The model
Qtc sets the shape
A sealed box adds the stiffness of its air to the driver’s suspension. The smaller the box, the stiffer that air and the higher both the resonance (Fc) and the total Q (Qtc). Qtc decides the shape of the roll-off; Fc and Qtc together decide where it sits.
The response and excursion come from the full 00 Simulator model in half space at 1 m, with the driver’s own losses and typical box losses, which damp the curve a touch more than the lossless formula: F3 lands a few per cent above Fc at Qtc 0.707. Like most box calculators, the curves leave out the voice coil’s inductance, which only trims the top of the range.
Qts, Vas and Fs come from the driver’s Bl, moving mass and compliance, the values the model runs on. When a spec sheet’s own numbers disagree, the calculator says so and uses what it simulates, so the Qtc it names is the one the curve shows.
Design
Reading the chart
The bright curve is your box, the dashed curves the same driver at the classic Qtc targets. Under it, the cone excursion at your input power against Xmax. A sealed box keeps the cone under control well below its roll-off, which is one reason it needs no protective high-pass.
When the box is chosen, take its net volume to the box volume calculator to turn it into panel sizes.
Questions
Sealed box questions
How do I work out sealed box volume?
From the driver's Qts and Vas and the Qtc you want: Vb = Vas / ((Qtc / Qts)² − 1). The example 12-inch driver (Qts 0.392, Vas 85 L) needs 37.6 L net for Qtc 0.707. That is net volume: add the driver's own displacement and any bracing to size the cabinet.
What is Fc?
The resonance of the driver in the box: Fc = Fs × √(1 + Vas / Vb). The box's air stiffens the suspension, so Fc sits above the driver's free-air Fs. The example's 31 Hz driver resonates at 56.0 Hz in its 37.6 L box.
Does stuffing make a sealed box bigger?
Filling the box with loose fibre makes it act a little larger, typically by 10 to 20% with a generous fill, and lowers Qtc slightly. The calculator gives the net air volume without fill; use the lower end of any gain when planning.
What Qtc should I choose?
0.707 gives the flattest response before the roll-off and is the usual choice. Lower (0.5 to 0.6) needs a bigger box and rolls off earlier but more gently, which suits a room that adds bass. Higher (0.8 to 1.0) fits a smaller box with a little lift above the roll-off: for the example, Qtc 1.0 needs 15.4 L against 72.5 L at 0.577.
Why is F3 not the same as Fc?
F3 is where the response is 3 dB below the passband, and it depends on Qtc as well as Fc. At Qtc 0.707 the two nearly coincide; a lower Qtc puts F3 above Fc and a higher one below it. The calculator reads F3 from the simulated response, as the simulator does: 58.9 Hz for the example.
Sealed or ported for a subwoofer?
Sealed boxes are smaller, simpler and roll off more gently, and they keep the cone under control below the roll-off. Ported boxes play lower and louder near their tuning for a bigger box and a port. The subwoofer box calculator compares both for your driver.