A friend was saying that there is not much difference between a 15" and 12" sub. But a big difference compare to a 8" because of the surface area of the cone. The middle part is about the same size. So generally, small subs are useless, since they have less area to pump air.

How true is that statement?

 

Actually, there is a pretty good difference between any size of sub in respect to surface area. As for movement of air, the size only tells you half of the story. For example, the amount a sub can move is called excursion. If you have 2 10" subs, one's Xmax rating is 12mm and the other is rated at 8mm, the first one would move more air because it has more excursion from peak to peak. So saying that there isn't much difference in the surface area of 2 different sized subs is not telling a complete story. There are many 15" competition subs that will move more air and reproduce higher volumes than 2 or even 3 12" subs together, just as there are many 12" comp subs that will out perform many 15" subs. Also, keep in mind that a lighter mass in smaller subs can reproduce sounds more accurately than their larger counterparts, it's all a matter of preference, loud, bragging about the most subs, or accuracy to hear the crashing of thunder in some test tracks.....