So 2.75 cu.ft for the 18? Then likely the 15 would need to be a tad smaller?
Yes it would. However remember with a sealed speaker the only thing moving air is the speaker cone. That is inefficient and more so the smaller the cone. With other alignments that is not so but appears so. What really matters is Fs, sensitivity/efficiency and displacement. Fs is really of over riding importance to bass extension, but not output. Increased cone size usually equates to heavier which means lower Fs also reduced excursion for the same output. Then another issue, the higher the flux density and therefore sensitivity the lower the bass extension. Horns, pipes and Helmholtz resonators are much more efficient couplers in that order. Pipes also have a property of encircling which enables them to defy the distance square law which makes them particularly effective for low bass duty.
This has really been brought home to me in our new very large lower great room space open to the upper level via the stairs. One Dayton 10" driver fills the space with little power, so much so that I have not stopped reducing its volume. The bass it totally even in the space. It takes little power. I'm now more convinced than ever that the optimal alignment for a sub is a reverse tapered aperiodically damped transmission line. The perceived quality of the bass is also much better than other alignments.
I have been convinced for a long time that the only reason to build a sealed sub is that you don't have room for something better.
