Center Channel Speakers In A Movie Theater?

Just one.

I thought I read one time that they often have at least two speakers up front.

Here's the trick...
Go into the different theater venues (including IMAX) and go peak under the screen! Then you'll know.

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Avgoround...I attempted that and was escorted out of the theater by security...with no refund.
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I used to be in charge of product for Cinema for JBL. Its the same speaker in the center as the left and the right-and that's how we did it at the Academy of Motion Pictures as well.

In a correct film mix, dialogue is the ONLY thing supposed to come from the center speaker. However, I hear from my scoring mixer friends that there are a few [film] mixes out there with non dialogue showing up in the Center. I think its harder to give motion or position since center and left (or right) aren't so far apart.

In IMAX they use a lot more channels than typical film as they use a vertical component of audio to imply an up and down motion (most film only uses left to right).

Someday maybe we'll have 12.1- level one LCR front and LCR surround and level two LCR front and LCR back. Give me a few more overhead and they can do 360 steering!

BTW, center channel in 5.1 music mixes are another story. All bets are off, you can do anything you want.