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CarAudioCalc

Enclosure calculator

Ported Subwoofer Box Volume Calculator

Quick answer

The QB3 vented alignment gives Vb = 20 x Vas x Qts^3.3, tuned to Fb = 0.42 x Fs x Qts^-0.96. A driver with Vas of 1.2 cubic feet, Qts of 0.42 and Fs of 32 Hz wants about 1.7 cubic feet tuned near 31 Hz.

Your numbers

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Result

Net internal volume 1.371 cu ft Published figure
Tuning frequency Fb 30.9 Hz Published figure
Predicted F3 28 Hz Published figure
In cubic inches 2369 cu in

This is NET volume after the port and the driver are subtracted. Work out the port next, then add its displacement back into the box you build.

The formula

Vb = 20 x Vas x Qts^3.3, Fb = 0.42 x Fs x Qts^-0.96, F3 = 0.26 x Fs x Qts^-1.4

QB3 only describes a useful vented box over roughly Qts 0.2 to 0.5. Outside that range this returns nothing rather than a number that looks authoritative and is not. A driver with a Qts above 0.5 generally wants a sealed box.

The figures above are computed with these exact inputs: Vas 1.2 cu ft, Qts 0.42, Fs 32 Hz.

What this calculator assumes

Some of these are published physics or manufacturer figures and some are conventions. They are not the same kind of number, so each one is labeled.

AssumptionKindWhere it comes from
Thiele and Small vented-box alignment approximationsPublished figureThiele and Small vented-box alignment approximations, quasi-Butterworth third order (QB3)

Gear that matches this result

Every card below is in the page source. The calculator re-ranks them against your numbers and marks anything that falls short of what you just calculated.

Frequently asked questions

How does the ported box volume calculation work?
Vb = 20 x Vas x Qts^3.3, Fb = 0.42 x Fs x Qts^-0.96, F3 = 0.26 x Fs x Qts^-1.4. QB3 only describes a useful vented box over roughly Qts 0.2 to 0.5. Outside that range this returns nothing rather than a number that looks authoritative and is not. A driver with a Qts above 0.5 generally wants a sealed box.
Are these figures published or are they rules of thumb?
Both, and the page labels which is which. The Helmholtz port relation, the Thiele/Small alignment equations, cone area geometry, Ohm’s law, conductor resistance and ampacity, and manufacturer specifications are published and are labeled as such. Amplifier efficiency, cabin gain, the farad per thousand watts rule, port area per cubic foot and coverage percentages are conventions with no measurement behind them, and they are labeled as conventions.
Why does this calculator sometimes refuse to give an answer?
Because there genuinely is not one. A port can be too large in area for the box and tuning asked of it, a vented alignment stops describing a useful box outside a Qts range, and a sealed box cannot produce a total Q below the driver’s own. In each case the calculator returns nothing and explains why, rather than extrapolating past the end of a published relation.
Do I still need an installer if I use this?
Many owners do their own 12V work, and these are the figures that work is sized from. What matters is that a car battery delivers hundreds of amps into a short without hesitating, so the main fuse goes within 18 inches of the battery positive terminal and the battery comes off before power wire goes in. If any of that is beyond what you want to take on, a professional installer is money well spent.

Researched guidance, not professional advice. These results are planning figures from published relations and manufacturer specifications. Fit the main fuse within 18 inches of the battery positive terminal, because that fuse protects the wire and the wire is what starts a fire. Disconnect the battery before running power wire, use a grommet through every piece of sheet metal, and remember that sustained high sound pressure causes permanent hearing damage.