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Subwoofer Box Design from T/S Parameters (Fs, Qts, Vas)

Type your sub’s Fs, Qts and Vas: sealed box for a Qtc, or a ported box with its tuning and F3.
Box type
Sealed
Ported
Sealed
Ported
Fs, free-air resonance (Hz)
From your sub’s spec sheet.
Qts
Vas (ft³)
1.5 ft3 or 42 L. A bare number is ft³.
Qtc (box Q)
0.6
0.707
0.8
0.9
1
0.707 is the flattest. Lower = bigger box, higher = smaller box with more punch.
Net volume (sealed)
0.71 ft³ (20 L)
Box resonance (Fc)
53 Hz
Low-end −3 dB (F3)
53 Hz
Formula
Vb = Vas ÷ ((Qtc ÷ Qts)² − 1), Fc = Fs × Qtc ÷ Qts
With
Fs = 30 Hz, Qts = 0.4, Vas = 1.5 ft³, Qtc = 0.707, α = 2.12
Starting points, not guarantees. Your sub’s manual wins: check its recommended box and tuning.

How it works

Type Fs, Qts and Vas from your sub’s spec sheet (Vas in ft³ or liters), then pick sealed or ported.
Example: Fs 30 Hz, Qts 0.40, Vas 1.5 ft³, sealed at Qtc 0.707: (0.707 ÷ 0.40)² − 1 = 2.12, so 1.5 ÷ 2.12 = 0.71 ft³ net, and Fc = F3 = 53.0 Hz.
Same sub ported: 15 × 1.5 × 0.40^2.87 = 1.62 ft³ net, tuned to 28.7 Hz, F3 about 28.1 Hz. Tap "Use this volume" to size the box.

Frequently asked questions

What are Fs, Qts and Vas?

Thiele/Small parameters from the sub’s spec sheet. Fs is the resonance in free air, Qts how damped it is, Vas the air volume as stiff as its suspension.

Should my sub go sealed or ported?

Rule of thumb by Qts: under about 0.4 suits ported, 0.4 to 0.6 works either way, above 0.6 goes sealed or free air. The sub’s manual wins.

What Qtc should I pick?

0.707 is the flattest response. A higher Qtc means a smaller box and more punch: the same sub at Qtc 0.8 needs 0.50 ft³ and Fc is 60 Hz.