Port Length Calculator – Subwoofer Port Length

Use this subwoofer port length calculator is used to measure the optimal length of a port in a ported (or bass reflex) subwoofer enclosure.

You're building a subwoofer enclosure with a volume of 2 cubic feet and want to tune it to 35 Hz using a 3-inch diameter port. Without a calculator, determining the correct port length would be a complex task involving multiple variables and equations. 

Port Length Calculation Chart

Port Length Formula

The Port Length Formula is derived from Helmholtz resonator physics and is typically expressed as:

L = [23562.5 (d² / (v f²))] - (0.732 * d)

Where:

• L = Port length (inches)
• d = Port diameter (inches)
• v = Enclosure volume (cubic inches)
• f = Tuning frequency (Hz)

Suppose we have an enclosure volume of 2.5 cubic feet (4320 cubic inches), a port diameter of 3 inches, and we want to tune it to 32 Hz. Plugging these values into the formula:

L = [23562.5 (3² / (4320 32²))] - (0.732 * 3)
L = [23562.5 * (9 / 4423680)] - 2.196
L ≈ 11.8 inches

The optimal port length for this configuration would be approximately 11.8 inches.

How Do You Measure the Length of a Port?

Measure the total length: Use a tape measure or ruler to measure from one end of the port to the other.

Account for flares: If your port has flared ends, measure from the inside edge of one flare to the inside edge of the other.

Subtract end correction: The effective acoustic length of a port is slightly longer than its physical length. Subtract 0.732 times the port diameter from your measurement.

Let’s say you have a port with a physical length of 12 inches and a diameter of 3 inches:

Effective port length = Physical length - (0.732 * diameter)
Effective port length = 12 - (0.732 * 3)
Effective port length ≈ 9.8 inches

This 9.8-inch effective length is what you’d use in your calculations or when comparing to the results from a port length calculator.

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