What are Thiele-Small parameters?

Thiele-Small (T/S) parameters describe the electromechanical properties of a loudspeaker driver. The key ones for enclosure design are Fs (resonant frequency in Hz), Qts (total Q factor) and Vas (equivalent compliance volume in litres). These are published in driver datasheets.

Should I choose sealed or ported?

Sealed enclosures are simpler to build, have better transient response and a gentler roll-off below cutoff. Ported enclosures extend bass lower and produce more output near the tuning frequency but roll off sharply below it. Drivers with Qts below 0.4 typically suit sealed; those from 0.3 to 0.5 suit either.

What is Qtc and why does 0.707 matter?

Qtc is the total system Q of a driver in a sealed enclosure. A Qtc of 0.707 (Butterworth alignment) gives the flattest possible response with no bass peak before roll-off. Lower values give flatter but less extended bass. Higher values add a bass hump that can sound boomy.

Do I need to add volume for bracing and driver displacement?

Yes. Bracing, the driver magnet and port tube all displace internal volume. Add approximately 10-15% to the calculated net volume to compensate. The calculator shows net internal volume — your external box should be larger.

Subwoofer Box Calculator — Sealed & Ported Enclosures

Calculate sealed or ported subwoofer enclosure volume from driver Thiele-Small parameters.

Enclosure Type

Sealed or ported box design

Driver Diameter

Nominal driver diameter (e.g. 250 for a 10-inch)

Fs (Resonant Frequency)

Driver free-air resonant frequency from datasheet

Qts (Total Q Factor)

Total Q factor from driver datasheet

Vas (Compliance Volume)

Equivalent compliance volume in litres from datasheet

How We Calculate This

Sealed Enclosure

Vb = Vas / ((Qtc / Qts)² − 1)

f3 = Fs × √((Vas / Vb) + 1)

Ported Enclosure (Keele)

Vb ≈ 15 × Vas × Qts^3.3

fb ≈ 0.42 × Fs × Qts^−0.9

Port length is calculated using the standard port resonance formula with end correction.

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Last updated: March 2026

All calculations are estimates based on standard formulas. Always verify results for critical applications.