The port and the air in the box form a Helmholtz resonator. The port's length and area set the moving air mass, the box volume sets the spring, and together they decide the tuning frequency.
The formula
Leff = N · Sp · c² / ((2π · Fb)² · Vb)
L = Leff − k · rN ports of area Sp each, speed of sound c = 343 m/s, net volume Vb in m³. r = √(Sp/π) is the port's equivalent radius and k is the end correction: 1.463 for one flanged and one free end, 1.70 for two flanged ends, 1.226 for two free ends.
End correction matters: the air just outside each port end moves with the port, so the physical port must be shorter than the effective length. Skipping it tunes the box several hertz low. The port length guide has a worked example.
Checks before you cut
- Port area: about 2.7 to 3.6 cm² per litre of box (12 to 16 in² per ft³) is a common rule of thumb for quiet ports in car audio. The real test is air speed at full power, which the full calculator shows.
- Port volume: the port takes space inside the box. Add it to the net volume.
- Pipe resonance: a long port resonates like an organ pipe. Keep your low-pass filter well below this frequency.