Subwoofer Enclosure Calculator

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Reviewed by David Chen, Audio Engineer

Use the Subwoofer Enclosure Calculator to determine the required port length ($L_v$) for your vented (ported) speaker box, or to calculate the necessary volume ($V_b$) or tuning frequency ($F_b$) when the other parameters are fixed.

Subwoofer Enclosure Calculator

($\text{ft}^3$)
(Hz)
($\text{in}^2$)
(in)
Calculated Result:

Subwoofer Enclosure Calculator Formula

The core relationship for a vented (ported) enclosure is used to solve for Port Length ($L_v$).

$$ L_v = \frac{K_c \cdot A_p}{F_b^2 \cdot V_b} – K_e \cdot \sqrt{A_p} $$

Where:

  • $L_v$ is Port Length (in)
  • $A_p$ is Port Area ($\text{in}^2$)
  • $F_b$ is Tuning Frequency (Hz)
  • $V_b$ is Enclosure Volume ($\text{ft}^3$)
  • $K_c \approx 8466$ (Constant adjusted for $\text{in}^2$, $\text{ft}^3$, and inches)
  • $K_e \approx 0.732$ (End correction factor)
Formula Source (Wikipedia)

Variables

  • Enclosure Volume ($V_b$): The net internal air space of the box, measured in cubic feet ($\text{ft}^3$).
  • Tuning Frequency ($F_b$): The frequency (in Hertz, Hz) at which the port and the air volume resonate. This is the low-frequency limit of the system.
  • Port Area ($A_p$): The cross-sectional area of the port opening, measured in square inches ($\text{in}^2$). Larger areas help prevent port noise (chuffing).
  • Port Length ($L_v$): The required physical length of the port tube/duct, measured in inches (in).

What is a Subwoofer Enclosure Calculator?

A Subwoofer Enclosure Calculator is an essential tool for designing custom speaker cabinets, particularly those utilizing a vented (or ported) design. Unlike sealed enclosures, vented boxes use a port to tune the system to a specific frequency ($F_b$), dramatically increasing low-frequency output near that point.

The calculator determines the precise physical dimensions required—primarily the port length—based on the desired acoustic outcome (tuning frequency) and the physical constraints of the enclosure volume and port size chosen. Using the correct port length ensures the system performs as intended, preventing distortion or excessive driver excursion at low frequencies.

How to Calculate Port Length ($L_v$) (Example)

Let’s find the required Port Length for a system with the following parameters:

  1. Input Variables: Enclosure Volume ($V_b$) = $2.0 \text{ ft}^3$; Tuning Frequency ($F_b$) = $30 \text{ Hz}$; Port Area ($A_p$) = $25 \text{ in}^2$.
  2. Apply the Constant Term: Calculate the numerator: $8466 \cdot 25 = 211650$.
  3. Calculate the Denominator: Calculate the term $F_b^2 \cdot V_b$: $30^2 \cdot 2.0 = 900 \cdot 2.0 = 1800$.
  4. Find the Main Ratio: Divide the numerator by the denominator: $211650 / 1800 \approx 117.58$.
  5. Calculate End Correction: Calculate $0.732 \cdot \sqrt{A_p}$: $0.732 \cdot \sqrt{25} = 0.732 \cdot 5 = 3.66$.
  6. Determine Final Length: Subtract the end correction from the main ratio: $117.58 – 3.66 = 113.92$. The required Port Length ($L_v$) is $113.92$ inches.

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Frequently Asked Questions (FAQ)

  • What happens if my calculated port length is too long? The port must be installed without bending or blocking the path. If the required length exceeds the longest internal dimension of the box, you must increase the enclosure volume ($V_b$), decrease the port area ($A_p$), or use an aeroport with a flared end.
  • Why is the Port Area ($A_p$) important? The Port Area controls air velocity. If the area is too small, air speed will be too high, leading to “port noise” or “chuffing.” Increasing the area reduces the required port length but may make the port too large to fit in the enclosure.
  • What is the difference between $F_b$ and $F_s$? $F_b$ is the enclosure’s tuning frequency, determined by the port and volume. $F_s$ is the driver’s free-air resonance frequency, a fixed parameter of the speaker itself. For a good design, $F_b$ is usually chosen to be close to or slightly below $F_s$.
  • Can this calculator be used for passive radiator enclosures? No. Passive radiator enclosures require a different formula focusing on the mass and compliance of the passive driver, not the physical length of a port.
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