Port length calculator

Tuning a box is really tuning a vent. Here is the formula, the end correction everyone forgets, and how to tell whether the vent you have drawn is going to whistle.

The formula

A vent and the box behind it form a Helmholtz resonator. Its resonance is:

Fb = (c / 2π) · √( Sv / (Vb · Leff) )c ≈ 345 m/s, Sv = vent cross-section, Vb = box volume, Leff = effective duct length.

Rearranged for a single round vent, in the units you will actually be working in:

L = ( 2.356 × 10⁷ · d² ) / ( Fb² · Vb ) − k · dL and d in centimetres, Vb in litres, Fb in hertz. k is the end correction.

And the same thing in inches and cubic feet:

L = ( 1,463 · d² ) / ( Fb² · Vb ) − k · dL and d in inches, Vb in cubic feet, Fb in hertz.

For n vents, use the total area: n identical vents behave as one vent of n times the area, which means each one is n times longer than a single vent would have been for the same tuning. Two vents are not a shortcut to a shorter duct.

The end correction

The air that moves as part of the vent extends a little beyond each physical end. The duct is acoustically longer than it is, so you cut it shorter than the raw calculation by k · d:

On a 100 mm vent that is 8.5 cm of correction — enough to move tuning by several hertz on a small box. It is the most commonly skipped term in the calculation and the usual reason a built box measures lower than it was designed to.

Vent shapes

• Round tube — easiest to build, worst use of space, and the most prone to chuffing at the ends unless flared. Sv = π·d²/4.
• Slot — a rectangular duct built from panels. Same maths on its equivalent diameter. Keep the gap at least 15 mm, or friction in the boundary layer starts to dominate and the vent becomes lossy.
• Wall slot — a slot whose run is the full width of the cabinet, formed by a partition set back from one wall. The most space-efficient shape in a car box: the cabinet supplies three sides, and the gap is the only variable to solve for.
• Corner vent — a triangular duct in the cabinet corners, often four at once. Very stiff, and it braces the box while it vents it.
• Aero / flared — a moulded tube with a radius at each end. The flare keeps flow attached at much higher air speeds, so a flared vent stays quiet where a straight tube of the same area is chuffing. It also raises the effective end correction, so use the manufacturer's figure when they give one.

Air speed: whether it will chuff

Peak air speed in the vent is what decides whether you hear the box breathing. The calculator reports it directly against frequency; these are the numbers to judge it by:

If the vent is too fast the only real fixes are more vent area (and therefore a longer duct), flared ends, or a lower target output. Making the vent shorter does not help — it just retunes the box.

Common tunings, checked quickly

Read across a row and the pattern is the whole problem in one line: doubling the vent diameter quadruples the area and so roughly quadruples the length. A 150 mm vent almost never fits, which is why slot and wall vents exist.

Frequently asked questions

How do I calculate port length?

Helmholtz, rearranged for length. For one round vent of diameter d cm in a box of Vb litres at Fb hertz:

L = (2.356 × 10⁷ · d²) / (Fb² · Vb) − k · d

with k ≈ 0.85 for a vent flush in the baffle and open inside. In inches and cubic feet, the constant is 1,463. For several vents, use the combined area — each vent then comes out longer, not shorter.

Does port length change the tuning frequency?

Yes — it is the main way to adjust tuning once the box exists. Longer port, lower tuning; shorter port, higher tuning, because the air mass in the duct is what resonates against the spring in the box.

It is not linear: Fb goes as 1/√L, so halving the tuning frequency needs roughly four times the length.

Why is my box tuned lower than I calculated?

Three usual causes:

• The end correction was skipped, so the duct is acoustically longer than intended.
• The vent's inner end is within about one diameter of the opposite wall, so it is restricted and behaves as if longer.
• The tuning used gross volume rather than net.

Check all three before you recut anything.

How do I stop my port from chuffing?

Chuffing is turbulence, so lower the air speed. More vent area (which means a longer duct at the same tuning), or flared ends, which keep flow attached where a straight-cut tube would already be turbulent.

Aim for peak air speed under about 17 m/s. Shortening the port does not help — it retunes the box and leaves the speed roughly where it was.

Can I use two ports instead of one long one?

You can — but it makes each duct longer, not shorter. Two vents behave as one of twice the area, and length scales with area, so each of the two is about twice as long as a single vent would have been.

Use several vents when you need the area to keep air speed down, or when two tubes physically fit where one large one does not.

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