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CarAudioCalc

Enclosure calculator

Slot Port Length and Area Calculator

Quick answer

A slot port is converted to an equal-area round port and solved the same way. A 12 by 2 inch slot has 24 square inches of area, which is an equivalent diameter of 5.5 inches, and tuning 2.0 cubic feet to 32 Hz takes about 32 inches of it.

Your numbers

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Result

Slot length 29.31 in Published figure
Port area 24 sq in
Area the convention suggests 25 sq in Convention
Peak port air velocity 8.9 m/s Convention

Air velocity is the useful test of port area. The square inches per cubic foot figure is a convention with no measurement behind it.

The formula

Equivalent diameter = 2 x sqrt(area / pi), then Lv = (1.463e7 x R^2) / (Fb^2 x Vb) - (k x R)

Slot ports fold along a box wall, and a wall of the box counts as a wall of the port. The 12 to 16 square inches of port per cubic foot figure is a convention rather than a measurement. The defensible test is air velocity, which this calculates when you supply cone area and excursion.

The figures above are computed with these exact inputs: Net box volume 2 cu ft, Tuning frequency 32 Hz, Slot width 12 in, Slot height 2 in, Driver cone area Sd 81.7 sq in, Driver Xmax 13 mm.

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
Port Area Per Cubic Foot: 12.5ConventionThe 12 to 16 square inches of port area per cubic foot guideline. Widely repeated, useful as a starting point, and not derived from any published measurement. The defensible version of this question is port air velocity
Max Port Velocity: 17ConventionA commonly cited maximum port air velocity of roughly 17 metres per second, about 5 percent of the speed of sound, above which port noise becomes audible. The threshold depends on port geometry and flaring and is not a published standard

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 slot port calculation work?
Equivalent diameter = 2 x sqrt(area / pi), then Lv = (1.463e7 x R^2) / (Fb^2 x Vb) - (k x R). Slot ports fold along a box wall, and a wall of the box counts as a wall of the port. The 12 to 16 square inches of port per cubic foot figure is a convention rather than a measurement. The defensible test is air velocity, which this calculates when you supply cone area and excursion.
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.