Cabinet Volume Calculator — Sealed & Ported Box Design

Sealed Cabinet (closed-box / acoustic suspension)

The box volume for a chosen target system Q (Qtc) comes from Small's closed-box theory:

Vb = Vas / ((Qtc / Qts)² − 1)

The system resonance in the box (raised above Fs by the air spring) is:

Fc = Fs × √(Vas / Vb + 1)

Qtc must be greater than Qts for a valid sealed box (Qtc = 0.707 is the maximally-flat Butterworth target; 0.5 is critically damped; 0.577 is the Bessel alignment; above ~0.8 gives a bass bump). The true −3 dB point of the resulting 2nd-order high-pass is:

F3 = Fc × √[(1/(2·Qtc²) − 1) + √((1/(2·Qtc²) − 1)² + 1)]

F3 equals Fc only when Qtc = 0.707; at lower Qtc the −3 dB point sits above Fc, at higher Qtc below it.

Ported Cabinet (vented — Margolis & Small QB3/B4 approximation)

Unlike the rough "Vas × 1.5" rule of thumb, a proper vented alignment depends on Qts (Margolis & Small, JAES 1981):

Vb ≈ 20 × Vas × Qts^3.3

Fb ≈ 0.42 × Fs × Qts^−0.96 (box tuning)

F3 ≈ 0.28 × Fs × Qts^−1.4 (approximate −3 dB point)

This approximation suits mid-Qts drivers (roughly 0.2–0.5); always confirm against modelling software for the final build.