Subwoofer box design calculator for online creating a high performance subwoofer enclosure. But if you have the necessary arrangements then it is a beast … Custom subwoofer enclosure build. Make a subwoofer enclosure plan. Online calculator subwoofer box volume, port length and other parameters Subwoofer Box Design for 15 inch Speakers. This is a monster for producing beats at high bass. 15 inch subwoofer box design, ported box plan, 3D model. Net internal volume 2.83 cubic feet, port area 31.00 square inches, tuning frequency 34 Hz. Subwoofer box … Most people do not prefer this design due to its extra large size and it takes up a lot of space. Build a ported box, sealed box.