Subwoofer Port Length Calculator

Direct answer. Enter net enclosure volume and the inside dimensions of a round or slot port. The tool can estimate tuning from a known physical length or solve the physical length for a target tuning.

Method. The rigid-cavity Helmholtz estimate is f = c/(2 pi) sqrt(nA/(V Leff)). Here n is the number of identical ports, A is one port's cross-sectional area, V is net box volume and Leff is physical length plus the entered end corrections. A slot is compared with a round port by equal area, not by assuming the shapes are acoustically identical.

Worked example. One 100 mm round port in a 50 L net enclosure, 200 mm long with total end correction of 1.6 equivalent radii, gives about 40.9 Hz in this model. Four otherwise identical ports raise tuning by a factor of two unless their length changes.

Input definitions. Net volume excludes the driver, bracing and the port itself. Port dimensions are clear inside dimensions. End-correction fields are multiples of the equal-area radius so you can apply assumptions appropriate to the actual flush, flanged, free or nearby-wall termination.

The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.

Solve for

Tuning from port length

Input units

Metric: L and mm

Port shape

Round

Net enclosure volume

50 L or ft3

Round-port inside diameter

100 mm or in

Slot inside width

200 mm or in

Slot inside height

40 mm or in

Identical port count

1

Physical port length

200 mm or in

Target tuning

40 Hz

Inside end correction

1 x radius

Outside end correction

0.6 x radius

Primary result: Physical port length: 200.00 mm.

• Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
• Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
• When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.

This is a lumped Helmholtz starting estimate. It does not model losses, port turbulence, flares, bends, nearby walls, cabinet compliance, driver response, leakage or higher modes. Slot aspect ratio can change real behavior even at equal area. Recalculate net volume after the final port displacement is known, confirm clearances physically, and validate the completed enclosure by impedance or acoustic measurement.