The response given above was basically correct, a bandpass does put a resonance chamber on the front of a closed box loudspeaker and this does increase the efficiency. It is a fairly narrow band design, which is fine for a sub, but I don't give any importance to group delay at LFs in small rooms.
They are hard to get working correctly, I will agree with that, but I have had a lot of experience with them so I can seem to make them work.
They are "passive" in that the amps are external. I have not found plate amps to be reliable and they are too limited in the controls. I recommend a DCX2496 and some rack amps for the LF subs.
I also don't worry too much about "frequency response" of a sub because the room changes it completely anyways. How the subs work in the room at the choosen location is what matters and this is far more determined by the room and placement than by any free field measurement of the subs. In the end the sub gets EQ'd all over the place anyways. What IS required is a great deal of linear output capability and bandpass does this when a robust driver is used. I measure this as a near field measurement of the output, but not anything free field.
I used multiple ports because there was no place to put one. Large areas require long lengths, but a small area has too much turbulence, noise and distortion. So I needed to devide the poart into smaller pieces that would fit.
The sub is about 18 x 16 x 14 inches and weighs about 45 lbs.
Such a simpledesign does make sense as a kit. There is no way for me to make a decent profit.
My queue is pretty long at the moment, about three months.
Earl Geddes