A ported box adds a tuned air mass, the port, that resonates with the box at the tuning frequency (Fb). Near Fb the port does most of the work, so you get more output and deeper extension than a sealed box of similar size.
How this calculator works
The suggested start uses Keele's classic fit to the QB3 alignment family, which gives a flat, well-damped response for drivers with a Qts around 0.3 to 0.5:
Vb ≈ 15 · Vas · Qts^2.87
Fb ≈ 0.42 · Fs · Qts^−0.9The response is Small's fourth-order vented-box model with typical leakage losses (QL = 10, as in the full calculator). Port length uses the Helmholtz formula with end correction for a port flush with the baffle on one end and free inside the box. See the port length guide for the math.
Before you build
- Use a subsonic filter a few hertz below Fb. Below tuning the cone is barely loaded and travel climbs fast.
- Port area decides port noise. A small port can be loud at high power; check air speed in the full calculator or read how big a port should be.
- Port volume adds to the box. Add it, the driver and bracing to the net volume before you work out panel sizes.