too dense will be really effective for HF. IIRC modern foam and fibreglass have roughly equal effectiveness.

I hope you find a good solution. Personally, I had to rearrange my room when I had a corner setup. Panels only went so far.

 

[andreasmaaan]

@edechamps in terms of physical solutions, I would try bringing the absorber out from the wall as far as possible, so that there's as much air as possible between the wall and the absorber. This will increase its effectiveness, particularly at lower frequencies. Alternatively, add additional absorbers to effectively double/triple the depth of the absorber. And place some low-density acoustic foam over the panel to ensure no high frequencies are being reflected (I think you mentioned you'd already tried this).

Failing that, adjusting interchannel levels/delays are probably the only (imperfect) alternative.

 

[andreasmaaan]

Here are Left/Right ETC comparisons:

Just to add to the previous post, and in case it's not already apparent, what is happening IMO is that your brain is interpreting that strong early reflection as a cue that the (phantom) source is close to the left wall. So to solve the problem, you'll really need to reduce that reflection's intensity to the point at which it stops giving your brain this incorrect cue.

Adjusting interchannel levels/delays may go part way to compensating for this other unwanted cue, but will ultimately be unlikely to provide a satisfactory solution IMO.

 

[edechamps]

@edechamps in terms of physical solutions, I would try bringing the absorber out from the wall as far as possible, so that there's as much air as possible between the wall and the absorber. This will increase its effectiveness, particularly at lower frequencies. Alternatively, add additional absorbers to effectively double/triple the depth of the absorber. And place some low-density acoustic foam over the panel to ensure no high frequencies are being reflected (I think you mentioned you'd already tried this).

It's unclear to me how much low frequencies (say, below 300 Hz) matters when it comes to this. Intuitively I'd think this part of the spectrum is not the main cue for localization, but maybe I'm wrong. I experimented with some low-pass/high-pass filters and it's much easier for me to pinpoint the issue on my test sample when it's high-passed with, say, a 250 Hz brickwall then when the opposite lowpass filter is applied.

(For those curious, here is the sample I'm using. It's a conversation (male vocals) that happens around the 12th minute of Devs S01E01 and makes the issue very apparent.)

Just to add to the previous post, and in case it's not already apparent, what is happening IMO is that your brain is interpreting that strong early reflection as a cue that the (phantom) source is close to the left wall. So to solve the problem, you'll really need to reduce that reflection's intensity to the point at which it stops giving your brain this incorrect cue.

Yes, this has been my main hypothesis thus far. That left wall reflection is so early and so loud that my brain is likely creating a phantom image between the right speaker and the left wall.

And place some low-density acoustic foam over the panel to ensure no high frequencies are being reflected (I think you mentioned you'd already tried this).

Indeed, and thus far that seems to have produced the best results, both in ETC measurements and in actual listening. It really looks like the flat, relatively smooth surface of the MondoTrap is ineffective at HF absorption, and the foam wedges perform significantly better (perhaps because the wedge shape provides some diffusion as well).

Now I'm wondering if I can improve even further on this solution and cover the 2 dB imbalance that still remains. The acoustic foam I used in that last experiment is pretty basic and cheap I believe - Pro Acoustic AFW305. Maybe I can get even better results by using something fancier, such as thicker foam from Auralex or something.

 

[Lorenzo74]

Can you add a wall on the right side?

indeed:
0) pull your speaker closer to you 10-15cm so you reduce the effect of side wall reiforcement
1) pull the left panel shown in first picture , use the mirror to center it. (tweeter must be in the vertical axis of the panel)
2) place the second absorption panel (you show it added on the left) on the right! make simmetrical side walls
3) measure with Rew and check in impulse response the reflections, (small spikes) now should be symmetric in left and right data (if you share mdat i will help)
4) this is my approach and is also suggested by ASR members.

looking forward
best

 

[andreasmaaan]

Now I'm wondering if I can improve even further on this solution and cover the 2 dB imbalance that still remains. The acoustic foam I used in that last experiment is pretty basic and cheap I believe - Pro Acoustic AFW305. Maybe I can get even better results by using something fancier, such as thicker foam from Auralex or something.

Without knowing the properties of the specific materials, my hunch is actually that lighter foam is likely to work better than thicker foam, since the most likely frequency range in which the panel is reflective is the (very) high-frequency range.

It's unclear to me how much low frequencies (say, below 300 Hz) matters when it comes to this. Intuitively I'd think this part of the spectrum is not the main cue for localization, but maybe I'm wrong. I experimented with some low-pass/high-pass filters and it's much easier for me to pinpoint the issue on my test sample when it's high-passed with, say, a 250 Hz brickwall then when the opposite lowpass filter is applied.

I'm having a hard time interpreting the website's absorption coefficients. They only give a single set of values for "mondo trap on wall", but then also state that MondoTraps are 4'-9". What is the thickness of your panel? And (rhetorical question) for what panel thickness are the given absorption coefficients?

In any case, creating an air layer between the trap and the wall will (at the lower end of the frequency range in which the panel is effective) lead to higher levels of absorption, and is certainly something I'd experiment with.

 

[edechamps]

What is the thickness of your panel? And (rhetorical question) for what panel thickness are the given absorption coefficients?

Actually I just remembered my panels are the "Fat" version of the MondoTrap, so it's 15 cm thick (4-1/4 inches). I don't see data for that particular variant, but I'd expect it to be a bit more effective than a standard MondoTrap (at low frequencies, that is).

 

[Inner Space]

Actually I just remembered my panels are the "Fat" version of the MondoTrap, so it's 15 cm thick (4-1/4 inches). I don't see data for that particular variant, but I'd expect it to be a bit more effective than a standard MondoTrap.

I learned two things from somewhat-related experiences: 1) Absorber panels are conceived and (I suspect) measured and tested in the raw, and putting cosmetic skins on them for domestic acceptability reduces their effectiveness, sometimes dramatically. And 2) @andreasmaaan is right about leaving a gap behind them. Not so much for frequency considerations, but because they're velocity absorbers - therefore putting them on the wall means you're "wasting" the depth by getting them as close as possible to where they're least useful, i.e. where velocity is zero, i.e. the point of reflection on the wall. Better to think of them as entirely freestanding arrays - and maybe using them literally that way ... can you prop them vertical, closer to the desk, maybe angled differently?

 

[andreasmaaan]

Actually I just remembered my panels are the "Fat" version of the MondoTrap, so it's 15 cm thick (4-1/4 inches). I don't see data for that particular variant, but I'd expect it to be a bit more effective than a standard MondoTrap.

At the top of the absorption coefficient tables here it states: Mondo traps are 4'-9". What does the 9" refer to?

I wouldn't expect a 4"-thick absorber to have an absorption coefficient >1 at 125Hz when placed directly against a wall.

 

[edechamps]

At the top of the absorption coefficient tables here it states: Mondo traps are 4'-9". What does the 9" refer to?

It's the length. "MondoTraps are 2 feet wide by 4'-9" tall."

I wouldn't expect a 4"-thick absorber to have an absorption coefficient >1 at 125Hz when placed directly against a wall.

Well, RealTraps use limp-mass membranes which are supposed to greatly improve low frequency absorption compared to raw fiberglass. I'm not an acoustic engineer so no idea if that actually makes sense.

By the way, the same article states "One big feature of RealTraps products is their large amount of absorption at very low frequencies, with intentionally less absorption at mid and high frequencies." Oh well…

 

[andreasmaaan]

By the way, the same article states "One big feature of RealTraps products is their large amount of absorption at very low frequencies, with intentionally less absorption at mid and high frequencies." Oh well…

Ah, I hadn't read the text, and the published absorption coefficients don't show this at all (absorption is actually highest in the mid frequencies according to the published chart). So something doesn't seem to be adding up IMHO.

But if it really is the case that the panels are less absorbent at mid-high frequencies than the chart suggests, then your earlier idea of using thicker foam makes sense.

And apologies for barging in earlier without making myself fully abreast of the technical properties of the panels. The apparent discrepancy between what they say and what the absorption coefficient table shows is a bit difficult to make sense of, though.

 

[Inner Space]

Well, RealTraps use limp-mass membranes which are supposed to greatly improve low frequency absorption compared to raw fiberglass. I'm not an acoustic engineer so no idea if that actually makes sense.

By the way, the same article states "One big feature of RealTraps products is their large amount of absorption at very low frequencies, with intentionally less absorption at mid and high frequencies." Oh well…

Limp-mass membranes are pressure absorbers (as opposed to velocity absorbers) - a LF wave tries to bludgeon through, thereby converting its acoustic energy to heat. A HF wave just bounces off. The two things need to be considered separately.

 

[DDF]

My office layout was asymetrical like yours and I had the same issue, even needing similar level change to center the image (2 to 4 dB). The balance setting needed was program dependant and the problem with this approach is that it doesnt address the root cause, so the image would wander depending on spectral content. Yours is a really tough case because the left wall is so close that the reflection gets very little reduction in level due to time of flight (6db per double distance)

I've read claims that time based room eqs (vs peq approaches) can help stabilize image wander due to asymetrical layout. Its worth a shot though in your near field case I imagine it might only help over a very tight listening space. Does dirac offer a limited trial, to check the concept? Matheq may be worth a shot as well.

 

[tmtomh]

You want a broadband absorber rather than a bass trap. Taking off from @andreasmaaan ’s and others’ comments above, you might be better off with less expensive, thinner, non-limp-membrane absorbers (which should absorb more in the mids and lower treble), mounted off the wall, so they absorb those frequencies coming and going: some will pass through the absorber and hit the wall, but then be absorbed on the way back as they re-enter the absorber. Mounting an absorber with a little airspace between it and the wall sort of gives you the equivalent of two absorbers in that way.

 

[DDF]

You want a broadband absorber rather than a bass trap. Taking off from @andreasmaaan ’s and others’ comments above, you might be better off with less expensive, thinner, non-limp-membrane absorbers (which should absorb more in the mids and lower treble), mounted off the wall, so they absorb those frequencies coming and going: some will pass through the absorber and hit the wall, but then be absorbed on the way back as they re-enter the absorber. Mounting an absorber with a little airspace between it and the wall sort of gives you the equivalent of two absorbers in that way.

I highly recommend a pegboard style absorber if going this route. It can provide a nice broadband absorption using that added depth.

 

[trl]

Mirror the mondo panels onthe other side too, like others suggested already.

Your existing panels seem very good, but you can also add on top of the mondo panels some panels like in this post https://www.audiosciencereview.com/...soundstage-early-reflection.19236/post-631421. Reason for this is to reflect & disperse & absorb midrange and trebles a bit more. Bass absorption should not be affected by adding additional "sponge-like" traps on top of existing mondotraps.

Source: https://realtraps.com/data.htm​