how do you figure out the cubic air space in a speakerbox?

Answer 1

You simply measure the inside height, width and depth of the enclosure and multiply the numbers together.
For example, if the box is 24" high, 12" wide and 8" deep - the box contains 24 *12 * 8 or 2304 cubic inches.
To convert cu. in. to cu. ft., divide by 1728 (12^3).