What does useful volume mean

so i want to build a speaker box for 2 alpine s-w10d2's but the only problem i have is where it gives total volume and useful volume is the useful volume the net volume or what?

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I have the same issue, anybody have an answer? My sub is ideal sealed enclosure 1.25 so am I targeting that as total or useful?

I manually calculated mine after getting the 3d drawing. It's useful volume is the one that matters, the "enclosure volume". I have no idea where they are getting the "total volume" or what that is.

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useful volume = Vb = internal net volume = the volume which "makes the sound", total volume is calculated by: internal net vol + displacements (ports, sub baskets, braces, etc...) + additional volume by material thickness

*Total Volume is overall area in cft.(ft³) or cin.(in³) available  that your enclosure will occupy.

*Useful Volume is total ft³ or in³ within the enclosure - net volume, speaker displacement, and port displacement combined.

*Net Volume is the  "Vb" the speaker requires not counting it's displacement and port displacement 

well said.