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Optimizing Center Channel Performance - Dispersion, Placement & Image

leveler

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One of the known disadvantages of a television compared to an acoustically transparent screen is that the center channel speaker must be mounted above or below the TV, causing the center channel image to shift below the left and right speakers. This image shift could be eliminated by using a phantom center image, however, this badly harms listeners outside the sweet spot. I assume the system will perform better by optimizing the performance of a center channel below the TV, rather than removing it entirely.

I have a few questions about how to optimize the performance of the center channel speaker, given the constraint that it be placed below a television, rather than behind an acoustically transparent screen.

1) I intend to use 3 matching bookshelf speakers as the left, center, and right channel speakers. The speakers will then be timbre matched and avoid the design issues inherent in horizontal center channels. Are there any issues I should consider when using a bookshelf speaker as a center channel speaker below a TV?
2) I intend to angle the center channel speaker up towards the listening position. What is the best solution for angling a center channel bookshelf speaker up towards the listening position? Are there any mounts that will fit below the television and allow for stable articulation/angling up towards the listening position?
3) What impact (if any) does wide vs narrow dispersion characteristics of a bookshelf speaker have on the localization of the center channel below the television? Is it possible that a wide dispersion speaker, angled up, will be perceived as 'more centered' in the screen than a narrow dispersion speaker? Would this effect be more pronounced in a horizontal center channel speaker, considering the inherently wide vertical dispersion characteristics.

Are there any other center channel optimizations I should consider when placing the center channel below the television?
 

alex-z

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Acoustically transparent screens aren't within downsides. The perforations still cause diffraction and loss of treble output. Not to mention the minor hit to video quality. Better than a normal screen, but the primary reason I decided against a projector.

If you are going to use a bookshelf speaker for the centre, make sure it is oriented the same direction as the L/R. Most bookshelf speakers have wildly different horizontal and vertical directivity. Directivity difference could mess with imaging.

Wide vs narrow dispersion primarily decides how much energy is reflected off the surfaces in your room. In theory there is a tradeoff, with wide dispersion having a sense of spaciousness, and narrow having greater imaging accuracy. I personally prefer wide dispersion, and then use acoustic treatment to manage decay times.

Beyond placing the speaker as close to the bottom of the TV as possible, there isn't special optimization. Same rules as your other speakers apply, distance effects speaker boundary interference. Close to the wall is usually best, SBIR is pushed into the 300-500Hz range where acoustic treatment works well.
 

KMO

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I assume the system will perform better by optimizing the performance of a center channel below the TV, rather than removing it entirely.

Should do, yes, unless your listening space is very narrow with only 1 seat in use. Vertical offset is annoying, but you get used to it. Try to get it as close in height to L+R as you can.

1) I intend to use 3 matching bookshelf speakers as the left, center, and right channel speakers. The speakers will then be timbre matched and avoid the design issues inherent in horizontal center channels.

In practice, they still won't fully match, particularly at lower frequencies, because the physical environment of the centre and the L/R will be quite different. But you'll certainly have a better starting point, and no chance of compounding differences from different speaker and placement.

2) What is the best solution for angling a center channel bookshelf speaker up towards the listening position? Are there any mounts that will fit below the television and allow for stable articulation/angling up towards the listening position?

I failed to find anything good acting as a simple extra "shelf", but assuming you at least have a space marked out...

If the speaker is going on or in a cabinet, you do want something to isolate it. You absolutely need to minimise resonances to try to stay near the timbre of the L+R on their lovely (I assume) stands.

Cheap option is a foam wedge - assorted varieties are available on ebay as "monitor speaker isolation foam pads" or similar, that will give you up to maybe 12-15 degrees of tilt. That's what I'm using (not least due to lack of space for anything fancier).

There are fancier things available from IsoAcoustics, and quite a few "studio monitor" stands that would be suitable to angle a bookshelf.

Are there any other center channel optimizations I should consider when placing the center channel below the television?

Avoid reflections from shelves - make sure the speaker baffle is at or beyond the front edge of any cabinet.

If you are going to use a bookshelf speaker for the centre, make sure it is oriented the same direction as the L/R. Most bookshelf speakers have wildly different horizontal and vertical directivity. Directivity difference could mess with imaging.

Unless it is a coaxial with very similar H/V directivity. Something like the KEF LS50 Meta is absolutely fine on its side, and that's what I have.
 

rgdawsonco

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I spent quite a bit of effort once trying to get my center channel to sound identical to a phantom center. Then I realized that that is not possible, even if mounted at the same height. But when the center is mounted lower than the left and right, it is worse. But if you, like me, are force to do that, I have found that tilting the center up so that it is aiming above ear (in my case about a foot) level helps, although I don’t know why.
 

KMO

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I spent quite a bit of effort once trying to get my center channel to sound identical to a phantom center. Then I realized that that is not possible, even if mounted at the same height.

Indeed - it will never sound identical. A phantom centre is audibly not a point source - it sounds wider. Part of the motivation of having a centre speaker (as early pre-commercial-LP stereo systems did) was to avoid the forced extra width of the phantom centre. Being used to single-speaker mono playback, phantom centre was unnatural for the pioneers - we're accustomed to it.

Good mixers will use the difference between centre-only and L+R as a dynamic artistic choice in a multichannel music mix, particularly for vocals.

When setting up, you just need to aim for L->C->R pans not going noticeably up and down, and not shifting in timbre.
 

youngho

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I spent quite a bit of effort once trying to get my center channel to sound identical to a phantom center. Then I realized that that is not possible, even if mounted at the same height. But when the center is mounted lower than the left and right, it is worse. But if you, like me, are force to do that, I have found that tilting the center up so that it is aiming above ear (in my case about a foot) level helps, although I don’t know why.

You might find the folllowing discussion interesting: https://www.diyaudio.com/forums/multi-way/277519-fixing-stereo-phantom-center.html
Also, https://www.aes.org/e-lib/browse.cfm?elib=14174, which is discussed in Toole's book

The precedence effect may explain your finding, due to image shifting from a stronger ceiling reflection
 

Teeter

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@leveler , In having auto speaker calibration on your AVR, it might even things out. Angle the center channel up towards the listening position with existing room acoustics, and the HT might just sound fine. If requires some balancing with the center channel, manually increase or decrease the level via your speaker settings in the AVR. Experiment a little and adjust accordingly. You can adjust the output level of the left and rights too.
Let us know how you made out.
 

Nwickliff

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One of the known disadvantages of a television compared to an acoustically transparent screen is that the center channel speaker must be mounted above or below the TV, causing the center channel image to shift below the left and right speakers. This image shift could be eliminated by using a phantom center image, however, this badly harms listeners outside the sweet spot. I assume the system will perform better by optimizing the performance of a center channel below the TV, rather than removing it entirely.

I have a few questions about how to optimize the performance of the center channel speaker, given the constraint that it be placed below a television, rather than behind an acoustically transparent screen.

1) I intend to use 3 matching bookshelf speakers as the left, center, and right channel speakers. The speakers will then be timbre matched and avoid the design issues inherent in horizontal center channels. Are there any issues I should consider when using a bookshelf speaker as a center channel speaker below a TV?
2) I intend to angle the center channel speaker up towards the listening position. What is the best solution for angling a center channel bookshelf speaker up towards the listening position? Are there any mounts that will fit below the television and allow for stable articulation/angling up towards the listening position?
3) What impact (if any) does wide vs narrow dispersion characteristics of a bookshelf speaker have on the localization of the center channel below the television? Is it possible that a wide dispersion speaker, angled up, will be perceived as 'more centered' in the screen than a narrow dispersion speaker? Would this effect be more pronounced in a horizontal center channel speaker, considering the inherently wide vertical dispersion characteristics.

Are there any other center channel optimizations I should consider when placing the center channel below the television?
What did you end up going with. I bought a cheap pair of Kef q100's and put one on it's side for my center speaker. Coaxial design allows for it to be a little closer to the tv since I can place it on it's side, and I have good vertical and horizontal dispersion throughout the seats because of the coaxial design. For music I use "center spread" which spread the center channel to the LR as well so not everything comes out of the center during music presentation.
 

nstzya

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one speaker above, the other below. acoustic source will be in the middle
This is my proposed solution for my situation similar to the OP - with in walls. Wondering if comb filtering, etc is as significant in the vertical plane (since interaural is not as applicable)
 
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Kvalsvoll

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This is my proposed solution for my situation similar to the OP - with in walls. Wondering if comb filtering, etc is as significant in the vertical plane (since interaural is not as applicable)
Center problems seems to be a recurring issue, with some quite inventive suggestions for a solution that will not work well.

This is actually a solved problem, and there are 2 ways to do it:

1. Use phantom center, only L and R speakers, this will work well for one center listening position, for all other the image will shift more or less towards the nearest speaker, but dialogue will still be clear and understandable when using properly designed speakers.

2. Use a properly designed center speaker and place it properly, in most cases this means above the screen to get away from obstructions.

A properly designed center speaker will place the image exactly on the screen, it will not sound like the dialogue comes from above or below the screen, and it will have a perfect coverage for all listeners in the room. It will allow for lowering the volume to a comfortable low level, and still deliver a clear, understandable, intelligible dialogue.
 

dasdoing

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This is my proposed solution for my situation similar to the OP - with in walls. Wondering if comb filtering, etc is as significant in the vertical plane (since interaural is not as applicable)

there will be "holes" in the vertical plane. since your head should be more or less in the middle of the screen I think this will be relevant starting from 5000Hz-ish. it can be somehow simulated with this https://www.usound.in.ua/graphs/
you can listen to how this sounds be listening to mono sources in a stereo setup and moving from left to right.
 

nstzya

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Are they as audible in vertical plane? No interaural and besides not as likely to be moving head up and down as left and right. ;-)
 

dasdoing

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Are they as audible in vertical plane?

no, since they will be closer together.

not as likely to be moving head up and down as left and right

that's why I said it becomes relevant around 5000Hz-ish where even a few cm matter. around 1000Hz for example you will have a big enough zone when having ears in the middle.
 

Kvalsvoll

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Is that an answer, or just restating the problem? ;)
It is the answer to the problem, it is solved.

There are some discussions on this forum on validity and usefulness of measurements of speakers. They do not seem to correlate with listening. Well, they do, when the speaker is defective by design.

Look at some of the center speaker measurements. Most of them show a speaker that has a usable listening window of less than +-10 degrees horisontal. You do not need a degree in electroacoustics to realize that this can never work well. The frequency response for all but the center position will be so bad that it severely compromises dialogue intelligibility.

If you do not play loud - i.e. listen at 0dB ref level when watching movies - you can do with a smaller monitor-type of speaker, mounted above the screen. Several options exist, and they generally will have excellent both on and off axis performance.

For further improvement, a speaker with more controlled radiation will improve clarity and intelligibility by reducing level of early reflections.

Then ideally the center speaker should have a similar sound character as the L-R mains. This can be achieved by using the same drivers and horns in all speakers, the speakers do not need to be exactly the same, just similar enough, especially at higher frequencies.

For a center to do justice, it should as a minimum provide a tonally equal listening window covering > +-30 degrees. It must have output capacity matching the required spl level actually listened at. And it should limit sound radiation outside this listening window, to maximize clarity by reducing early reflected sound.

This center speaker must also be designed so that it presents the sound image to appear as coming from the screen, not from the speaker itself. And this was also possible to achive. Indeed, the image is consistently placed fixed on the screen, even whren moving around in the room to other listening positions.

Such a center speaker do exist, I have had one in the media room for many years now. You can not but this one, but you can go to a shop and ask for such speakers, and if enough people ask for them, eventually some manufacturer will look into how to acquire the knowledge to make such speakers, and then you will be able to buy one.
 

JRS

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Although I have never used a center, what I like is a modern recliner in leather that allows me to mount the display about 4' to the lower edge, and tip back enough so that I'm looking more or less straight on. I favor a scissor mount that will bring the TV 2 feet off the wall, where a bookshelf would have lots of room. I never once felt like the sound source was too low--these are small monitors in a 15L cabinet, adjusted so the tweeter is ear level. If there was a center the illusion might be broken, but it is remarkable to the extent ones brain can fool itself.
 

nstzya

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Such a center speaker do exist, I have had one in the media room for many years now. You can not but this one, but you can go to a shop and ask for such speakers, and if enough people ask for them, eventually some manufacturer will look into how to acquire the knowledge to make such speakers, and then you will be able to buy one.
Why can one not buy one? Your company makes speakers, does it not?
 
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Thomas_A

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What I've learned. Either you have to place one below and one above the screen and use summing location to fix it to the screen. Or you need you take advantage of psychoacoustic cues to raise the image closer to the center of the screen, if you use a center below the screen. Such as peaking around 8 kHz.
 
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