Impact-Site-Verification: adc34a7b-9f49-42e9-b7a9-b49fdb888532

Sine Design

Enclosure calculator

Port length calculator

Find the vent length that tunes a ported box to your target frequency — round or slotted, single or multiple ports — and check for chuffing.

Enclosure tuning
Port geometry

Rectangular/square ducts use the engine's equivalent-diameter end correction (the standard equal-area treatment, as in WinISD) — not a separate rectangular-duct model.

Air velocity / chuffing (optional)

Add the driver's cone area Sd and peak one-way excursion to check the peak port air velocity (chuffing) at tuning.

How port length works

A vented (bass-reflex) box is a Helmholtz resonator: the air in the port acts as a mass and the air in the box acts as a spring. Together they resonate at the tuning frequency Fb. For a given box volume and port area, the port must be a specific length to hit that tuning — a larger port area needs a longer port, and a higher tuning needs a shorter one.

This calculator solves the standard Helmholtz vent-length relation, generalised to slot ports and multiple ports through an equivalent diameter, and applies the end correction for how the port terminates. It uses the same engine as the enclosure designer, where you can model the full response and impedance of the finished box.

Avoiding port noise (chuffing)

If the air moving through the port gets too fast it becomes turbulent and audible as “chuffing.” Peak velocities under about 17 m/s are generally quiet; 17–34 m/s is audible under hard drive and benefits from a flared port; above 34 m/s the port is too small. Enter the driver's cone area and peak excursion above to estimate the peak velocity for your design.

Results are modelled estimates from published (nominal) parameters — verify by measurement. Looking for another calculator? See all tools & calculators.