I've read that the typical horizontal MTM center channel suffers from impaired horizontal dispersion, which is usually countered by raising the tweeter in relation to the two woofer/midrange drivers. Given my lack of enclosure constraints, I thought it made more sense to build a simple vertical two-way center channel. However, I was also concerned about the low impedance of my drivers, and the modest power handling of the 5" Westra (20w RMS, 40w max). So I decided to address both issues by wiring the drivers in series (tweeter-tweeter, mid-mid) and building side-by-side two-ways in a single enclosure: hence the "D2W" (Double Two-Way) center channel speaker. This design presents my amplifier with an 8-ohm load and doubles the center channel speaker power handling capability to a level that works well with a 100w/channel amplifier.

People who believe that potential interference between closely placed identical drivers is to be avoided at all costs will probably stop reading right here. This seems to be a controversial area, and I liked the D2W design concept, so I thought it was worth an empirical test.

I modeled a pair of the Westra drivers in a sealed enclosure in WinISD. Although the optimal enclosure was 0.9 cu. ft., there seemed to be very little drawback to using a somewhat smaller enclosure. I settled on external dimensions of 13.5" wide, 9.75" tall, and 12" deep, which I could construct from one 12" x 6' piece of material. I used 5/8" particle board (would MDF be that much better?), so the internal dimensions are 1.25" less than the external ones, with an enclosure volume of 0.65 cu. ft. The vertical centerlines of the two-ways are 6.5" apart horizontally. Each tweeter and woofer are placed as close together vertically as possible, and each pair is vertically centered in the enclosure, for a simple symmetrical design: