Subwoofer Box Calculator — Sealed & Ported Enclosures

Sealed Enclosure

Vb = Vas / ((Qtc / Qts)² − 1)

Fc = Fs × √((Vas / Vb) + 1) — system resonance

f3 = Fc × √( ((1/Qtc² − 2) + √((1/Qtc² − 2)² + 4)) / 2 )

The −3 dB point equals Fc only at Qtc = 0.707 (Butterworth); for other alignments f3 and Fc differ.

Ported Enclosure (Keele QB3)

Vb ≈ 20 × Vas × Qts^3.3

fb ≈ 0.42 × Fs × Qts^−0.96

f3 ≈ 0.28 × Fs × Qts^−1.4

Port length uses the Helmholtz vent formula Lv = (c²·Sp)/(4π²·fb²·Vb) − 0.732·d, where d is the port diameter and 0.732 is the end-correction for one flanged plus one free end (c = 343 m/s at 20 °C).

Suggested dimensions are net internal, with the internal width and height never smaller than the driver diameter + 50 mm so the driver fits the baffle. Bracing and driver/port displacement reduce usable volume, so the calculator also shows the external box size for your chosen wall thickness — add a further 10–15 % to net volume for displacement if not modelled separately.