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originally published: 9/20/2012

Inan 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 nodiseases, no poverty and no money. But,you do have to worry about the occasional Borg assimilation or Dominiontake-over of the Alpha quadrant. Inreality, our viewing screen prohibits us from having a tall center channelspeaker and our lack of warp drive keeps us far enough away from the badaliens.

Mountinga speaker horizontally, like nearly all center channel speakers, almost alwayssacrifices performance for convenience. Our ears are more sensitivetothe acoustical interference caused by horizontal lobing errors than they are tovertical lobing errors of multiple drivers being physically separated while sharingthe same signal and bandwidth of operation. (Don’t worryif that sentence was too dense, we will unpack it in this article) Listed below are the most common centerchannel design layouts employed by the majority of manufacturers and theirassociated pros and cons.

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

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Comparisonof Different Center Channel Design Topologies

MTM Configuration (Figure1.a)

Theclassic vertical MTM design has been popular for loudspeaker designs for anumber of years and for good reasons. Whenvertically oriented, they do a great job of reducing horizontal lobing betweenthe tweeter and midrange drivers thus producing a very smooth response. They also can produce more dynamic outputthan a conventional two-way design since they have two mid bass drivershandling 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 centerchannel speakers. Because of their lowprofile when oriented horizontally, MTM's became quite popular to use asdedicated center channels and were often times sold in sets of three (deemedLCRs) where the user would flip one on its side and place it directly under thedisplay to be used for center channel duties. The results can be quite good provided the actual speaker is designedwell, but there are limitations in off-axis performance that shouldn’t beignored.

Wethoroughly covered these limitations in the following three articles:

Nested MTM Configuration (Figure1.b)

Asthe demands for better performance were placed on the center channel speaker,manufacturers began looking at ways to improve upon the very popular MTMconfiguration. The goal was to reducethe spacing between the horizontally mounted woofers to minimize lobing errorsand improve off-axis response. Of coursedoing this involved different cutouts in the cabinet design. This meant that manufacturers would have to produce a specific cabinet for center channelduties vs the traditional cutouts used for the classic vertical mountedMTM. Hence the nested configuration wasborn. The end result is a slight improvement ofoff-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 dedicatedbass 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 (Figure1.c)

TheW(T/M)W horizontal center channel has recently become a favorite amongmanufacturers and consumers. It’s basicallya 3-way loudspeaker design which allows the larger woofers to be crossed overat a low enough frequency where acoustical interference between drivers playingthe same bandwidth becomes a non-issue. Ina W(T/M)W design, a dedicated midrange driver is mounted directly under thetweeter to improve off-axis performance. This can widen the area of coverage for more consistent sound across thelistening seats. The downside cansometimes be restrained dynamics by having a smaller, single mid-frequencydriver instead of two from the classic MTM design. The other disadvantage is that this type ofspeaker requires more vertical real estate, which can often be a challenge whentrying to cram a speaker under a display. Some users instead opt to just use a vertically oriented two-waybookshelf speaker. This can be a greatoption 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 (Figure1.d)

Wedon’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 verticalspace while still making a speaker with multiple drivers that can play very loudly andabsorb a lot of power. The problemhowever, is that it involves physically separating high frequency drivers at adistance much greater than the wavelength where acoustical interference betweenthe tweeters becomes a significant factor. This is problematic when not sitting directlyon-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 areequidistant from both tweeters, this won’t be much of an issue. However, the user must determine how faroff-axis they plan on positioning other seats as this type of design producesthe worst lobing errors (especially the TMMMT version) of all four designs discussed here and thus results inthe 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.

CenterChannel Variants

Wecan’t obviously cover every permutation of design involving center channelspeakers. But most of the major designsare 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, butattempts to average them out for broader coverage and better off-axis performance. Theadvantage of such designs is to reduce distortion, increase power handling andoverall 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 lobingerrors. There are also compromises heresince the horizontal plane radiation pattern becomes asymmetrical as aresult. These designs were popular a decade ago but aren't as common these days.

Conclusion

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

Usingthe guidelines presented in this article and our three previously referenceddetailed articles on center channels will help you determine what kind ofcenter channel is right for your needs. It’s important to try to match the front three LCR speakers as closelyas possible to ensure you achieve the most seamless transition betweenthem. Hence this is why it’s typically agood idea to stay within a certain series of products from the manufacturer ofyour choice to ensure your front LCR speakers have similar drivercompliments, quality of components and output capabilities.

Acoustically Transparent Screen courtesy of SMX

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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.