Understanding Thiele-Small Parameters
Thiele-Small parameters describe a loudspeaker driver's electromechanical properties. They're the foundation of all enclosure simulation and optimization.
The Big Three
Fs (Resonant Frequency) — The frequency at which the driver naturally resonates in free air. Lower Fs generally means better deep bass potential. Measured in Hz.
Qts (Total Q Factor) — How "damped" the driver is at resonance. It combines electrical damping (Qes) and mechanical damping (Qms). Higher Qts means a more resonant peak; lower means more damping.
- Qts < 0.4: Good for vented enclosures
- Qts 0.4–0.7: Versatile, works in sealed or vented
- Qts > 0.7: Best suited for sealed enclosures
Vas (Equivalent Compliance Volume) — The volume of air that has the same compliance (springiness) as the driver's suspension. Larger Vas generally requires larger enclosures. Measured in litres.
Other Important Parameters
Re — DC resistance of the voice coil (ohms). Always lower than the rated impedance.
Le — Voice coil inductance (mH). Higher Le means more impedance rise at high frequencies.
Xmax — Maximum linear excursion (mm). How far the cone can move before distortion becomes significant.
Sd — Effective cone area (cm²). Combined with Xmax, determines maximum air displacement.
BL — Force factor (T·m). The product of magnetic flux density and voice coil length. Higher BL means more control over cone motion.
Mms — Moving mass (g). Includes the cone, voice coil, and air load. Lower mass generally means higher sensitivity.
Pe — Power handling (W). Maximum continuous power the driver can handle without damage.
How RokketBox uses these
When you select a driver in RokketBox, these parameters feed directly into the simulation engine. The sealed, vented, and bandpass models all derive their frequency response, impedance, and excursion curves from these values.
The optimizer uses Fs, Qts, Vas, and Xmax as the primary inputs for determining optimal enclosure dimensions and tuning.