What tuning frequency should I use for a ported subwoofer?

For car audio, most builds tune between 28 and 38 Hz. Tune to approximately 0.8 to 0.9 times Fs for a QB3 alignment, or lower for extended bass. Simulate first - the right Fb depends on your driver's Qts and available box volume.

How do I calculate port length for a ported box?

For a circular port: Lv = (23562.5 x Dv^2) / (Fb^2 x Vb) - (0.732 x Dv), where Dv is diameter in cm, Fb is tuning in Hz, Vb is net volume in litres. RokketBox calculates this automatically.

How big should the port be?

A common car-audio rule of thumb is 12 to 16 cm2 of port area per litre of box volume. A 12 inch driver in a 40 L box works out to roughly 480 to 640 cm2 total - for example a slot port about 40 cm wide by 12 cm tall (480 cm2), or six 4 inch round ports (about 80 cm2 each). The real target is keeping port velocity under about 20 m/s at your power level, so simulate in RokketBox rather than relying on the rule alone.

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Ported Subwoofer Box Calculator

A well-designed ported subwoofer enclosure can deliver 3 to 6 dB more output in the bass region than a sealed box of the same size. A poorly-designed one produces port noise, a one-note resonance peak, and poor extension below tuning. The difference is getting the volume and tuning relationship right for your specific driver.

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How It Works

Enter driver specs
Select from the driver database or enter your own Thiele-Small parameters (Fs, Qts, Vas, Xmax, Re, BL).
Simulate frequency response
The circuit-domain engine calculates SPL, port velocity, and excursion across 500 frequency points - see the tuning tradeoff before you cut.
Export cut sheet
Every panel dimension, port length, and corner detail, print-ready for the workshop.

How ported boxes work: Helmholtz resonance

A ported enclosure uses a tuned port to create a resonance that supplements driver output at the tuning frequency Fb. At Fb, air in the port is in phase with the cone, acting as a second acoustic source. Below Fb, the port goes out of phase and driver excursion increases rapidly - which is why ported boxes require a subsonic filter at all times. The Helmholtz resonance formula: Fb = (c / 2π) x √(Sp / (Vb x Lv)), where c = 343 m/s, Sp is port area, Vb is box volume, Lv is effective port length.

Box volume and tuning frequency: the tradeoff

For most car audio drivers with Qts 0.3 to 0.5, the Butterworth (QB3) ported alignment means tuning Fb to approximately 0.7 to 0.9 times Fs with a box volume around 0.8 to 1.5 times Vas. Tuning lower than QB3 extends bass but requires a larger box. Tuning higher maximises output at a single frequency at the expense of extension. RokketBox simulates the actual frequency response so you can see the tradeoff before cutting.

Port diameter and port velocity

Port cross-sectional area determines air velocity at a given power level. Too small and turbulence produces audible chuffing at high SPL. Rule of thumb for car audio: 12 to 16 cm2 of port area per litre of box volume keeps port velocity under approximately 20 m/s. A 3 inch round port has about 45 cm2; a 4 inch port about 80 cm2. Flared port ends reduce velocity at the aperture, allowing smaller ports at high power.

Subsonic filter: required for all ported builds

Ported boxes provide no mechanical excursion control below Fb. A subsonic filter set to approximately 0.65 to 0.75 times Fb is required at all power levels. Without it, bass content below Fb pushes the driver beyond Xmax without producing useful SPL - risking coil rub and driver failure. RokketBox displays your simulated port tuning and the recommended subsonic filter setting.

Frequently Asked Questions

What tuning frequency should I use for a ported subwoofer?: For car audio, most builds tune between 28 and 38 Hz. Tune to approximately 0.8 to 0.9 times Fs for a QB3 alignment, or lower for extended bass. Simulate first - the right Fb depends on your driver's Qts and available box volume.
How do I calculate port length for a ported box?: For a circular port: Lv = (23562.5 x Dv^2) / (Fb^2 x Vb) - (0.732 x Dv), where Dv is diameter in cm, Fb is tuning in Hz, Vb is net volume in litres. RokketBox calculates this automatically.
How big should the port be?: A common rule of thumb is 12 to 16 cm2 of port area per litre of box volume. A 12 inch driver in a 40 L box works out to roughly 480 to 640 cm2 total - for example a slot port about 40 cm wide by 12 cm tall (480 cm2), or six 4 inch round ports (about 80 cm2 each). Keep port velocity under about 20 m/s at your power level, so simulate in RokketBox rather than relying on the rule alone.

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