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CarAudioCalc

Enclosure calculator

Subwoofer Port Length Calculator

Quick answer

Port length is (1.463e7 x port radius squared) divided by (tuning frequency squared x net box volume in cubic inches), minus the end correction. A 4 inch port tuning 2.0 cubic feet to 32 Hz needs about 15.3 inches.

Your numbers

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Result

Port length 14.89 in Published figure
Port area 12.57 sq in
Tuning frequency 32 Hz
Net volume 2 cu ft

The port displaces air inside the box. Add its outside volume back when you build, or the box ends up smaller than the figure you designed around.

The formula

Lv = (1.463e7 x R^2) / (Fb^2 x Vb) - (k x R)

R is the port radius in inches, Vb the NET internal volume in cubic inches, Fb the tuning frequency in Hz. The end correction k is 0.732 with both ends in free air, 0.823 with one flanged end, and about 1.7 with both flanged. Use the net volume the driver sees, not the gross volume of the box.

The figures above are computed with these exact inputs: Net box volume 2 cu ft, Tuning frequency 32 Hz, Port diameter 4 in, Port ends oneFlanged.

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
The Helmholtz resonator relationPublished figureThe Helmholtz resonator relation, in the form commonly published for vented loudspeaker enclosures: Lv = (1.463e7 x R^2) / (Fb^2 x Vb) minus the end correction

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 port length calculation work?
Lv = (1.463e7 x R^2) / (Fb^2 x Vb) - (k x R). R is the port radius in inches, Vb the NET internal volume in cubic inches, Fb the tuning frequency in Hz. The end correction k is 0.732 with both ends in free air, 0.823 with one flanged end, and about 1.7 with both flanged. Use the net volume the driver sees, not the gross volume of the 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.