Asymmetrical First Reflection Setup and Control

[Jsmith]

My back wall (speaker wall) has a large window. This window is not centered on the wall but shifted to the one side by about a foot. Right now I have my speakers centered on the window and not on the room. I set up a grid from the middle of the window back to my listening position and have the speakers aligned to this. This means my one speaker is about one foot closer to the side wall than the other. I selected this arrangement because it felt unnatural centering my listening position in the middle of the room and then looking off center into the window.

 

I have acoustic panels on my back wall and corner bass traps and panels for the first reflection points on order and to be delivered next week. My question is the following. Since the one speaker is closer to the wall the first reflection point appears to be different than the other. This seems to make sense to me. Therefore, my first reflection panels would need to be asymmetrical to align with the speaker.

 

Is there any inherent issues with an asymmetrical panel setup as long as they align with the speaker? I am open to shifting my listening position to center of the room to avoid any potential issues, but if it is a non-issue will leave where it is.

 

 

[firedog]

I think you need to set up the panels acc'd to where you sit. The reflection points change if either the speaker positions or your position changes.

 

My non professional advice - get a friend to do the "mirror test" while you are in your normal listening position and you'll see where your first reflection points are. That worked great for me.

 

In other words, I think in your terms it is a "nonissue".

 

Who did you buy the panels from? They should be able to advise.

 

[Jsmith]

I purchased the panels from ATS. I will ping them and ask for their input as they have been pretty helpful so far.

 

While I don’t have the panels in hand, I did do the mirror and laser test. That is how I noticed the first reflection points appeared to be asymmetrical and lead me to this.

 

I am fine hanging the panels where the reflection appears to be, but wanted some input first as I would like to limit the number of holes I have to put in my wall from moving them.

 

 

[barrows]

Is the best way to determine the real first reflection point. Additionally, with an assym set up, you are going to have more spl from the speaker closer to the corner of the room, and will probably need to adjust the balance to suit.

I have a similar asym room, with an asym window placement on the wall behind the speakers, and am working on a solution getting a large curtain made to cover most of the wall (the window faces NW, and the room already gets plenty of light from other windows). I set up my speakers symmetrically to the sidewalls, which easily allows for a central listening position, then deal with evening out the reflections from the wall with the curtain and DIY absorbers.

 

 

[Jsmith]

Barrows – you touched up something that may greatly help. The first reflection point may not cause an issue, but as you said the speaker closer to the wall / corner may have higher SPL. This may be the root cause of my other issue. It seems like my image is always slightly bias to that side. I had not considered the closeness as a possible culprit. It seems so obvious now.

 

I think I am going to wait until I get my corner bass traps in. Then I will have a listen, and if there still seems to be the bias I may adjust to the room center before installing the first reflection panels.

 

-Thanks

 

 

[barrows]

:

 

"It seems like my image is always slightly bias to that side. I had not considered the closeness as a possible culprit. It seems so obvious now."

 

This is exactly what happens when the corner is a little closer (consider the corner acts as a horn), the image will shift. if you have a balance control, you can adjust it with the balance. A test disc with a solid central image is ideal for doing this adjustment by ear. If you have a voltmeter, you can do an even finer adjustment with a test disc and measuring voltage at the speaker. Stereophile's "editor's choice" disc has the test tracks that would make this possible, as does the Nordost test disc.

 

[Jsmith]

OK, so I am almost convinced I need to center my speaker grid to the middle of the room to avoid any unwanted bias effects. I measured and the difference and I only need to shift the speakers 3 inches if I keep my current setup for speaker to listening and speaker to speaker. This now means the panels I have on my back wall would be off center by 3 inches. Just out of curiosity I wanted to see if this would make a difference.

 

I put on a pink noise track, plugged in the external mic to the iPad, and fired up the SPL meter app. I aimed it at the back wall approximately where my ears would be. I was surprised at what I found. As I moved off center from the panels the SPL increased. I would expect this as I got close to the edges since the mic would start to pick up reflections off the uncovered wall. My covered area is four foot square. Even going three inches off center yield and increase in about .5 dB. I not sure how impactful this would be, but considering the bare wall to the center only increased by 1.2 dB it seemed to be to be enough to justify centering the panels on the new spot.

 

Centered - 65.6 dB

3” off Center - 66.1 dB

Bare wall - 66.8 dB

 

I did a quick double check with the RadioShack SPL. It only does whole dB. I could not measure the exact same, but it did increase one dB as I got close to the edge.

 

Will I notice a .5 dB increase with the three inch off center? I am not sure. What I do know is I have invested too much time and money to chance it and will therefore, center my back wall panels on the new grid.

 

[barrows]

Oops, just re-read my post and realized I recommended measuring voltage at the speaker binding posts! Clearly this will not help you balance the system output at the listening position with an asym set up! Sorry. The approach of measuring spl at the listening position is what you want to do. I would average 5-10 readings in a grid around the listening positions, perhaps a 24" grid or so and then try and get those measurements equal for both speakers.

Then confirm with pinknoise for a solid central image by listening. And finally with real music which features a solid central image.