Quick answer
Enter your driver's Fs, Qts and Vas. The calculator returns a sealed box volume for your target Qtc and an EBP figure that tells you whether a ported design makes sense. Use the result as a starting point — always account for bracing and driver displacement, and verify the final design with measurement.
How this calculator works
The subwoofer box calculator translates driver Thiele/Small parameters (Fs, Qts, Vas) into a starting sealed enclosure volume and alignment guidance for sealed vs ported choice. EBP (efficiency bandwidth product = Fs/Qts) indicates which type of enclosure the driver suits best.
Sealed volume uses the classic Qtc formula. Ported designs require additional port modeling — use the bass reflex port calculator or slot port calculator after choosing Vb and tuning frequency Fb.
Formulas used
- Sealed Vb = Vas / ((Qtc / Qts)² − 1)
- Sealed system Fc ≈ Fs × (Qtc / Qts)
- EBP = Fs / Qts
- Starting ported Vb ≈ 20 × Vas (rough estimate only — verify with full vented alignment modeling)
Worked example
Driver: Fs = 28 Hz, Qts = 0.42, Vas = 80 L. Target Qtc = 0.707:
- Sealed Vb = 80 / ((0.707/0.42)² − 1) ≈ 25 L, Fc ≈ 47 Hz
- EBP = 28/0.42 ≈ 67 → sealed or moderate ported depending on goals
- Starting ported Vb ≈ 50–80 L at Fb ≈ 28–35 Hz
After confirming volume, cut panels accounting for 18 mm walls and bracing. Use the speaker box calculator to verify net internal volume from external dimensions.
Key terms
- Sealed box — smaller volume, controlled second-order roll-off, higher power handling at resonance.
- Ported box (bass reflex) — more output near Fb, steeper roll-off below; requires correct port area and length.
- Qtc — total Q of driver in the sealed box; determines low-frequency shape.
- EBP — efficiency bandwidth product. Below 50: sealed. 50–100: either. Above 100: ported preferred.
- Fs — free-air resonant frequency of the driver.
- Vas — equivalent compliance volume; the air volume that has the same compliance as the driver suspension.
Limitations and assumptions
- The Qtc formula assumes a lossless, rigid enclosure. Real boxes have panel flex, leaks and absorption.
- The ported volume estimate (Vas × 2.2) is a starting point only — full ported design requires modeling with driver parameters and port dimensions.
- Room gain (typically +3 to +6 dB per octave below room dimensions) affects in-room response and is not included.
- Driver parameters change with break-in; measure before finalizing EQ or passive components.
- Always subtract bracing volume and driver displacement from the target Vb before cutting.
Related tools
- Subwoofer Design Wizard — guided design from a driver or target frequency, with port sizing, warnings and cut lists.
- Speaker Box Calculator — calculate net internal volume from external cabinet dimensions.
- 3D Speaker Box Calculator — same with 3D cabinet preview.
- Bass Reflex Port Calculator — port length from Vb, Fb and port diameter.
- Slot Port Calculator — slot port dimensions for vented designs.
- Thiele/Small Calculator — derive enclosure recommendations from driver measurements.
- Speaker Building Basics Guide — overview of the DIY speaker design process.
Frequently asked questions
- Sealed or ported for home subwoofer? Sealed for tight, accurate bass with controlled roll-off. Ported for more output near Fb if the driver EBP supports it (EBP > 80–100).
- What Qtc should I target? Qtc ≈ 0.707 (Butterworth) is a balanced starting point. Lower Qtc (0.5–0.65) gives tighter transient response but requires a larger box.
- Does this calculate port length? No. Use the bass reflex port or slot port calculator after choosing Vb and target Fb.
- Why does the built sub sound different? Room gain, placement near walls, driver break-in and port noise all affect real-world response. Measure and EQ if needed.