Ported enclosure and subsonic filter

[Raven]

I'm told to go with a ported enclosure, you'll benefit from a subsonic filter.

This is probably a dumb question, but ebay turns up this (http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=14931&item=5732520391), a passive adjustable subsonic filter. However, all the frequencies are a multiple of five. Would I have to port the box to the same frequency, or would a Xhz enclosure and X-4hz subsonic filter work fine?

EDIT: Just to make myself sound more like an idiot: this thread (http://forums.caraudio.com/vb/showthread.php?t=78429&highlight=subsonic+filter) answered this, but I have no idea what a 1/2 octave would be.

[Acidburn]

one octave below is half of the frequency
so 1/2 octave is half way between that frequency and the frequency of one octave below

[Raven]

Here we go.

On full octave down from 100 hz would be 50 hz, right? One full octave up would be 200. One octave up = double the frequency, one octave down = half the frequency.

Therefore, since I want to port it near 30, I would have to filter out 15 and below. That peticular crossover will handle down to 20. It should be alright.

[Acidburn]

yes, one octave below 30Hz is 15 Hz, but 1/2 an octave below 30Hz is 22.5 Hz since 22.5 is half way between 15 and 30