Thiele-Small Parameters And Model Limits
Canonical Definition
Thiele-Small parameters are a lumped small-signal model of an electrodynamic driver near its rest position. The values depend on measurement level, temperature, mounting, fixture resistance, air load, added mass or test-box accuracy, and the model used to fit the impedance and motion data.
Measured And Derived Terms
| Group | Examples | Reporting rule |
|---|---|---|
| Electrical | Re, Le and the chosen inductance model | Remove fixture and cable error; state coil temperature. |
| Mechanical | Mms, Cms, Rms, Bl, Sd | State how motion, added mass, or test volume was measured. |
| Derived | Fs, Qes, Qms, Qts, Vas, efficiency or sensitivity estimates | Keep the underlying assumptions and reference conditions attached. |
Qts Does Not Choose The Box
Qts describes total damping at the measured small-signal condition. It influences possible alignments, but the final choice also depends on Vas, Fs, Bl, moving mass, excursion and thermal capability, available volume, output target, passband, filters, losses, vent feasibility, vehicle transfer, and acceptable compression.
Two drivers with the same Qts can require very different boxes and can reach very different limits.
Measurement Contract
- Record driver identity, orientation, ambient condition, and cool or warmed state.
- Record stimulus level, bandwidth, series resistance, cable resistance, and calibration.
- State whether parameters came from added mass, test box, laser displacement, or another method.
- Report fit quality or repeatability where the instrument provides it.
- Repeat after a controlled warm-up when the design is sensitive to parameter shift.
Where The Linear Model Ends
At higher excursion and temperature, Bl, suspension stiffness, inductance, losses, rest position, and voice-coil resistance can change. Thermal compression and nonlinear displacement can dominate even when the low-level model looked ideal. Use large-signal displacement, distortion, compression, and temperature data when the design approaches those limits.