Bracing is more difficult but not overly so IMO. I disagree that subs don't need cross bracing, because even though the resonances are out of the passband, pressure based vibrations aren't. Not many people build critically damped subs.
Tuned to 15hz, let's say
9 cu ft
5.3" x 8.5" x 55" slot port
14.5hz 2nd order high pass filter
Will do about
115.7db at 23hz (driver excursion limited)
115db @ 18hz (vent velocity limited)
108db @ 15hz (vent velocity limited)
107.5db @ 13.5hz (vent velocity limited)
1st port resonance is at 122hz. A 60hz crossover point is nessesary to keep it out of the sub's passband.
Attempting to do louder than that may produce vent compression and chuffing.
If OP has mains which are comfortable down to 35hz or so with high output, it may work.
The passive radiators will allow a higher crossover point because there is no vent resonance. Although I recommend a 6.5 inch box earlier, this time for comparision's sake I will also use an equivalent 9 cu ft box for the passive radiators (which will still be smaller because there is no port) to tune it to 15hz. In retrospect I really don't remember WHY i used a smaller box in the first place... I must have had a reason.
9 cu ft
3 APR15 + 1200 grams added mass
15hz high pass filter
Will do
115.7db at 23hz (driver excursion limited)
116.5db at 18hz (driver excursion limited)
113db at 15hz (PR excursion limited)
109.5db at 13.5hz (PR excursion limited)
I never said the vented box won't perform. Dollar for dollar it may be a better solution. BUt I do think the PR box will have stronger infrasonics.
