About the Subwoofer Box Calculator
This subwoofer box calculator turns the three key Thiele/Small parameters printed on every subwoofer spec sheet — free-air resonance Fs, total Q (Qts) and equivalent compliance volume Vas — into a box size. Choose a sealed enclosure to get the net volume for a target system Q, or a ported (bass-reflex) enclosure to get a recommended volume, tuning frequency and the length of port needed to hit that tuning.
It is meant for car-audio and home-theatre DIYers planning a build, and for anyone checking whether a pre-fab box suits a particular driver. You can override the ported volume and tuning to fit the space you have, and the calculator adds driver and port displacement to give the gross internal volume you should actually build.
Sealed results use the standard closed-box equations. The ported starting point uses D. B. Keele’s well-known approximations of a flat (QB3-type) alignment, which assume a driver with Qts roughly between 0.2 and 0.5; outside that range, model the box in dedicated software. Box losses, stuffing and cabin gain are ignored.
With the default inputs, the net internal volume is 40.9 L. Change any value above to recalculate instantly.
How to use the subwoofer box calculator
- 1Copy Fs, Qts and Vas from your subwoofer’s spec sheet (convert Vas from ft³ to litres if needed).
- 2Choose a sealed or ported enclosure.
- 3For sealed, set the target Qtc — 0.707 is the usual flat choice.
- 4For ported, keep the recommended volume and tuning or enter your own, then set port diameter and count.
- 5Enter the driver displacement and build to the gross volume shown.
Formula and method
In a sealed box the trapped air acts as an extra spring. The compliance ratio α = (Qtc ÷ Qts)² − 1 sets how stiff that spring must be, so the box volume is Vas ÷ α, while resonance and Q both rise by the factor Qtc ÷ Qts. F3 for a second-order closed box follows from Qtc; at Qtc = 0.707 it equals Fc exactly.
For a ported box this tool uses Keele’s approximations for a maximally flat alignment, a reliable starting point for drivers with Qts around 0.2–0.5. A port is a Helmholtz resonator: its effective length is c²A ÷ (4π²Fb²Vb), where A is the total port area and c = 343 m/s. The physical length subtracts an end correction of about 0.732 × diameter (one flanged end, one free end).
- Fs, Qts, Vas
- Driver free-air resonance, total Q and equivalent compliance volume
- Qtc
- Target Q of the driver in the sealed box
- Vb
- Net internal box volume
- Fc, Fb
- Sealed-box resonance; ported-box tuning frequency
- A, D, Lv
- Total port area, port diameter and port length
- c
- Speed of sound, 34,300 cm/s
Worked examples
12-inch sub in a sealed box (Qtc 0.707)
With Qts 0.45, α = (0.707 ÷ 0.45)² − 1 = 1.468, so the box is 60 ÷ 1.468 ≈ 40.9 L (1.44 ft³). Resonance rises to 30 × 0.707 ÷ 0.45 ≈ 47.1 Hz, which is also the F3 for a Qtc of 0.707. Adding 2.5 L of driver displacement gives 43.4 L to build.
Same driver, ported with one 10 cm port
Keele’s alignment gives Vb = 15 × 60 × 0.45^2.87 ≈ 91.0 L tuned to 0.42 × 30 × 0.45^−0.9 ≈ 25.9 Hz, with F3 ≈ 23.9 Hz. A 10 cm port (78.5 cm²) needs an effective length of 38.5 cm; minus the 7.3 cm end correction it is about 31.2 cm long. Port and driver push the gross volume to about 95.9 L.
Ported box squeezed into 2 ft³ at 32 Hz
Two 3-inch (7.62 cm) ports give 91.2 cm² of area. Tuning 56.6 L (2 ft³) to 32 Hz needs an effective length of about 46.9 cm, so each port is 46.9 − 0.732 × 7.62 ≈ 41.3 cm long — long enough that a slot port may fit better.
Frequently asked questions
Sealed or ported: which subwoofer box is better?+
Sealed boxes are smaller, with tighter, more accurate bass and a gentle 12 dB/octave roll-off. Ported boxes play louder and deeper near the tuning frequency for the same power but are larger and fall off faster below tuning.
What do Fs, Qts and Vas mean?+
Fs is the driver’s free-air resonance, Qts its total damping (Q), and Vas the volume of air with the same stiffness as its suspension. Together they predict how a driver behaves in any box.
What is net versus gross box volume?+
Net volume is the air the driver actually works against. Gross volume is the empty internal space you build, which must also hold the driver’s magnet and basket and any port or bracing.
How big should the port be?+
Larger ports reduce air velocity and chuffing noise. A common rule is roughly 12–16 square inches (80–100 cm²) of port area per cubic foot of box for a high-power car sub, but larger area needs a longer port for the same tuning.
What Qtc should I choose for a sealed sub?+
A Qtc of 0.707 gives the flattest response. Around 0.5–0.6 sounds tighter and benefits from car cabin gain; 0.9 and above gives a bump of punch at the cost of boomier, less controlled bass.