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[recon440]

666WMD,
Basically I am not to concerned about having the subwoofers strongly produce above ~65Hz. The reason behind the 55Hz tuning is that given the box constraints in terms of size I wanted to have solid output in the 35-50Hz range (I try to peak around 45Hz as this is my favorite frequency). Tuning lower (say 48-50Hz) would reduce the peak and 55-65Hz response would suffer severely. The other reason for picking 1CF each for the sealed chambers is that this seems to be the optimal size vs. low-end extension. I ran tried 1-1.5CF and the low end was compromised too much (essentially the roll-off was much faster that of the ported box). I want a 2.5-3CF only seemed to improve the response in the 20-30Hz region with minimal improvement in the critical 35-50Hz band.

In regards to subwoofers, I have on hand a pair of SWR-1243d's but these don't model as flat as the Rockfords. Additionally I have a single Diamond audio TDX15 and that woofer has an EBP of like 39 which would make it great for a bandpass but I'm not certain I could fit that in my trunk.

Frankly speaking, I'm none too sure why people claim bandpasses are more efficient. From all the modelling I did, the ported box will dominate a bandpass box if the volumes of the boxes are the equal(even if both are very large). The only reason I'm doing this is to direct the output of the woofer to a location to best exploit boundary/hornloading. I'm also under the impression that the lower frequencies with a wavelength of over 40ft will be enchanced since they will have time to resonate off the rear glass many times to develop up to quarter wavelength before reaching the ear.

If the back seat were to be optimal position I would just Build a bigger ported box there. However I have a very strong gut feeling that there is significant cancellation in that location since the subwoofer cones is almost in the middle of the Car maybe 4" behind the B-pillar. (Also it sounds stupid but theres also the fact that a sealed setup sounded louder and hit harder (vibrated the mirrors to make them change position)).

smfonic,
The suggestion of using a ported box with the port running through the deck is not feasible (see photo of the worst possible trunk ever; double rear firewall, gasoline tank, and small trunk opening). This is why I'm running the ported box on the rear seat since there was almost 10dB attenuation by having it in the trunk (even with the stock 8" subwoofer open to help it breathe). The 4th order port will have to be external and snake between the the 15cm gap between the gas tank and rear deck. It will also have to be of sufficient length to get to the rear deck (maybe 25-30cm) while not being too long (keep tuning around 50Hz). This is why I'm restricting myself to a ported section of around 3CF since a larger ported section will required shorter runs of port or a wider port area (which is tight as it is (max. 15cm diameter tube will fit). The box also much be removable so it must fit through the trunk opening.