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FIELD GUIDE / 02

How to read Thiele-Small parameters

Turn a driver data sheet into a useful starting point, without asking the numbers to promise too much.

4 min read / A practical starting point

A small-signal description

Thiele-Small parameters describe a driver's low-frequency, small-signal behavior. The most useful starting values are fs, Vas, Qes, Qms, Sd, and Re.

The six useful starting values

  • fs is the driver's free-air resonance under the stated measurement conditions. It is not the finished subwoofer's cutoff frequency.
  • Vas expresses suspension compliance as an equivalent air volume. It is not a recommended box size.
  • Qes and Qms describe electrical and mechanical damping. Their combination is Qts.
  • Sd is effective radiating area, not nominal frame diameter.
  • Re is DC voice-coil resistance, not the driver's nominal impedance.
Qts=QesQmsQes+QmsQ_{ts}=\frac{Q_{es}Q_{ms}}{Q_{es}+Q_{ms}}

Beyond the basics

Other useful values include moving mass Mms, compliance Cms, force factor Bl, voice-coil inductance Le, and one-way excursion Xmax. Our parameter converter shows how the consistent small-signal values relate.

Xmax is one-way peak travel here. Manufacturer definitions differ; it is not a universal distortion threshold.

Measure when the tolerances matter

Published parameters vary with sample, temperature, test mounting, and break-in. For a close-tolerance project, measure the driver you will actually install.