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I haven't thought much about a passive radiator though. I guess I would be concerned with stored energy. Just from the physics of it wouldn't a radiator "ring" and to some degree attempt to drive the driver... almost like intermodulation?
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PR's are considered bass reflex alignments, just like a traditional port. Output drops off a bit faster below tuning than a traditional ported box of the same volume and tuning. No worries about port noise. More expensive to build, but the most space efficient way to tune a small cabinet to a low frequency.
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Do you think it would matter if I faced the driver backward?
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It would be no different than downfiring, all things being equal (distance to wall/floor and crossover frequency). Maybe slight variations if your floor is carpeted, etc. Forward with a grill should be the gold standard.
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It sounds like you are really against doing an LT. It seemed to me to be the best for a small enclosure considering room gain. The sealed enclosure has such a shallow slope.
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Nothing personal against LTs, or any other alignment. I just like to try to find "natural" solutions that will accomplish my design goals. For instance, I don't like to count on enclosure stuffing to allow me to build a smaller than optimum sealed box, either. Just personal preference, KISS principle, whatever you want to call it.
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So, revised plan:
390HO in 2.3 cubes. 3db of eq + 6db of room gain should get me pretty flat out to 20Hz. And it looks like 180w is sufficient to fully use the Xmax. And if the 6db of room gain is right then I only need 22.5w to reach my 100db target. That sounds too low though... WinISD Pro is showing 180w to reach 103db @20Hz. If I am assuming 6db room gain, then it would be 109db. And 1/8th of the power brings it down to 100db. But that is only 180/8 = 22.5w. Anyone care to sanity check my math?
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WinISD seems to calculate SPL/Excursion etc. by adjusting the specified "system input power" by the amount of boost added by your LT, but there's nothing to indicate how much power the driver is then receiving in the model. Apparently, the assumption is made that your amp can deliver the additional power needed above the input power specified...this might even be in the Help file, but I haven't looked. Regardless of EQ, SPL is a function of cone travel and cone travel is a function of input power. So, with a bit of playing with EQ/No-EQ models, you can determine how much power is actually needed from your amp.
My WinISD Pro v.50a7 model of the 390HO in 2.3 ft^3 sealed with no EQ has a Qtc of .661. With 180watts at 20hz, SPL is 97.06 dB at 1m and excursion is 8.4mm. Excursion limited power is 365 watts. If I use 180 watts as the input power with 3dB of boost at 20hz (PEQ 20hz/+3dB/Q=.1), WinISD will then show 100.06dB/11.9mm at 20hz since +3dB requires 2x power. I think you're assuming WinISD is including the +3dB in your 180 watt input when it's really just the baseline that the boost is then applied to.
-Brent