Subwoofer Port Length Calculator
Calculate the length of a round vent for a ported subwoofer box from the enclosure volume, tuning frequency and port diameter.
How it works
Lv = (23562.5 · d² · N) ÷ (Fb² · Vb) − 0.732·d (Lv, d in cm · Vb in litres · Fb in Hz)
A bass-reflex port is a Helmholtz resonator: the air in the vent acts as a mass and the air in the box as a spring, and together they resonate at the tuning frequency. Longer or narrower ports lower the tuning; a bigger box also lowers it. The final 0.732·d term is the end correction — the air just outside the port mouth moves too, so the acoustic length is longer than the physical tube.
Worked example
For a 40 L box tuned to 32 Hz with a single 10 cm port: 23562.5 × 100 × 1 ÷ (1024 × 40) = 57.5 cm, minus the 0.732 × 10 = 7.3 cm end correction, giving a physical port about 50.2 cm long.
Frequently asked questions
What happens if the port is too small in diameter?
A narrow port makes the air move faster, and above roughly 17 m/s it "chuffs" — you hear turbulent noise on bass notes. If the calculated length becomes impractically long or the port whistles, use a larger diameter or add a second port to increase total area.
Why subtract the 0.732·d term?
That is the end correction. The slug of air that resonates extends slightly beyond both ends of the physical tube, so the tube behaves acoustically longer than it is. Subtracting it gives the length you actually cut. A port flush with an internal wall on one end uses a smaller correction.
Can I use a slot port instead of a round one?
Yes, but the formula changes because a slot has a different cross-section and end correction. Convert the slot area to an equivalent diameter (d = √(4A/π)) only as a rough starting point, then verify the tuning by measurement.
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This calculator is for educational purposes. Double-check important results before acting on them.