For this new system I decided to step up to probably the best dipole woofer money can buy, the Acoustic Elegance Dipole 15. I ordered four of these drivers and a few weeks later they arrived. The Acoustic Elegance Dipole 15 is the work of a real craftsman with a machined cast frame, a huge magnet, and a stiff paper cone with a large silver phase plug at the center. However, the Acoustic Elegance woofers presented a different crossover challenge compared to the Eminence Alpha 15A woofers. The Acoustic Elegance Dipole 15 woofer has a lower fs of 22 Hz and a Qts of 0.8 which when combined with a 12 dB/octave low pass crossover, my standard recipe for the Alpha 15A woofers, results in a rolled off low frequency response which does not reach very deep. The 6 dB/octave dipole roll-off starting above 100 Hz really attenuates the deep bass output, the dipole response has rolled off the woofer's bass output by 12 dB by the time you get down to 25 Hz (two octaves below 100 Hz). My solution was to use a 6 dB/octave low pass filter set to about the driver's fs to counter the dipole roll off yielding a flat low frequency response. So far so good but nothing is ever without a trade-off, the penalty is that this crossover configuration also lowers the SPL/W/m output to about 83 dB for a pair of Dipole 15 woofers wired in parallel. The Acoustic Elegance drivers are not so efficient to start with at about 90 dB/W/m, so you need to use an active crossover to boost the low end to match the output of the Lowther full range driver which has an SPL/W/m of about 97 dB.
I looked at a lot of configurations for OBs, U frames, and H frames keeping in mind that I wanted a flexible system that could be repositioned and paired with different full range drivers pulled from my growing collection. Building four U or H frames seemed to make the most sense and I ended up compromising and building an offset H frame that is probably closer to a U frame design. The H frames are 11.75" deep and 17.25" square (outside dimensions) with the baffle offset back two inches from the front. The offset is intended to provide some protect for the Acoustic Elegance woofer's cone. Each Acoustic Elegance Dipole 15 woofer had its dual voice coils wired in series and then the pair of H frames per channel were connected to the amplifier in parallel. The active crossover applied a 6 dB/octave low pass filter set at 30 Hz and then boosted the power to the paralleled Acoustic Elegance woofers by 18 dB. The H frame's calculated SPL response matches the Lowther driver's SPL/W/m and is flat with -3dB points at 25 Hz and 240 Hz. ..
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