Ported Box F3 Calculator
A vented (ported) loudspeaker enclosure uses a tuned port to reinforce the lowest octaves, extending bass below what a sealed box of the same size could reach. Designing one means choosing a box volume and port tuning matched to the driver's Thiele-Small parameters. This calculator applies a widely used quasi-Butterworth alignment approximation to estimate the box volume, the port tuning frequency Fb, and the resulting -3 dB cutoff F3 from your driver's Fs, Vas and Qts. Enter your driver's published parameters to get a starting design for a vented enclosure.
Vented alignment formulas
Vb = 20 * Vas * Qts^3.3 (litres)
Fb = Fs * (Vas / Vb)^0.31 (Hz)
F3 = Fs * (Vas / Vb)^0.44 (Hz)
(Quasi-Butterworth approximation)
These are empirical alignment approximations. Verify the final design with full simulation; small drivers and high Qts may suit sealed boxes instead.
Vented design notes
- Vented boxes extend bass below a sealed box of the same volume.
- Required volume rises steeply with Qts.
- Below Fb the response rolls off at about 24 dB per octave.
- Use the driver's published Fs, Vas and Qts, not generic values.
- QB3 alignments suit Qts roughly 0.3 to 0.4.
Ported box F3: frequently asked questions
What is F3 in a vented speaker box?
F3 is the frequency at which the loudspeaker's low-frequency output has fallen 3 dB below the passband, the conventional definition of bass extension. For a ported (vented) box, F3 depends on the driver's resonance frequency Fs, its total Q (Qts), and the box and port tuning. A lower F3 means deeper bass.
What is a quasi-Butterworth or QB3 alignment?
Vented box design uses standard alignments that relate box volume and tuning to the driver's Qts. A common one for drivers with Qts around 0.3 to 0.4 is the QB3 (quasi-Butterworth 3rd order). This calculator uses a widely used approximation: box volume Vb = 20 * Vas * Qts^3.3 and box tuning Fb = Fs * (Vas / Vb)^0.31, with F3 estimated from Fb and Fs.
What Thiele-Small parameters do I need?
You need the driver's free-air resonance frequency Fs in Hz, the equivalent compliance volume Vas in litres, and the total Q factor Qts. These are published by the driver manufacturer on the datasheet. They are real measured properties of a specific driver, so enter the values for your driver rather than relying on generic figures.
How big a box does a vented design need?
Vented boxes are usually larger than sealed boxes for the same driver. The required volume rises steeply with Qts: a driver with Qts 0.3 needs a much smaller box than one with Qts 0.4. The calculator returns the suggested internal volume in litres and the tuning frequency the port should be set to.
Why is vented (ported) bass deeper than sealed?
A port resonates with the air in the box, reinforcing output near the tuning frequency Fb and extending the low-frequency response below where a sealed box of the same size would roll off. The tradeoff is a steeper 24 dB per octave roll-off below Fb and reduced cone control at very low frequencies, so port tuning must be chosen carefully.
Official sources
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.