Speaker Box Volume Calculator
Calculate speaker enclosure volume for sealed and ported boxes using Thiele-Small parameters.
Find ideal box size, port dimensions, and tuning frequency.
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 the driver’s ability to store energy against its ability to dissipate it 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). A sealed box can only raise a driver’s Q, never lower it, because the trapped air adds a second spring. That is why a target Qtc below the driver’s own Qts asks for a box of infinite size. Higher Qtc means less damping, a smaller box, and a bump in output just above the system resonance. Below about 0.5 the box gets very large and the bass turns dry.
Fc and F3 are not the same number Fc = Fs × Qtc / Qts is the system resonance. F3 is where output has fallen 3 dB. They coincide only at Qtc = 0.707. At Qtc = 0.5 the true F3 sits about 55% above Fc, so a design quoting Fc as its −3 dB point is claiming far more bass extension than it has. Plenty of published box charts do exactly that.
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, and the driver is unloaded at the 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 a very large box; may not sound good sealed, so use ported Qts 0.3–0.5: good sealed/ported; target Qtc = 0.5–0.707 Qts > 0.7: designed for sealed; may not work well ported
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