I recently, (foolishly?) have been thinking about constructing PA speaker cabs.
I've been looking at bin designs horn loaded bass bins in particular, and 1 thing strikes me as odd about the designs of bass horns I have been looking at.
They seem to have a very small sealed cavity of air behind the driver.
This would seem to be at odds with the general principle that bass speakers require large enclosures or tuned ports, and I would have thought this would create significant resistance on the cone movement and reduce the efficiency of the driver. This isn't the case with scoops and many sub designs, yet many tried and true bass horn cabs seem to have this very small cavity behind the driver
......They seem to have a very small sealed cavity of air behind the driver.
This would seem to be at odds with the general principle that bass speakers require large enclosures or tuned ports, .......
The enclosure behind the driver is there for exactly what you think, to add resistance. This keeps the driver frum unloading and other such things and actually won't muck with the sensitivity or efficency as much as you think.
The part you are missing is the horn, which is doing a lot more than you might think. It actually acts like an impedence matching transformer. The driver is made of dence, rigid material while air is very much the opposite of that. Any time you radicaly change mediums while transducing a signal, you loose a good bit of energy doing it. What the horn does is keep a small, fast moving and densly packed (because of the other air behind it) bit of air near the driver (to try to closer match the "impedance") and then gently match this to the outside air in the room with a large opening.
My advice would be that if you don't have the skill and patience (I might have the former, but certainly don't have the latter) to build LAB subs, you are best to stick with used, store-bought boxes like JBL SRX728s, EV QRX218, and Peavey QW218.