This is a matter of efficiency and low frequency extension. A larger cone moves more air than a smaller one, and therefore does not need to move as much in order to achieve the same SPL level as a smaller cone. With a larger cone, it is easier to achieve a low free-air resonant frequency, and therefore better low frequency extension. A small cone can have as low a free-air resonance, and reproduce low frequencies, but a 12" cone would have to move much more distance back and forth in order to reach the same SPL at the same frequency as an 18" driver. It would reach it's excursion limits at the lowest frequencies way before the 18" driver for equal SPL. This also means that the 12" driver would be more prone to non-linear behavior because of the distance the cone must travel, and therefore distort more. In addition, any port noise would be higher with the smaller woofer and cabinet.

I use four 18" subs, each in an 8 cubic foot cabinet in order to match the efficiency and output capability of my mains, which have efficiency of 106db/watt. My room also is 6,600 cubic feet. It would be impractical to have enough 12" subs to achieve the SPL and low frequency extension as the 18" subs. With my seup, response extends to 16Hz at 120+db SPL (my 1/2" Bruel & Kjaer instrumentation microphone clips before I can measure higher SPL).

If you don't need high output capability combined with good low frequency extension, a smaller subwoofer will work, however, as output and low frequency extension demands rise, the only practical solution is a large speaker driver, in a large cabinet.


[This message has been edited by soundhound (edited April 07, 2003).]