The 2227 fifteen-inch cone driver should be thought of as a low-midrange-not really a woofer. Yes it's a big driver with a big voice coil. In fact, I use two of them in my bass guitar rig, but its QTS and moving mass are so low that when you put it through Thiele-Small calculations and plot curves on a computer as I did countless times, the device ends up looking more like a midrange itself. To be accurate, the Keele exponent-corrected program I use (because it gives me systems that measure the same as the model predictions) calls for about 1.5 cubic feet per driver, tuned to about 85 Hz-not exactly organ pedals. I ended up opting for a slight over-damping of two units in a 3 cubic foot volume tuned to 80 Hz. Even so, the unassisted output of the box is flat to 65 Hz and droops only slightly at 40 Hz, in the middle of a 40,000 cubic foot room (my test lab at Imagineering R&D).

Be sure the high-pass filter on the amplifier driving the two 2227H's is set to 80 Hz and rolls off at a rate of at least 18 dB per octave. The 2227H drivers are high-efficiency, limited linear excursion devices (in fact, they are one of the highest efficiency cone drivers made anywhere). The crossover frequency of 80 Hz is the design target to limit cone excursion and produce a good transition to the subwoofers.

The enclosure is built as rigid and non-resonant as possible and then lined with fiberglass over the entire interior surface area, except around the ports where air turbulence might tear off pieces of fiberglass and spray them around. The woofer portion of the enclosure is the only real structure. The midrange tube is extremely rigid and exceptionally non-resonant. My goal in designing the woofer section was to minimize spurious panel-vibration and acoustic output-within reason. There is more panel output, in fact, from the thick, ribbed metal back cover of the compression driver.

I used four two-by-fours for bracing inside the woofer compartment. These were counter-drilled for wood screws and glued with aliphatic resin glue, on-edge, to the compartment panel interior surfaces. I tried to space the braces at random-so that no two unbraced panel areas were the same size-thus randomizing panel section resonances. I also glued the two cutout discs from the woofer holes to the outside of the back panels to make the total panel thickness 2 inches, plus braces!