I looked at the Ultimax 18 vs SI24 and ended up with four Ultimax 18 and also two SI 24 that are waiting for boxes to be built. The 24's are probably going to be nearfield subs.
Using MFG specifications:
The 18s displacement is 2682cm^3 vs the SI 24 is 7896cm^3 (one 24 = three 18)
One 24 or three 18 could give 100 dB ,101dB respectively at 10Hz (free space).
The 18s are 89dB and the 24s are 95dB (it takes 4x the power to get an 18=24)
(three 18s put close together give 98dB with 1w each. A 24 makes 98dB with 2w)
For high frequencies at 101dB the 24 needs 4W and the three 18s need 2w each.
The 24 builds to a sealed box of Qtc=0.7 with 34Hz resonance and -3dB (roughly)
The 18 builds to a sealed box of Qtc=0.7 with 26Hz resonance and -3dB
The box for a 24 needs 250L the box for the three 18s needs 855L (8cf vs 30cf).
The dB ratings above are for frequencies higher than resonance (mass controlled oscillation)
At 10Hz the oscillation is spring controlled.
The 24" is at -24dB when playing 10Hz and would need 1000w to Play 100dB.
The 18"s are at -19.5dB when playing 10Hz and need 100w each to play 100dB.
However you can put more than 3x the bass into a given room using 24" subs than when you put 18s in because the box is smaller compared to, xmax*Sd, displacement.
Sub costs are $1300 each 24 vs $975 per three 18s.
So if your limit is money the amps and subs are both less per 10Hz bass SPL with 18s.
If your limit is % room volume taken by subs the 24s are the better choice.
Adding a 4th 18 brings the price up to the single 24 (rougly).
Cutting the cabinet volume in half gives 20cf box volume with four 18s and can play 103dB at 10Hz using about 150w each driver. That's what I have in my HT.
Last edited by artsci2; 01-12-2019 at 02:30 AM.
Reason: corrected value of Ultimax 18 displacement is 2682cm^3