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[germanyt 10+ year member]

Learn math. You can't just add diameters together to figure out equivalent area. The easy way to do it is to get the Sd for each driver. The harder way is to get the actual cone diameter and figure out the area yourself.
A=pi*r^2.

Your average 12" sub only has about a 10" cone because of the width of the surround (the surround does not contribute apprecably to the surface area of the sub) and a 15" is about 13".

Using the above equation a 12 has about 78 in^2 of surface area and a 15 has about 132 in^ of surface area. 4 12s is 312 in^2 and 2 15's is 264 in^2. The 12s DO have more cone area but according to your math, 3 12s would have more area than 2 15s and if you actually do the math you will see that that isn't the case.

I was gonna say that, you beat me too it.

 

[Insomniac119 10+ year member]

- 4 SX 12"s- 2 SX 15"s

best.