2 10's or 1 15?

[squeak9798 5,000+ posts]

SPL is Sound Pressure Level. So in effect, its a measurement of how much your speakers are pressurizing the air molecules inside your car.
Power is only one part of the overall output equation. For example, driver efficiency and driver/enclosure efficiency affect the power input part of the equation. There is also a phenomenon called power compression that basically means as you increase power input to the driver, efficiency drops slightly (it will not use a second 500 watts as efficiently as it did the first 500 watts) due to power loss in the form of heat.

Bah....you jumped ahead //content.invisioncic.com/y282845/emoticons/tongue.gif.6130eb82179565f6db8d26d6001dcd24.gif The point I was trying to make with the original example is that, all things being equal, two subs moving the exact same amount of air in the exact same way will have the same SPL, regardless of how much power it takes to make each sub get to the point of where they are moving the exact same amount of air.