Subwoofer Port Length Calculator

What Is a Bass Reflex Port?

A bass reflex (ported) subwoofer enclosure uses a vent to reinforce low frequencies at the tuning frequency. The port length must match the box volume, port diameter, and desired tuning frequency for accurate bass response.

Subwoofer Port Length Formula

$$L = \frac{23562.5 \times D^2 \times N}{V \times F^2} - k \times D$$

$L$ is port length in cm, $D$ is port diameter in cm, $N$ is the number of ports, $V$ is box volume in liters, $F$ is tuning frequency in Hz, and $k$ is the end correction factor (typically 0.732 for one flanged end).

Example: a 50 L box tuned to 35 Hz with a single 10 cm port gives approximately 31 cm port length.

Frequently Asked Questions

What is the end correction factor?

It accounts for the effective length added by port end conditions. Use 0.732 for one flanged and one free end, 0.850 for both flanged, or 0.614 for both free.

Does a longer port lower tuning frequency?

Yes. A longer port lowers the tuning frequency while a shorter port raises it, assuming volume and diameter stay the same.

What units should I use?

Use centimeters for port diameter and length, liters for box volume, and hertz for tuning frequency.

Can I use multiple ports?

Yes. Enter the number of identical ports. The formula scales with the port count.

How do I measure port length?

Measure along the centerline of the port from the inner opening on one side to the inner opening on the other, including bends.

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Port Length Calculator

What Is a Bass Reflex Port?

Subwoofer Port Length Formula

Frequently Asked Questions

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