Cone
Audio, speakers, and home theater calculators
Net volume from the outside of the box, a sealed or ported starting point from the driver, and the port that tunes it.
How these calculators work
The subwoofer box calculator is three jobs on one page. The outside width, height, and depth, minus the wall thickness, are the gross internal volume. The driver, the port, and a bracing allowance come off that to leave the net volume. A ¾ inch wall is the usual MDF default, about 19 mm.
Thiele-Small parameters size the air, not the wood. A sealed box uses Richard Small’s closed-box relations: pick a Qtc and the page returns the net volume, or pick a volume and it returns Qtc, Fc, and F3. A ported box starts from D. B. Keele’s QB3 alignment. That is a starting point, not a finished design, and it is a poor start when Qts is above about 0.5. Bandpass boxes are not on this page.
Port length is the Helmholtz resonator with an end correction: 0.732 diameters when one end is flanged and one is free, 0.85 diameters when both are flanged. The air in the port is subtracted from the net volume. Check the result in a modelling program such as WinISD before cutting.