Thiele/Small parameters

The set of electromechanical parameters that describe a moving-coil driver well enough to predict its behavior in an enclosure before building one. Named for A. Neville Thiele (Australian Broadcasting Commission) and Richard H. Small (University of Sydney), who formalized the small-signal model (Small’s JAES articles from 1972) ts-parameters-wikipedia. They are the bridge between a driver’s datasheet and a designed box: feed them to a calculator like micka-enclosure-calculator and you get a predicted response curve ts-parameters-explained.

The standard behind the numbers, and its expiry

A datasheet’s T/S figures are not just “measured” — they are measured to a standard, and that standard has changed. IEC 60268-5 (1972, and in its 2003 form the document that specified these parameters, the anechoic chamber, the IEC baffle and the test boxes) has been withdrawn, replaced by IEC 60268-21 (output-based acoustical measurement), IEC 60268-22 (electrical and mechanical measurement on transducers) and IEC 63034 (micro-speakers) iec-60268-5-replacement. The reason matters here: 60268-5 treated a loudspeaker as “entirely passive,” which an active/DSP speaker is not.

Practical consequence for anyone reading a datasheet: numbers produced under the old and new regimes are not guaranteed comparable, and nothing in this corpus says how a builder tells which regime a given datasheet used.

And the unit-to-unit spread has been measured. ts-parameter-manufacturing-variation (AES Convention 125, 2008) took twenty nominally identical drivers and measured each one’s parameters, on the premise that “manufacturing tolerances can have a large impact on the variability and quality of loudspeakers produced” — with a follow-on phase correlating that spread with failure mode and lifetime. That upgrades this wiki’s measure-your-own-driver advice from builder folklore to something an AES paper set out to quantify, though the figures themselves sit behind the e-library and are not held here.

The core parameters

• Fs — free-air resonance frequency (Hz); a practical floor for useful output. A 40 Hz‑Fs woofer handles 45 Hz but not 30 Hz ts-parameters-explained.
• Qms / Qes / Qts — damping (Q factors) at resonance: mechanical, electrical, and the combined total. Qts is the headline figure for matching a driver to a box type.
• Qtc — the system Q of the driver once sealed in a box (a design output, not a driver constant).
• Vas — the equivalent air compliance volume: the air volume whose springiness matches the suspension. The key number for sizing an enclosure.
• Cms — suspension compliance (mechanical springiness); Vas is its acoustic restatement.
• Re / Le — voice-coil DC resistance and inductance (Le shapes high-frequency rolloff).
• Bl — motor strength, the product of magnetic flux density and coil length in the gap.
• Mms — moving mass of the cone assembly (incl. the air load).
• Sd — effective cone area (the radiating piston).
• Xmax — linear excursion limit before distortion; with Sd it bounds clean output / displacement.
• SPL / efficiency — sound pressure per watt at a reference distance.

How they’re measured

The datasheet numbers aren’t handed down — they come from a free-air impedance sweep around resonance ts-parameters-wikipedia. Re is read with the cone blocked (a free cone picks up ambient sound); Fs is the impedance peak; the Q factors fall out of the curve (Qes from Re + peak impedance, Qms from its shape, Qts their parallel combination). Accuracy rises across three rigs — constant-current (large series resistor), constant-voltage, and the small-series-resistor (~10 Ω) method that computer measurement systems use. Vas needs a second reading that shifts the suspension’s apparent stiffness — added mass on the cone or sealing it in a known box, then re-measuring the moved resonance. Caveat: these are small-signal snapshots that drift with drive level, voice-coil temperature, and age, and vary unit-to-unit within tolerance — so a careful builder measures their own driver rather than trusting the spec sheet.

Why they’re the foundation of this wiki

They convert speaker design from trial-and-error into calculation: the same numbers decide sealed vs. vented, the box volume, and the port tuning. Wattage ratings alone say nothing about in-box performance — the T/S set is the actual spec ts-parameters-explained.

Related

loudspeaker-driver · loudspeaker-enclosure · micka-enclosure-calculator · ts-parameters-explained · ts-parameters-wikipedia · loudspeaker-primer-hackaday · iec-60268-5-replacement · ts-parameter-manufacturing-variation · synthesis