What are Thiele-Small parameters?

Thiele-Small parameters describe the low-frequency behaviour of a speaker driver. The key ones for cabinet design are: Vas (equivalent air compliance volume), Qts (total Q factor), and Fs (free-air resonant frequency). These are provided in the driver datasheet.

Sealed or ported — which is better?

Sealed cabinets are simpler, have tighter transient response, and roll off at 12 dB/octave. Ported cabinets extend lower with less power, but roll off at 24 dB/octave below tuning. Sealed suits music accuracy; ported suits PA where maximum bass output matters.

Vas is the volume of air that has the same compliance (springiness) as the driver suspension. A large Vas means a compliant suspension that typically needs a larger cabinet. Vas is specified in litres and is one of the most important parameters for cabinet design.

What is Qts and how does it affect cabinet size?

Qts is the total quality factor of the driver, combining electrical and mechanical damping. Low Qts (0.2-0.4) suits sealed boxes. Medium Qts (0.3-0.5) works for ported designs. High Qts (0.5+) may not suit either design well. Qts determines the optimal sealed box volume ratio.

AudioCalcs

Cabinet Volume Calculator — Sealed & Ported Box Design

Calculate sealed and ported speaker cabinet internal volume from Thiele-Small parameters.

Vas (equivalent volume)

Qts (total Q factor)

Fs (resonant frequency)

How We Calculate This

Sealed Cabinet (Butterworth Q = 0.707)

Vb = Vas / ((Qts / 0.707)²)

F3 = Fs × √(1 + Vas / Vb)

Ported Cabinet (estimate)

Vb ≈ Vas × 1.5 (typical starting point)

F3 ≈ Fs × 0.8 (approximate -3 dB point)

Frequently Asked Questions

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Last updated: February 2026

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