Thiele/Small parameters explained (AudioJudgement)
A DIY-oriented walkthrough of the Thiele/Small parameters — the electromechanical numbers on a driver’s datasheet that let you predict how it behaves before you build anything. One of the three founding sources of this wiki. T3: an enthusiast site, but the parameter definitions are standard and the worked cases are concrete.
What it covers
Defines the full parameter set and groups it by what it tells you:
- Resonance. Fs — the driver’s free-air resonance frequency (Hz); below it, output falls off.
- The Q factors — damping at resonance: Qms (mechanical losses), Qes (electrical, via the motor), and Qts (total, the parallel combination) — plus Qtc, the system Q once the driver is in a sealed box.
- Compliance & air-spring. Cms (suspension compliance) and Vas (the volume of air whose springiness equals the suspension’s) — Vas is the headline number for sizing an enclosure.
- Electrical. Re (DC voice-coil resistance), Le (voice-coil inductance, shaping the top end).
- Motor & mechanics. Bl (motor strength = flux density × coil length), Mms (moving mass), Sd (effective cone area), Xmax (linear excursion limit), and the SPL/efficiency rating.
The real-world cases (why the numbers matter)
- Buying blind. Picking a “300 W 10‑inch subwoofer” on wattage alone, with no T/S data, leaves a designer unable to predict the in-box result — the parameters are the actual spec.
- Sealed vs. bass-reflex. Qts and Vas drive the enclosure choice: low-Qts drivers suit vented boxes, higher-Qts drivers suit sealed.
- Reach vs. resonance. A woofer with Fs ≈ 40 Hz won’t usefully reproduce 30 Hz but does fine at 45 Hz — Fs sets a practical floor.
Related
thiele-small-parameters · loudspeaker-driver · loudspeaker-enclosure · micka-enclosure-calculator · synthesis