originally published: 9/20/2012

In
an ideal world we would have three (3) identical speakers with a vertical arrangement of drivers for the front left,
front right and center speakers; hence the term “matching LCR’s”.  In this idealistic world, there are no
diseases, no poverty and no money.  But,
you do have to worry about the occasional Borg assimilation or Dominion
take-over of the Alpha quadrant.  In
reality, our viewing screen prohibits us from having a tall center channel
speaker and our lack of warp drive keeps us far enough away from the bad
aliens. 

Mounting speaker drivers horizontally almost always sacrifices performance for convenience.

Mounting
a speaker horizontally, like nearly all center channel speakers, almost always
sacrifices performance for convenience. 
Our ears are more sensitive to
the acoustical interference caused by horizontal lobing errors than they are to
vertical lobing errors of multiple drivers being physically separated while sharing
the same signal and bandwidth of operation. (Don’t worry
if that sentence was too dense, we will unpack it in this article)  Listed below are the most common center
channel design layouts employed by the majority of manufacturers and their
associated pros and cons.

Center Channel Speakers: Which Design is Best for Home Theater?

Comparison
of Different Center Channel Design Topologies

Center Diagram 2

MTM Configuration (Figure
1.a)

Polk CenterThe
classic vertical MTM design has been popular for loudspeaker designs for a
number of years and for good reasons.  When
vertically oriented, they do a great job of reducing horizontal lobing between
the tweeter and midrange drivers thus producing a very smooth response.  They also can produce more dynamic output
than a conventional two-way design since they have two mid bass drivers
handling the bandwidth instead of one. 
Some more upscale designs incorporate an additional pair of midrange drivers in a MMTMM arrangement to offer broader coverage and increased output capability.

With the advent of 5.1 surround sound, people quickly began looking for center
channel speakers.  Because of their low
profile when oriented horizontally, MTM’s became quite popular to use as
dedicated center channels and were often times sold in sets of three (deemed
LCRs) where the user would flip one on its side and place it directly under the
display to be used for center channel duties. 
The results can be quite good provided the actual speaker is designed
well, but there are limitations in off-axis performance that shouldn’t be
ignored. 

We
thoroughly covered these limitations in the following three articles:

Atlantic2Nested MTM Configuration (Figure
1.b)

As
the demands for better performance were placed on the center channel speaker,
manufacturers began looking at ways to improve upon the very popular MTM
configuration.  The goal was to reduce
the spacing between the horizontally mounted woofers to minimize lobing errors
and improve off-axis response.  Of course
doing this involved different cutouts in the cabinet design. This meant that manufacturers would have to produce a specific cabinet for center channel
duties vs the traditional cutouts used for the classic vertical mounted
MTM.  Hence the nested configuration was
born.  The end result is a slight improvement of
off-axis response over classic MTM’s. 

More elaborate designs can employ an additional pair of midranges in an M(MTM)M configuration for broader coverage and greater output capability.  Some manufacturers even offer a 3-way variant W(MTM)W where dedicated
bass woofers are located on each side of the nested MTM arrangement.  This helps extend the bass response and power handling of the center channel. 

Infinity center2

W(MTM)W Center Channel Courtesy of Infinity

W(T/M)W Configuration (Figure
1.c)

B&W CenterThe
W(T/M)W horizontal center channel has recently become a favorite among
manufacturers and consumers.  It’s basically
a 3-way loudspeaker design which allows the larger woofers to be crossed over
at a low enough frequency where acoustical interference between drivers playing
the same bandwidth becomes a non-issue.  In
a W(T/M)W design, a dedicated midrange driver is mounted directly under the
tweeter to improve off-axis performance. 
This can widen the area of coverage for more consistent sound across the
listening seats.  The downside can
sometimes be restrained dynamics by having a smaller, single mid-frequency
driver instead of two from the classic MTM design.  The other disadvantage is that this type of
speaker requires more vertical real estate, which can often be a challenge when
trying to cram a speaker under a display. 
Some users instead opt to just use a vertically oriented two-way
bookshelf speaker.  This can be a great
option especially if the same speaker as the front left/right speakers is used.

We have also seen W(M/T/M)W versions where the center channel is a 3-way design employing a vertical MTM driver arrangement with dedicated bass woofers on each side of the MTM array.  This can be a great approach to increase dynamic output over a similar W(T/M)W design but it increases vertical real estate, and cost. 

WTMMTW or TMMT Configuration (Figure
1.d)

We
don’t see these type of center channel designs often.  The W(TMMT)W is a 3-way design where the outer woofers primarily operate in the bass frequencies while the inner dual or sometime triple midranges handle vocal duties.  Budget and more space conscious designs will scale this back to a 2-way design such as the example pictured below or a TMMT or TMMMT variant.  

HiVi

W(TMMT)W Center Channel

Center channels that horizontally separate multiple tweeters can be problematic when sitting off-axis.

These types of center channels offer a good option for saving vertical
space while still making a speaker with multiple drivers that can play very loudly and
absorb a lot of power.  The problem
however, is that it involves physically separating high frequency drivers at a
distance much greater than the wavelength where acoustical interference between
the tweeters becomes a significant factor. This is problematic when not sitting directly
on-axis to the speaker and equidistant to the tweeters (ie. the sweet spot). 
If the listener is positioned in the sweet spot seat and his/her ears are
equidistant from both tweeters, this won’t be much of an issue.  However, the user must determine how far
off-axis they plan on positioning other seats as this type of design produces
the worst lobing errors (especially the TMMMT version) of all four designs discussed here and thus results in
the narrowest usable listening window. 
If the installation requires a speaker with a small vertical footprint,
then a single tweeter MTM or a MMTMM (for more output) is usually a much better choice.  We caution anyone considering a TMMT or TMMMT center channel if you’re sitting anywhere but directly on-axis to the speaker.

Center
Channel Variants

We
can’t obviously cover every permutation of design involving center channel
speakers.  But most of the major designs
are based on 1 of these 4 approaches. 
The most common variants we discussed are based on MMTMM, W(MTM)W and W(M/T/M)W driver topologies as we previously mentioned.

Some companies offer multi woofer and tweeter designs such as (WTWTWTW)
which still produces the lobing errors found in any multi driver design, but
attempts to average them out for broader coverage and better off-axis performance.  The
advantage of such designs is to reduce distortion, increase power handling and
overall output but these tend to be very big, bulky and expensive alternatives.  Other center channel designs include 2-1/2 way MTM’s where one driver is bandwidth-limited to reduce lobing
errors.  There are also compromises here
since the horizontal plane radiation pattern becomes asymmetrical as a
result.   These designs were popular a decade ago but aren’t as common these days.

Conclusion

Acoustical
interference caused by multiple drivers playing the same bandwidth is a non-issue if the radiating driver’s cutoff frequency is low enough so that the
distance between adjacent drivers is small relative to the wavelengths they are producing in equal or near equal proportions. (This is known
in filter-speak as the “transition band” or “crossover band”).  As the order of the crossover is increased (6
dB to 12 dB to 18 dB to 24 dB or beyond), the amount of phase shift within the
crossover band increases, so the likelihood of the two speakers being out of
phase at any given angle off-axis increases, while the frequency range over
which this is an issue decreases.  This
is why it can be acceptable to horizontally place multiple woofers and
midranges (if the center to center distance between them is closer than the
highest wavelength of operation) but, typically NOT tweeters.

Using
the guidelines presented in this article and our three previously referenced
detailed articles on center channels will help you determine what kind of
center channel is right for your needs. 
It’s important to try to match the front three LCR speakers as closely
as possible to ensure you achieve the most seamless transition between
them.  Hence this is why it’s typically a
good idea to stay within a certain series of products from the manufacturer of
your choice to ensure your front LCR speakers have similar driver
compliments, quality of components and output capabilities.  

screen

Acoustically Transparent Screen courtesy of SMX

If you absolutely want to use identical matching vertical LCRs or towers and don’t want to block your image, consider an acoustically transparent screen which will allow you to place the center channel speaker behind it with minimal degradation of sound quality.  Alternatively, if your theater room is small and doesn’t require large high output speakers, respectably good performance can be achieved by using identical two-way bookshelf speakers all vertically oriented which should still be small enough to tuck below your screen or display without causing any visual ugliness. 

gene posts on April 13, 2016 15:29


Vh4_3EefmJ8

fuzz092888 posts on March 17, 2014 21:55

walter duque, post: 1023842
This might sound crazy to you but I told my wife that when I go down for good to give you a call and you can pick my system up. All you need to do is just replace it with something simple to operate, like a little Bose system or something. I have nobody that can appreciate my set-up. My 2 dummy sons listen to earbuds on their I’pod and that’s about as far as good sound for them goes.

That does sound crazy. Well, it’s a good thing we won’t have to think about that for a good long while. As for replacing it with something simple to operate, I think I can handle that.

charmerci posts on March 17, 2014 21:54

ArnoldLayne, post: 1023817
I recently increased my screen size such that the only place for a center is behind the screen. I’m not going to spend several thousand on an acoustically transparent screen however. In fact, my 120“ fixed 1.1 screen was about $400 and is great. My front speakers are 6.5” pedestal mounted bookshelf type . So they are vertically centered on the screen so the sound emanates correctly instead of from below the screen. I like it. Along with an SVS PB12-NSD, side and back surrounds.
cheers

Weird – I just chose to play Arnold Layne, then came here and saw your “name.”

walter duque posts on March 17, 2014 21:52

fuzz092888, post: 1023841
Well if I did buy that set, I’m pretty sure my GF would murder me in my sleep so then you’d get it anyways

This might sound crazy to you but I told my wife that when I go down for good to give you a call and you can pick my system up. All you need to do is just replace it with something simple to operate, like a little Bose system or something. I have nobody that can appreciate my set-up. My 2 dummy sons listen to earbuds on their I’pod and that’s about as far as good sound for them goes.

fuzz092888 posts on March 17, 2014 21:41

walter duque, post: 1023840
And I always thought you and I are OK.

Well if I did buy that set, I’m pretty sure my GF would murder me in my sleep so then you’d get it anyways