Pet peeves

One of the most irritating trends in home theater technology is the marketing of HT speakers, especially center channel speakers, of an MTM design. The MTM design is a two-way speaker with a tweeter in the center flanked by two identical woofer/midranges. What's wrong with this incredibly common design approach represented below?

First, all else being equal, a 3-way speaker will always sound better than a 2-way design. Since 3-way speakers divide the incoming signal among 3 drivers rather than 2, each driver is responsible for reproducing a smaller percentage of the acoustic spectrum. More importantly, in a 3-way design the all-important midrange is handled by one dedicated driver optimized for midrange reproduction and freed from having to reproduce the bass. The result is generally that the sound is much more dynamic and punchy than a similar 2-way design, and the midrange is more natural and open. Also, speakers of this design tend to avoid a kind of chestiness when reproducing voices for which MTM designs are notorious.

However, there is a more fundamental design defect to the MTM type of speaker beyond the fact that it's merely a two-way design. After all, there are many excellent sounding two-way designs on the market. The bigger problem is what's called "lobing." This is a phenomenon whereby the sound waves coming from the left most speaker create interference patterns with the sound coming from the right most speaker, canceling each other out. The result is a frequency response that exhibits successive peaks and valleys as the listener moves across the horizontal plane.

The MTM design is less of a problem with the L/R and surround speakers, because they are typically placed in a vertical alignment. The lobing still occurs, but now in the vertical plane, which is less critical in the average listening environment.

So, if the MTM design does not work well for HT center channels, what does? Actually, it's based on the MTM design, except that it's 3-way, as represented below.

This 3-way version of the common MTM design solves all of the problems discussed above. It exhibits all the dynamics and midrange clarity of 3-way designs. It also avoids most of the problems with lobing, because the leftmost and rightmost speakers are now reproducing a much smaller percentage of the overall sonic presentation, focusing exclusively in the bass region where the wavelengths are much longer. With long wavelengths the consequences of the offset between the two speakers is negligible.

Several well-known manufacturers of quality speakers offer this 3-way MTM design, NHT and Paradigm come to mind, but there are many others. There's no reason for you to accept inferior 2-way MTM's so common on showroom floors.