Subwoofer box calculator

Work out the enclosure a subwoofer actually needs — sealed, ported or bandpass, one driver or four, for a car boot or a living room — and see the response, the excursion and the cut list before you buy the MDF.

Diameter tells you almost nothing

The single most common question — what size box does a 12-inch sub need? — has no answer, because the number on the label is not a parameter. Two 12-inch drivers can want boxes that differ by a factor of three. What decides the box is the driver's compliance and damping: Vas and Qts, with Fs setting where it all happens.

A stiff, heavily damped competition driver (low Vas, low Qts) wants a small box and a lot of power. A loose, lightly damped driver (high Vas, higher Qts) wants a large one and gives more output per watt. The Thiele/Small parameters page goes through which is which.

Sealed, ported or bandpass for a sub

If you only ever build one sub box, build a sealed one: it is the smallest, the most forgiving of a volume error, and the hardest to make sound bad. Build a ported one when you want the output and have the space. Build a bandpass one when a narrow band loud is exactly what you want and you accept that everything else is gone.

Car audio: the cabin is part of the system

A car interior is small compared with the wavelengths a subwoofer makes, so below roughly 50–60 Hz it stops behaving like a room and starts behaving like a sealed volume being pressurised. That is cabin gain, and it is worth something like 12 dB per octave as frequency falls — in a small hatchback, more than the driver's own roll-off.

Two consequences that decide real builds:

• You do not need to tune as low as you think. A sealed box rolling off at 12 dB/oct below 45 Hz, plus 12 dB/oct of cabin gain, is roughly flat to well below that in the car. Chasing a 25 Hz tuning often buys excursion problems and nothing audible.
• The box can be smaller than a home sub. The cabin supplies the bottom octave, so the box only has to get you to the point where it takes over.

A home subwoofer gets none of this until much lower in frequency, which is why the same driver sensibly ends up in a bigger, lower-tuned box indoors.

More than one driver

Volume scales with the number of drivers, and nothing else does — two drivers that each want 30 litres want 60 litres between them, whether that is one 60-litre chamber or two 30-litre ones.

• Shared chamber — simplest, one less panel, and the drivers share the air load. If one driver fails, the other is now in twice the box it wanted, so the response changes.
• Separate chambers — each driver keeps its own alignment whatever the other does, and the divider braces the box for free. This is the safer choice and costs one panel.
• Vent area scales too. Two drivers need roughly twice the vent area for the same air speed, which means a much longer duct at the same tuning. This is where ported multi-driver builds run out of room, and where a slot or wall vent usually wins over round tubes.

Where sub builds actually go wrong

1. Using gross volume instead of net. The driver, the vent, the bracing and the panel thickness all take up room. An 18 mm-walled box loses roughly 10–15% of its outside volume to the walls alone, and the driver basket and vent take more. Tune for net.
2. Ignoring excursion. A flat curve tells you nothing about whether the cone survives. Look at excursion against Xmax at the power you will actually use, especially below tuning in a ported box.
3. Undersized vents. Air speed over roughly 17 m/s starts to be audible as chuffing, and a vent that noisy is also compressing your output. Aim for vent area near a tenth of cone area.
4. No subsonic filter on a ported box. Below tuning the vent stops loading the cone and excursion climbs fast. A high-pass a few hertz below tuning is not optional.
5. A leaky box. A sealed box with a leak is a badly tuned ported box. Seal every internal joint, not just the outside.

Frequently asked questions

What size box does a 12-inch subwoofer need?

There is no single answer, because box size follows from the driver's parameters rather than its diameter. As a rough starting point a typical 12-inch car subwoofer lands around 25–45 litres (0.9–1.6 ft³) sealed, or 55–85 litres (2–3 ft³) ported and tuned near 32–35 Hz.

Treat that as a sanity check on your calculation, not a substitute for it — two 12-inch drivers can want boxes three times apart.

Is a bigger box always louder?

Up to a point, and only in part of the band. A bigger sealed box lowers system resonance and gives more deep bass, but it also lowers Qtc — so output just above resonance falls, and past a point the bass sounds thin rather than deep.

A bigger ported box at the same tuning needs a longer vent and narrows the region where the port is helping at all. Past the alignment the calculator suggests, extra litres mostly buy a bigger box.

Do I need a subsonic filter?

On a ported or bandpass box, yes. Below tuning the vent stops loading the cone, excursion climbs steeply, and the driver can pass Xmax on content you cannot hear at all. Set the high-pass about 3–5 Hz below the box tuning.

A sealed box does not need one — the air spring keeps loading the cone all the way down.

Can I put two subwoofers in one box?

Yes. Give the chamber the sum of what each driver wants — two drivers needing 30 litres each need 60 litres together.

A shared chamber is simpler; separate chambers keep each driver on its own alignment if the other fails, and the divider stiffens the box for free. If it is ported, vent area scales with driver count too, which usually pushes you to a slot vent rather than round tubes.

Does the shape of the box matter?

For the low-frequency response, almost not at all. A sub box is far smaller than the wavelengths it makes, so only internal volume and tuning matter.

Shape matters for three other reasons: long thin boxes get internal standing waves in the upper bass, large unsupported panels resonate and need bracing, and the box has to fit where it is going. Avoid a cube and avoid a long tube, then brace the big panels.

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