Idea: dedicated woofer for 2.1 system. Opposing drivers to cancel vibrations.
Kallax compartment is about 335x335x390mm. A 335x335mm baffle could just about fit an 8″ passive radiator and a 6.5″ driver, but it’s very close – 8″+5″ would be a better fit. Assuming 12mm plywood, internal volume using maximum dimensions is 3.11×3.11×3.66=35.4L. Subtracting driver volume, that’s about 34L cabinet.
Let’s run some options in Hornresp (including max SPL, quarter space radiation)
| Driver (2x) | Drone (2x) | F3 | F6 | F12 | SPL@35Hz | SPL@60Hz |
| SB16PFCR25-4 | SB20PFCR-00 | 54 Hz | 42 Hz | 33 Hz | 100 dB | 110 dB |
| Visaton W130X | SB20PFCR-00 | 48 Hz | 33 Hz | 29 Hz | 97 dB | 103 dB |
| Dayton DA135-8 | SB20PFCR-00 | 47 Hz | 37 Hz | 29 Hz | 95 dB | 101 dB |
| W5-1138SMF | SB20PFCR-00 | 46 Hz | 35 Hz | 29 Hz | 107 dB | 113 dB |
| W5-1138SMF | none | 66 Hz | 46 Hz | 29 Hz | 101 dB | 110 dB |
| SB20PFC30-4 | none | 69 Hz | 54 Hz | 36 Hz | 104 dB | 113 dB |
Ok so assuming I can do some EQ in the low end to bring up the response, for the same money I can better just put in a pair of more powerful drivers and no passive radiators.
Looking at smaller drivers, I’m intrigued by a 2×5″ SB13PFC25-4 sub that could potentially do 91+ dB (quarter space) down to 30 Hz at xmax and would need less than 30W for it – at only 6.1L internal volume. That could fit in a 22cm cube, including space for a TPA3221 amplifier and a laptop charger to power it. But if I have the space in a Kallax compartment, 2×8″ SB20PFC30-4 looks great, much louder so probably cleaner at lower levels, and at Qtc of 0.75 should be relatively easy to EQ and crossover.