Sealed Box Calculator
Find the ideal sealed enclosure volume for your subwoofer from its Thiele/Small parameters and see the resulting frequency response and group delay.
Inputs
Equivalent compliance volume
Free-air resonance
Total Q of the driver
0.707 = flat, 0.8–0.9 = punchy
| Qtc | Net volume | F3 |
|---|---|---|
| 0.500 | 362.4 L / 12.80 ft³ | 48.3 Hz |
| 0.577 | 132.0 L / 4.66 ft³ | 45.7 Hz |
| 0.707 | 57.9 L / 2.04 ft³ | 44.0 Hz |
| 0.800 | 39.3 L / 1.39 ft³ | 44.7 Hz |
| 1.000 | 21.6 L / 0.76 ft³ | 48.9 Hz |
How it works
A sealed (acoustic-suspension) box traps air behind the cone. That air acts like an extra spring that stiffens the driver’s suspension, raising its resonance from Fs to a new system resonance Fc and its damping from Qts to Qtc. The smaller the box, the stiffer the spring, the higher both numbers climb.
α = Vas / Vb = (Qtc / Qts)² − 1
Vb = Vas / [(Qtc / Qts)² − 1]
Fc = Fs · (Qtc / Qts)
F3 = Fc · √{ [ (1/Qtc² − 2) + √((1/Qtc² − 2)² + 4) ] / 2 }The curve above is the response of a 2nd-order high-pass filter: 12 dB per octave below Fc. At Qtc = 0.707 (Butterworth) the response is maximally flat; above that you get a hump around Fc (more “punch”, worse transient accuracy); below 0.5 the bass rolls off early and sounds dry.
The group-delay curve shows how late the lowest notes arrive compared with the highs. High Qtc values ring longer, that’s the “boomy” sound. In a car, cabin gain adds roughly 6–12 dB per octave below 50–70 Hz, so many installers choose Qtc = 0.8–1.0 for the slightly smaller box, and let the cabin fill in the low end.
Tips: Use net internal volume (after subtracting driver, bracing and any port). Lightly stuffing the box with polyfill can make it behave ~10–15% larger. Parameters are only approximate for real drivers, cheap woofers vary ±15% from the spec sheet.