The info on the Delta 15a is completely contradictory! Furthermore, it does not appear to be suitable for bass cabs. If anyone has any real-world experience with it, please chime in.
The Alpha 15a on the other hand is a true sealed bass driver by vintage standards @ 2.8 cu.ft. stuffed, and a 100 Watt mechanical limit (F3@57Hz).
The Alpha has an efficiency rating of 97dB.
This is an interesting project, but there are some performance aspects that may be worth considering.
1) The bass is strongly peaked: 8 db at 90 Hz. The peak is fairly broad, so that fundamentals and harmonics between about 70 and 140 Hz will be reproduced at a much higher level than the remaining harmonics.
2) With a system Q (Qtc) around 2.7, the bass resonance of the system is poorly damped. Notes definition will be extremely poor (beyond soft, fat, mushy and pillowy) and low notes will audibly boom.
3) Power handling will be fine until the system resonant frequency (around 110 Hz), below which excursion limited power handling will plummet: less than 100w between 70 and 95 Hz. There is a lot of energy in that range.
People need to decide what they want performance-wise in a cab and design accordingly. That would include the tonal quality of the bass (articulate, well defined, rich, fat, etc.), bass extension (do you want the deep fundamentals, or how deep do you want fundamentals to be reproduced), and how much power must the cab take to produce the volume you need (how big is the venue, and how loudly do you play). You also need to consider the mids and highs (will a mid or tweeter be needed), and their impact on the tone of the cab.
Until the above questions are answered there is little point in working on a design (except maybe to learn, and that is a good one!).
