How many cubic feet do I need?

Just a FYI on how you can decide on sealed vs. vented for any given woofer:  find out the EBP (a number) by using Fs (resonant frequency) and Qes (electrical Q).  The equation is Fs / Qes = EBP.  An EBP of 50 and under is best sealed, higher numbers in the 70's and above are best vented.  But you can work it out either way (with trade-offs) if the number falls between 50 and 90.

ex. Fs- 38.10   Qes- 0.40  = EBP of 95 which says this example needs to be ported.

stevdart,

when using the equation you posted, what are the trade-offs of using a woofer in an enclosure different from what the EBP is telling you? (ex an EBP 50 sub in a vented enclosure). Does it have to do with performance of the sub (SPL or SQ) or can you damage your sub?

chris, you can always damage your sub by using a vented enclosure if set up without safeguards (subsonic filter, incorrect volume of box/port, etc.).  With that out of the way, to answer your question as best as I can....no, you won't do any harm.  In what I've seen, if a subwoofer's EBP calls for a sealed box it means that the sub will respond well to the lowest frequencies that are useable without needing the help of venting.  Whereas, a woofer that calls for venting just won't perform well if placed in a sealed box, generally losing a lot of low-end extension.  50 is on the line between the two, and I would look at it in WinISD if it were me to see which way would be more workable in the particular car.  Going vented normally means the box will be a lot larger, and with a low EBP such as 50, the port size might have to be unreasonably large.  I would only be guessing if I were to try to figure out what differences there would be in the quality, if any, but I know that the overall volume sizes would be much larger in the vented box.

I'm just a newbie, though.  I know some of the information but not all the reasons why.   ;#)

Yes you're right.  The only thing I'm worried about is the sub might not be as screw-tight as the first time.  But I'll try it and maybe like you said I might not even hear any difference.  No matter what I still think it's going to be time-well-spent!!  Thanks.