Speaker Box Volume Calculator

Thiele-Small Parameters Thiele-Small (TS) parameters describe a loudspeaker’s mechanical and electrical characteristics. Named after A. N. Thiele (Australia, 1961) and R. H. Small (Australia, 1972). Key parameters for box design: Vas — equivalent compliance volume (liters): how “springy” the driver is Qts — total Q factor: ratio of driver’s ability to store vs. dissipate energy Fs — resonant frequency (Hz): the driver’s natural resonance in free air

Sealed Box (Acoustic Suspension) Sealed boxes are simple, well-damped, and have a gentle roll-off (−12 dB/octave). Optimal volume: Vb = Vas / (Qtc²/Qts² − 1) Where Qtc = target system Q (0.5–0.707 typical; 0.707 = maximally flat Butterworth). Higher Qtc → smaller box, more low-bass boost, more audible roll-off. Best for music with extended bass lines; predictable, tight bass.

Ported Box (Bass Reflex) A ported (vented) box uses a tube resonator to extend bass output below the driver’s Fs. Adds a second resonance — driver unloaded at port tuning frequency (Fb). Classic alignment: Fb ≈ 0.7 × Fs (produces flat response for Qts ≈ 0.4). Typical ported volume: Vb ≈ 20 × Qts³·³ × Vas (alignment-dependent). Ported boxes are louder and go lower than sealed, but cut off sharply below Fb. Best for home theater subwoofers, car audio, bass-heavy music.

Port Dimensions Port length: Lp = (23562.5 × Nd × Dp²) / (Fb² × Vb) − 0.732 × Dp Where Nd = number of ports, Dp = port diameter (cm), Fb = tuning frequency (Hz), Vb = volume (L). Port diameter guideline: ~5–10 cm for home subwoofers. Round vs. slot ports: functionally equivalent; slots easier to fit in small boxes.

Q Factor Guidelines for Sealed Boxes Qts < 0.3: needs very large box; may not sound good sealed — use ported Qts 0.3–0.5: good sealed/ported; target Qtc = 0.5–0.7 Qts > 0.7: designed for sealed; may not work well ported