The pick
The driver first, then your goal
Each box is a defensible starting point, not a finished design: a sealed box at Qtc 0.707, a smaller sealed box for tight spaces, and a ported box sized for your goal and tuned where the model finds the lowest flat F3, with the usual high-pass at three quarters of the tuning.
The pick follows the driver’s EBP. When the driver works either way, the goal decides. Open any box in its own calculator to change it, or in the simulator to put them side by side on one graph.
Reading the chart
At your power, and at the limit
The top panel is each box at your input power: the passband level, where the bass rolls off and how steeply. The lower panel is the most each box can play before the cone reaches Xmax or the coil its thermal limit, which is where a ported box earns its size.
Everything comes from the full 00 Simulator model in half space at 1 m, with the driver’s losses and inductance.
Questions
Subwoofer box questions
Should my subwoofer be sealed or ported?
It depends on the driver and on what you want from the box. A driver whose efficiency bandwidth product (Fs / Qes) is below about 50 suits a sealed box, one above about 100 suits a ported box, and in between either works and your goal decides. The example 12-inch driver has an EBP of 74: ported, it reaches an F3 of 31.3 Hz in 79 L; sealed, 58.9 Hz in 38 L.
How big a box does my subwoofer need?
It follows the driver's Vas and Qts. The example driver needs 38 L sealed at Qtc 0.707, or 20 L as a compact sealed box at Qtc 0.90. Ported, it takes 50 L, 79 L or 125 L for a compact, balanced or deep box. All are net volumes: add the driver, port and bracing to size the cabinet.
How is the maximum level worked out?
At each frequency, the level where the first limit is reached: the cone at Xmax, or the voice coil at the heat it takes in its power rating test. It is the simulator's Max SPL, at 1 m in half space. Port noise is not counted as a limit here; the port velocity calculator checks the air speed.
What is EBP?
The efficiency bandwidth product: the driver's Fs divided by its Qes. A strong motor gives a low Qes and a high EBP, and has the control a ported box needs to turn its extra output into flat bass. A weaker motor keeps the EBP low and suits the gentler sealed roll-off. It is a rule of thumb for where to start, which is why the calculator shows both boxes.
Is a ported subwoofer louder than a sealed one?
Near and above its tuning, yes. At its limit the example's ported box plays 10 dB louder at 30 Hz than the sealed box (114 dB against 104 dB at 1 m), because the port does much of the work there and the cone barely moves. Below the tuning the port stops helping, so a ported box needs a high-pass while a sealed one keeps rolling off gently.
What does the goal change?
The size of the ported box: the smallest box that reaches about 1.25, 1 or 0.8 times the driver's resonance, each tuned where the model finds the lowest F3 without a hump. And, when the driver works either way, which box is the pick: compact leans to the small sealed box, balanced and deep to the ported one.