Port length and air speed?

That short of a port length could be possible since the box is so large, the length depends on both the size of the box and the desired tuning frequency.

What is your port area and are you planning on round or a slotted port? I'm guessing that your port is too small...

ach81090 said:

I don't understand why people are still using MDF. Cabinet grade pine is about the same price, but half and weight(more important for car audio), twice the strength, and easier on blades. Birch is even stronger, but more expensive. Use good material and common sense, you won't have to go too crazy with bracing and multiple baffles.(I'm not saying don't brace at all)

Good luck with the build!

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Lighter weight means a comparatively higher resonance frequency, bracing shifts resonance so that it is less or completely inaudible. So light weight really only saves your back when you have to move the sub. Also, MDF comes in 4'x8' sheets that can be found in about any corner of the modern world

BD55 said:

Hmm. From what I remember reading, (and from a physics standpoint) bracing increases the resonant frequency of each panel above the spectrum of frequencies the subwoofer will see or produce. If you think about it, a large panel will have a lower resonant frequency than a smaller panel, so by bracing, you would actually be increasing the resonant frequency. That would also explain why boxes that aren't well braced sound "boomy"; there is some destructive resonance occurring there.

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You are correct, I stated that wrong. Adding bracing reduces a side of the box into smaller portions, which then resonate at a higher frequency. Once they are beyond what the subwoofer produces, the box will no longer interact so much.