How long should my port be?

It depends on the box volume, tuning frequency and port diameter. Use the formula Lv = 23563·d²/(Fb²·Vb) − 0.732·d (cm/L/Hz) for a single round port, or let the designer solve it for round/slot/multiple ports.

Why is my port so long?

Smaller-diameter ports tune with less length but raise air velocity; larger ports stay quieter but get long. If the port is impractically long, use a larger box, higher tuning, or a slot/flared port.

Ported box calculator: volume, tuning and port length

Calculate a bass-reflex box: enclosure volume (Vb), tuning frequency (Fb), and the exact port length, with port air-velocity (chuffing) warnings.

Designing a ported box means choosing the net internal volume (Vb) and the tuning frequency (Fb), then sizing the port so the box actually tunes there. Sine Design solves the full 4th-order vented transfer function, so the response curve is exact for whatever Vb/Fb you choose — not just a table point.

Port length

For a single round port, port length Lv = 23563·d² / (Fb²·Vb) − 0.732·d (cm, litres, Hz), where the 0.732·d term is the end correction for one flanged + one free end. Bigger or more ports need more length for the same tuning; the calculator handles round, slot, single and multiple ports.

Avoid chuffing

Peak port air velocity should stay below about 17 m/s for quiet operation; 17–34 m/s becomes audible under hard drive (flare the port); above 34 m/s is unacceptable. The designer plots velocity vs frequency and warns at these thresholds.

Frequently asked

How long should my port be?: It depends on the box volume, tuning frequency and port diameter. Use the formula Lv = 23563·d²/(Fb²·Vb) − 0.732·d (cm/L/Hz) for a single round port, or let the designer solve it for round/slot/multiple ports.
Why is my port so long?: Smaller-diameter ports tune with less length but raise air velocity; larger ports stay quieter but get long. If the port is impractically long, use a larger box, higher tuning, or a slot/flared port.