Pros & Cons of Various Center Channel Designs

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 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

MTM Configuration (Figure 1.a)

The 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:

• Vertical vs Horizontal Center Channel Designs
• Vertical vs Horizontal Center Channel Designs – An Alternate Perspective
• Center Channel Speaker Designs Additional Considerations

Nested 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. 

W(MTM)W Center Channel Courtesy of Infinity

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

The 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.  

W(TMMT)W Center Channel

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.  

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.