Origins

A. N. Thiele and Richard H. Small wrote a series of papers published in the Journal of the Audio engineering Society in the early 1970s. In them, they applied well-developed electrical filter theory to the behavior of resonant speaker drivers mounted in any of several enclosures, most also resonant. In theory, Thiele-Small techniques can be used for mid and high frequency drivers, but in practice these techniques are most useful at low frequencies. As a result design of sealed and ported enclosures became not only easier but more precise.

Nearly all system designers have used their techniques since then. Thiele-Small identified several 'alignments' as short cuts to the full Thiele-Small procedure, and these are often used as a further design effort shortcut. The Thiele-Small model require several physical and electrical paraamters as inputs; Thiele-Small techniques are so popular that many driver suppliers provide them as part of their specifications. The measurements can be made in the field, but are sufficiently tedious that many designers do.

The Thiele-Small techniques don't apply to all possible system designs, however. For instance, horns, and to a lesser extent the drivers used for them, already had a well developed mathematical model. And transmission line enclosures have only recently gained a similarly useful mathematical model.

This site has a comprehensive list of descriptions of the individual parameters: https://www.speakerplans.com/page89.html

Have fun!

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