Screens // Center Channel Best Practice

Make sure all three channels across the front are identical no matter what.  Not simply from the same manufacturer or series, but identical model numbers.  This is the most critical speaker in your room.  Here we'll collect the various sound advice from audio/video publications relevant to the center channel.  In a world of no guarantees, you can always count on these facts:

Home Theater

"Set up the speakers so that their drivers are aligned vertically.  This will provide the smoothest off-axis response.  Horizontal center speakers are an exception, but this arrangement can compromise the performance.  The horizontal driver configuration in most center speakers is driven by aesthetic and marketing considerations, not the best sonic performance."
Speakers: Where Do I Put Them?, Darryl Wilkinson, Feb 2010

"Horizontal dedicated centers are a throwback to the days when bulky TVs ruled the roost.  Their persistence in the market is rooted in the popular misconception that only a horizontal speaker can serve in the center position.  The people who design and market them know better.  Dedicated centers usually have a left-to-right woofer-tweeter-woofer driver array that's prone to horizontal lobing.  Lobing is the result of closely spaced drivers' propensity to sum and cancel each other, which causes uneven response from seat to seat around the room.  Better center speakers mitigate the problem with a vertically stacked midrange and tweeter or different crossover points for the individual woofers.  When a woofer-tweeter-woofer speaker operates vertically, what had been undesirable lobing can be seen as a possibly desirable limiting of vertical dispersion, which can reduce ceiling and floor reflections.  The best center speakers duplicate the sonic characteristics of your front left and right speakers.  An exact duplicate, placed with the same orientation, works especially well."
Loudspeakers Buyer's Guide, Don't Underinvest in Your Surround System, Mark Fleischmann, Feb 2010

Home Theater

Audioholics

"In every case where we measured a center channel speaker with redundant horizontal drivers we were able to improve the smoothness of its horizontal frequency response in that range by reorienting the speaker vertically."
"To get the most cohesive performance out of perhaps the most important channel in your home theater, strive for getting a center channel that is identical to your mains."  
Vertical vs Horizontal Center Speaker Designs, Chris Seymour, Audioholics, December 2009

"Indeed, a horizontal in-wall center speaker is a monument to ignorance."  "While it's true that dual horizontal midrange drivers covering the same frequency range will overlap and 'picket-fence' with each other at some angle off-axis, what is also true is that in the crossover region, where the midrange driver is crossing over to the tweeter, their output also overlaps and causes interference in the horizontal plane.  So it's not just the dual 5-inchers, it's also where the 5 inch crosses over and overlaps with the tweeter.  Even if it were a single 5 inch mid crossing over to a 1 inch tweeter, if they were mounted side-by-side, there would be a lot of crossover-region interference in the horizontal plane."  Note on the drawing.  Yes Cletus, drivers radiate in all directions, but your seating is arranged horizontally. This illustration shows the wave interference across the horizontal plane and how to fix it by simple rotation.  In addition to the crossover zone illustrated, wave interference will occur between the woofers depending on their spacing and high frequency limitations.
Center Channel Speaker Design Additional Considerations, Steve Feinstein, Audioholics, April 2012

"Probably the biggest compromise in speaker type and placement in a home theater is often unavoidable. I am speaking of the dreaded “horizontal center channel speaker.” Nearly all horizontal centers, even the better ones, have the wrong dispersion pattern. When the drivers are horizontally configured, the speaker tends to have narrow horizontal dispersion and broad vertical dispersion; the opposite of what is ideal."
Optimizing Front LCR Speaker Placment, Paul Scarpelli, Audioholics, July 2015

Audioholics

Widescreen Review

Widescreen Review editor Gary Reber:  "How should the center channel loudspeaker be implemented?"
BG Radia Loudspeaker principle designer Igor Levitsky:  "Horizontally positioned center channel with two midwoofers flanking a tweeter is a compromise and should be avoided if possible for serious home theatre systems.  If one has no choice but to use a horizontally positioned center channel, then use loudspeakers with vertically or coaxially positioned tweeter and midrange drivers.  The ideal setup, though, is to use a center channel loudspeaker that is identical to the left and right loudspeakers and positioned behind the screen."
Widescreen Review, June/July 2013

Widescreen Review

Height-Ranked Center Channel Options

As height is typically the limiting factor, here are some height-ranked options that all have smooth, quality horizontal response without lobing or comb filtering, all under a foot tall when set on its shortest side.  Our choice would be the JTR Single 8HT.

  3.94", Tannoy HTX101 XP  Not currently available in North America, but the rest of the world can be saved from horizontally redundant drivers.
  5.98", Tannoy Revolution XT Mini
  7.09", KEF Q150
  7.09", KEF R100
  7.50", Omega Loudspeakers Super 3i  While listed as pairs, I bet they'd make you a third.
  7.87", KEF LS50  A Stereophile review is here.
  8.27", KEF Q350
  8.27", Tannoy Autograph Mini 
  8.67", Technics SB-C700  You'd need to attach rubber feet on its side as it's curved.  A Stereophile review is here.
  8.70", Tannoy Revolution XT 6  Awkward to lay on its side due to its silly base.
  9.00", Omega Loudspeakers Super 7 Monitor Mk2 
  9.75", JTR Single 8HT  These excellent-sounding, US-made speakers are an easy recommendation.  Their subs are incredible.  This would be our pick from this list.
  9.84", Tannoy DC6i 
  10.5", Hsu Research CCB-8
  10.7", Tannoy DC8  This design does not lend itself to easily placing on its side, but it's still a horizontally-responsible center channel option.
  11.5", Seaton Sound Spark XC  Without the Spark's dual radiators, this purely-coaxial is orientation independent.
  11.0", Omega Loudspeakers Super Alnico Monitor
  11.6", Tannoy DC8i  With a more easily rotated box, although taller

If you have more available height than this, then the entire world of bookshelf or even tower speakers open up to you.  Get three, even if you have to toss an extra in the closet.  It's what George Lucas and Abbey Road Studios did.

Honorable mentions. Pioneer-owned TAD speakers have a great sounding coaxial mid-high driver, but lack any short options.  You could lay their (or any) bookshelf on the side, but there will be horizontal wave interference in the crossover overlap zone.  Not ideal, but at least it keeps the mids and highs from being horizontally arranged.  Thiel used to have a great coaxial driver but after Jim's passing they no longer have any short options.

Finally, there are many options with the midrange and tweeter vertically oriented.  This is better, but there will still be (less audible) wave interference in the woofer to midrange crossover zone and at least harmonically within the horizontal woofers.  Pushing this redundancy as far down the frequency spectrum helps, as wave interference ideally doesn't occur within the 1/2 wavelength that's being reproduced.  For example, at 1kHz the 1/2 wavelength is 6.7".  That's not much space to fit anything horizontal, so having horizontal redundancy that high in frequency will result in poor off-axis response.  The 1/2 wavelength of 400Hz is 16.7", enabling the speaker designer to design the crossover and more space to duplicate horizontal woofers with less audible effect.  Here is a pretty good lecture on how wave interference occurs.  If you can't respect acoustics at least to this degree, then you may be Shawn Byrne, the semi-literate conciliatory admin in a feral Facebook group.¹

Blu-ray Forum

Here is for the most part a summarization of our previous Audioholics article, but the member "Big Daddy" has filled out some of the perspectives more, and of course who wouldn't be fascinated by another 27 pages of forum discussion?  Center Channel Speakers, Big Daddy, Blu-ray Forum, March 2009

Blu-ray Forum 

¹"Some companies do better than others. That's it. There's no right or wrong. There's just is."