🔉 Subwoofer Box Calculator

A subwoofer's sound depends as much on its box as on the driver. The sealed box is the simplest: a closed, airtight volume that adds a spring to the driver's suspension, lowering the resonant frequency and giving a smooth, deep roll-off. The ported (vented) box adds a tuned duct — the port — that reinforces output around the tuning frequency, extending low-frequency output with up to 3 dB more efficiency at the cost of a steeper roll-off below tuning. Both designs start from the driver's Thiele-Small parameters, especially Vₔₘ (equivalent air volume) and F₝ (free-air resonance).

Rule-of-thumb starts: a sealed box near Vₔₘ gives a tight, musical sub; going to 0.5–0.7×Vₔₘ tightens it further but sacrifices depth. A ported box near 1–1.5×Vₔₘ tuned to about 0.7–0.9×F₝ gives the classic car-audio boom with authority. The calculator returns an internal volume in liters and cubic feet, plus a port length for the tuning frequency you pick.

For a sealed box the calculator starts from the driver's Vₔₘ and a chosen alignment ratio (default 0.9) to derive the internal volume. For a ported box it uses the volume and tuning frequency to compute a port length via the standard end-corrected vent equation, L = (c²×S)/(4π²f²V) − 0.732D, where S = πD²/4 is the port cross-section, given the port diameter you choose. The port area rule of thumb (at least 5–10 cm² per liter) is included as a check so the port does not chuff.

The derived port end correction and the minimum port area flag help you avoid the two classic build failures: a port that is too long to fit the box, or too small and chuffs at high excursion. When pairing the box with an amplifier, the decibel calculator helps you compare output levels and the RMS calculator for signal levels.

1. Enter the driver's Vₔₘ (equivalent air volume) in liters.
2. Enter the free-air resonance F₝ in Hz.
3. Choose sealed or ported.
4. For ported: pick the tuning frequency and port diameter.
5. Click Calculate to see the internal volume in liters/cubic feet and the port length.
6. Check the port-area guideline to avoid chuffing.

What are Thiele-Small parameters?

A set of electro-mechanical driver measurements (F₝, Vₔₘ, Qₔₘ, etc.) that model how a driver behaves in an enclosure. Vₔₘ is the air volume with the same compliance as the suspension; F₝ is the free-air resonance. Box design essentially trades these against the enclosure volume and tuning.

Sealed or ported — which is better?

It depends on the goal. Sealed: smaller box, tight accurate bass, gentle roll-off, works in any orientation. Ported: up to 3 dB more output around tuning, deeper apparent bass for the same driver, but a bigger box, a steeper roll-off below tuning, and possible chuffing or boomy one-note bass if the tune is wrong.

Why does my ported box sound boomy at one frequency?

When the tuning frequency is too high relative to the driver and box, output peaks around Fₘ and sounds one-note. Lower the tune to 0.7–0.8×F₝ and make the box closer to the driver's recommended volume to flatten the response.

How long should the port be?

The calculator derives the length from tuning frequency, box volume and port diameter. Roughly, lower tunes and smaller boxes need longer ports. If the port ends up longer than the box interior, use a larger diameter or a slot/bend.

What if I have only the driver's datasheet without Vₔₘ?

Most reputable driver datasheets list Vₔₘ and F₝. If not, estimate Vₔₘ from similar drivers of the same size, or use the manufacturer's recommended enclosure volume directly — they publish one for good reason.

Does box volume affect power handling?

Indirectly. A smaller sealed box increases the spring and can reduce the driver's mechanical excursion at low frequencies (protecting it), while a ported box unloads the cone below tuning, where excursion can rise sharply and damage the driver. A subsonic filter on the amp is a good idea for ported designs.

Measure your driver's Thiele-Small parameters from the datasheet — do not guess Vₔₘ and F₝, as identical-looking drivers differ wildly. Seal the box completely; even a small leak converts a sealed box into an uncontrolled open one. For ports, use a minimum of about 8–10 cm² of area per liter of box volume to prevent chuffing, and keep the port at least one port-diameter from any wall. Add bracing, which steals volume — subtract the brace and driver displacement from the gross volume. In a car, cabin gain (about 12 dB/octave below 60 Hz) means a slightly higher tuning often sounds best, while a home theater sub in a big room wants the lower tune.