You can really only compare displacement in the same subwoofer alignment. Even then it is still subject to amplifier power availability.
To figure displacement you take the Sd in cm squared and multiply times twice the X-Max and divide by 10,000 to get the number in liters. You can convert the mm to cm for the X-max and then divide by 1000, but I think it is easier to just divide by 10,000.
Sd x 2(X-Max)/10,000=Displacement
A 15" driver usually has an Sd of around 830 cm squared and an 18" driver of around 1180 cm squared.
Two (2) 15" drivers with 19 mm of X-max have a displacement of 6.31 Liters.
One (1) 18" driver with 30 mm of X-max have a displacement of 7.08 Liters.
Four (4) 15" drivers with 19 mm of X-max have a displacement of 12.62 LIters.
Two (2) 18" drivers with 30 mm of X-max have a displacement of 14.16 Liters.
Eight (8) 15" drivers in my IB have a displacement of 24.57 Liters.
If HuskerOmaha gets all four of his subs running, he will have about 32 Liters of displacement
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