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Port Tuning Frequency Calculator

Enter your port area, port length, and box volume to find the tuning frequency.

How it is calculated

The tool applies the Helmholtz resonator formula to a single port in inch units. First the port's effective length, which is longer than the physical tube because the air just outside the ends also moves: Leff = L + 0.823 × √A, with A the port cross-section in in² and L the port length in inches. Then Fb = (c ÷ 2π) × √(A ÷ (V × Leff)), where V is the internal box volume in in³ and c = 13,504 in/s is the speed of sound used (so c ÷ 2π is about 2,149 in/s). Fb is shown in Hz to one decimal. A wider port or a smaller box raises the result, a longer port lowers it. All three inputs must be above zero. It does not account for port shape, driver displacement, bracing or stuffing; 1 ft³ is 1,728 in³.

Worked example

A box has an internal volume of 1,728 in³ (1 ft³) and a port with a 12 in² opening that is 6 in long. Effective length: 6 + 0.823 × √12 = 6 + 2.851 = 8.85 in. Ratio: 12 ÷ (1,728 × 8.85) = 0.000785. Square root: 0.0280. Multiplying by 2,149 gives 60.2 Hz, the tuning frequency the tool reports. Making the port 12 in long instead raises Leff to 14.85 in and lowers the result to 46.5 Hz.

Try this example →

Common mistakes

  • Entering metric sizes. Area must be in square inches, length in inches and volume in cubic inches, so a box of 28 litres is about 1,709 in³, not 28.
  • Using the external box volume. The volume should be the net internal air volume, after subtracting the driver, the port tube and bracing.
  • Entering the port diameter in the area field. Area is the cross-section, π × (diameter ÷ 2)², so a 3 in round port is about 7.07 in².
  • Adding the end correction to the length yourself. The tool adds it automatically, so give the physical tube length only.
  • Using one port's area for a box with several ports. Add the areas together, and use the length of one port only if all ports are the same length.

About this calculator

A vented, or ported, speaker enclosure uses an air-filled tube to reinforce bass output around a specific frequency, and that frequency is set by the classic Helmholtz resonator equation, which depends on the port area, port length, and internal box volume. Get the port dimensions wrong and the box either booms at the wrong frequency or loses the deep bass extension it was designed for.

Car audio installers, home subwoofer builders, and speaker cabinet designers calculate this tuning frequency (Fb) before cutting any wood, since it determines how the enclosure’s bass response will sound and how well it matches the driver’s specifications. This calculator applies the Helmholtz formula to your entered port area, port length, and box volume to return the resulting tuning frequency in Hz.

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