The model
Tuned by the model, not a table
For any box, the calculator tries tunings in the full 00 Simulator model and keeps the one with the lowest F3 whose bass stays within 0.75 dB of the driver’s passband. The starting points are the smallest boxes that, tuned that way, reach 1.25, 1 and 0.8 times the driver’s resonance, so they suit a hi-fi woofer and a low-Qts pro woofer alike.
Curves are in half space at 1 m with the driver’s losses and typical box losses. Like most box calculators they leave out the voice coil’s inductance, which only trims the top of the range; the simulator includes it. Qtc, Fs and Vas come from the driver’s Bl, moving mass and compliance, the values the model runs on, which a few spec sheets do not quite match.
Design
Reading the chart
The bright curve is your box, the dashed one the same driver sealed at Qtc 0.707. Under it, the cone’s excursion against Xmax, then the air speed in the port against 17 m/s.
Near the tuning the port does the work and the cone barely moves, which is where a ported box gets its output. Below it the cone is on its own, which is what the high-pass is for.
Questions
Ported box questions
How do I work out the size of a ported box?
The calculator starts from the bass you want. Its three starting points are the smallest boxes that reach an F3 of 1.25, 1 and 0.8 times the driver's resonance, each tuned where the simulator's model finds the lowest F3 without a hump. For the example 12-inch driver (Fs 31 Hz, Qts 0.39, Vas 85 L) that gives 50 L at 32.9 Hz, 79 L at 30.8 Hz and 125 L at 25.3 Hz.
What frequency should I tune a ported box to?
Near the driver's Fs for a box that reaches about Fs, lower in a bigger box and higher in a smaller one. Tuned too high for its volume, the bass rises into a hump that sounds boomy; tuned too low, it rolls off early and gently. The calculator shows the best tuning for any volume you type and warns about a hump.
How big should the port be?
Big enough that the air stays under about 17 m/s at full power, where port noise often starts. The calculator sizes the starting port for that at your power, 125 mm for the example at 100 W (peaking at 13.5 m/s), and offers a larger size or more ports when the air runs faster.
Ported or sealed?
A ported box plays lower and louder near its tuning, for a bigger cabinet, a port to build and a high-pass. A sealed box is smaller and rolls off more gently. The example driver reaches an F3 of 31.3 Hz ported in 79 L, against 58.9 Hz sealed in 38 L. The subwoofer box calculator compares both for your driver.
What about the classic formula, Vb = 15 × Qts^2.87 × Vas?
That is the maximally flat alignment, and for a hi-fi woofer the calculator lands close to it: 86 L for the example. For a low-Qts pro woofer it gives a tiny box tuned high (about 16 L for an 18-inch driver with Qts 0.17) that barely plays below 100 Hz. Those drivers are built for bigger boxes tuned lower, with bass that sits a few dB under their passband, which is what the calculator gives them.
How long should the port be?
As long as the tuning needs for the port's area: Fb = c / 2π × √(S / (V × Leff)), where the effective length includes an end correction at each end, so the tube you cut is shorter. The example's 125 mm port is 413 mm long. Twice the area needs roughly twice the length, and a port longer than the box is deep turns through an elbow, which the side view shows.
Does a ported box need a high-pass filter?
Usually. Below its tuning the port stops holding the cone back, so excursion climbs steeply for sound that is barely audible. A high-pass around three quarters of the tuning (23 Hz at 24 dB per octave for the example) keeps the cone inside Xmax. The excursion panel shows the cone with the filter you set.