http://cgi.ebay.com/ws/eBayISAPI.dll...1890606&rd=1,1

anyone know how to calculate this?

 

There is not enough info at that link to calculate the internal volume of the tube. Thickness must be known in order to calculate it as well as any protrusions that are inside the tube.

 

You need to calculate the ID area of the circle, then multiply that by the interior length of the enclosure.
Measure the inner diameter (ID) of the tube. Divide the diameter by 2 to get the radius of the circle. Multiply the radius squared by pi (3.14 is accurate enough to get your enclosure into the 100ths). The given calculation is the ID area of the tube.
Now, multiply the ID area by the inner length of the tube. This will give you the volume of the enclosure.
Using the measurements given (which are probably exterior measurements and are not an accurate representation of the enclosure's actual volume), the volume of this enclosure is approximately .522 cubic feet.

 

is it possible to use polyfill in a basstube? i'm looking for about .5 cubic feet so i'm guessing i should be right in that range.

 

don't see why not, however, poly fill has the reverse affect. It makes the box appear bigger to the sub. why? don't know, I recall hearing that somewhere. Or maybe I am full of sh*t. If anything, make a round plate to cut off a small portion of the box.

 

Polyfill makes the woofer act like the enclosure is larger by slowing down the movement of air inside the enclosure. It can be used in any type of enclosure to fool the woofer into responding as though the enclosure or chamber were larger.
Keeping in mind that my last calculations were based on the measurements given, which I assume are exterior dimensions, the enclosure's internal volume is going to be smaller than .5 cubic feet. If the tube is 1/2" thick, and there is a 1/2" base at one end that is countersunk into the tube, then the approximate volume drops to .38 cubic feet.
You also need to keep in mind that the tube is vented and the volume of the vent has not been accounted for. These are approximate calculations. Accurate calculation of volume cannot be accomplished without the specific measurements of the enclosure itself.