Dual Centers

[DougMac]

I have 340's for L/C/R in my home theater. My original plan was to use and acoustic screen, but when the room got laid out, the screen would be too close to the viewing area. I ended up painting a screen on the wall, which has worked great. The screen is 120" diagonal.

The only problem is center speaker placement. I toyed with above or below and ended up choosing below. It's usually not a problem, but sometimes the dialog is "too low".

I recently read about someone who used two centers, one above and one below to center the dialog vertically. Has anyone tried this?

I have a few concerns. Few receivers have dual center outs. Mine doesn't. If I wired the speakers in parallell, I'm halving the impedence. I don't think my Onkyo likes 4 ohms. I'm also concerned about possible phase problems and I'm wondering if Audyssey would compensate in the time domain.

Any thoughts?

[Reflections1]

I have 340's for L/C/R in my home theater. My original plan was to use and acoustic screen, but when the room got laid out, the screen would be too close to the viewing area. I ended up painting a screen on the wall, which has worked great. The screen is 120" diagonal.

The only problem is center speaker placement. I toyed with above or below and ended up choosing below. It's usually not a problem, but sometimes the dialog is "too low".

I recently read about someone who used two centers, one above and one below to center the dialog vertically. Has anyone tried this?

I have a few concerns. Few receivers have dual center outs. Mine doesn't. If I wired the speakers in parallell, I'm halving the impedence. I don't think my Onkyo likes 4 ohms. I'm also concerned about possible phase problems and I'm wondering if Audyssey would compensate in the time domain.

Any thoughts?

Not sure how well it would work if you wired them in series. It would give you approximately 6 ohms. Have only used that for alarm speakers and that works fine.

[curtis]

Dual centers could also cause cancellation problems.

[DougMac]

Dual centers could also cause cancellation problems.
I didn't express it, but that was my concern. I did think about it though. How would that be different from two channel where a voice placed in the center comes from the same signal on the L/R channels?

[curtis]

I didn't express it, but that was my concern. I did think about it though. How would that be different from two channel where a voice placed in the center comes from the same signal on the L/R channels?
To be honest, I am not entirely sure, but I think it has something to do with how the material was record, and how it is meant to be playback.

Using the voice in the center example, the source was recorded in two channel and meant to playback in two channel.

With the dual center, you are taking what was recorded as one channel, and playing it back in two mono channels.

BTW...I have seen pictures of setups with dual center channels, and I have read that it could cause cancellation problems, but I really don't know much after that. :)

[GirgleMirt]

To be honest, I am not entirely sure, but I think it has something to do with how the material was record, and how it is meant to be playback.

Using the voice in the center example, the source was recorded in two channel and meant to playback in two channel.

With the dual center, you are taking what was recorded as one channel, and playing it back in two mono channels.
Isn't voice, for example, recorded exactly the same way, whether it will be played in stereo or for the center channel? That is, one mic? :p To play a voice in the center with stereo, the same sound is simply played from both speakers... Unless they use different processing to make some alteration for center material in 2ch (two channel = stereo), I don't think there's a difference in recording...

Here's my take on 2 centers: Take 2ch vs mono, just a voice playing. If you're sitting 100% in front of 2 speakers playing the same thing, it should theoretically sound the same as mono... Actually, maybe not... As there's two sound sources instead of one, and you might probably be able to hear a difference... Argh...

Ok, let's just say that with 2 speakers, and you sitting in the center, you'll definitely hear one voice, coming from in front. Same for 1 speaker in mono. So here, it's all good. Let's ignore different reflection issues (way sound bounces off side walls and create 'sound reflections'), both are rather fine.

BUT, that's sitting 100% in front. If you're sitting off angle with stereo, closer to one speaker, let's say left, then there will be a (very) slight delay as it'll take longer for the sound coming from the right to reach you, and there will also be a difference in level, the sound coming from the left will be slightly louder than the one from the right. Also the sound waves from the left might interact with the sound waves from the left, and the left with the right, each causing issues with the other (changing what gets to your ears)... Then there's the reflections, you'll get more reflections from the left, less from the right...

Anyway, all this to basically say that when you move off center, you get many disadvantages, and stereo becomes much less 'perfect' when compared to a single speaker playing mono. With stereo, let's just say you really want to be in the center...

If you have two centers, one on top of the screen, one on the bottom, here are the issues I see:

1) Different distance to your ears from both centers, which is like not being in the center for 2 speakers, which will cause:
2) combing effect: http://homepage.mac.com/planet10/forum/comb-filter-diagram.gif Meaning, two sound waves will interact with each other and cause reinforcement/cancelations of frequency/amplitude...: http://www.ndt-ed.org/EducationResources/HighSchool/Sound/interference.htm Add all the new room reflections, now you have twice the ceiling reflections (one for each speaker), twice the floor reflections... That's a lot of new bugs you've introduced...

Anyhow... It would cause more issues than it would solve imho...

http://docstore.mik.ua/orelly/web2/audio/ch02_01.htm

To sum up all the above in one sentence... I think dual centers might have the effect of smearing the sound... You introduce possible issues... Ok that was two sentences... sigh...

[GirgleMirt]

Holy crap I'm awesome... Don't know why last time I looked I didn't find much... Here's some vids talking indirectly about the issue:

http://www.youtube.com/watch?v=2QPAFJyVb7U

http://www.youtube.com/watch?v=EfFuMKj15B4&feature=related

http://www.youtube.com/watch?v=-8a61G8Hvi0&feature=related

Btw, the speed of sound is like 350m/s, so a difference of 1m while not huge, probably still has significant effect with 5000, 10000 or 15000hz (cycles per second)... What would be the math... Say you're 5m away from the screen, top center is 2.5m up, that's like what, 0.5m difference at listening position? .5/350 = 0.0014s delay, which could definitely affect the mids and highs? Think then it enters the psycho-acoustic realm, way brain/ear perceives sound (and small delays), not sure how well/badly we'd perceive this, although maybe it's all insignificant given the room interaction and all of the other factors...

Actually, you can hear the 1ms delay in the 1st vid I posted...

[scape]

the onkyo should be able to push 4, 6, and 8 ohm loads. it's usually advertised on their manuals though. you might also be able to use the channel b outs and redirect center through those.
if you run them in parallel, you have to come to terms with the fact that one channel is pushing two sets of speakers with a single discrete amp, while left and right channels obviously get their own; if you push it hard this may become apparent on some level-- but then again maybe not; you should check the ohm loads and their corresponding watt amps per channel.
what i think girglemirt pointed out is the tweeter issue, and i believe it may have something to do with lobing; very few speakers even have two tweeters, let alone running in parallel.
without the option to work on delay if running in parallel, it may be better not to even go forward. maybe using channel b/zone2 out for second center and working on delay might work.
I gave up on the center channel, maybe i'll go back-- but having it above or below my tv just never sounded right; i do a phantom center setup of L/R and a sub and hopefully can put surrounds back up this week.

[Blutarsky]

Maybe tilt the center channel up a bit?

[hearing specialist]

my opinion is this: the more drivers, the more possible issues, the more drivers, the more processing needed to cut, reduce, and shape. Set it up as your thinking then dis-connect one of the centers to hear the difference. The receiver may try to address the dual setup by degrading the sound completely to deal with the calibration. Which form of Audyssey does your receiver have?

[DougMac]

Which form of Audyssey does your receiver have?
Multi-EQ. I do have my center angled up slightly.

Thanks everyone for your responses. I still suspect dual centers may cause more problems than they solve. I've got my center as high as possible without shadowing the screen. My wife doesn't notice the voices being low and it's only on rare occasions that I do. I have toyed with the idea of trying a phantom center, but that's down on my priority list. I don't fiddle with my electronics and speakers much, I'd rather just enjoy them.

[Dr. House]

The two biggest issues here is comb filtering and the low impedance load the speakers will present to the amplifier wiring them in parallel. Having one above and below a large screen would improve "anchoring" the dialogue but I highly doubt it would improve speech intelligibility and clarity.

Some information on comb filtering.

http://books.google.ca/books?id=sGmz0yONYFcC&printsec=frontcover&dq=floyd+toole++sound+reproduction&hl=en&ei=pgPETd3ZLcrXgQfg9OjKBA&sa=X&oi=book_result&ct=result&resnum=1&ved=0CDEQ6AEwAA#v=onepage&q=comb%20filtering&f=false

I think most would agree with a competently designed center speaker, only one is needed.