# Thiele/Small Parameters

In loudspeaker design the Thiele/Small Parameters are a set of electromechanical parameters that determine the low-frequency performance of loudspeaker drivers.

The parameters are named after two Australian loudspeaker engineers: Richard H. Small and A. Neville Thiele.

The parameters are divided into three categories: electromechanical parameters (or fundamental parameters), small-signal parameters and large-signal parameters.

## Electromechanical Parameters

These parameters are the physical properties of the loudspeaker driver, measured at small signal levels. They are often very difficult to measure in a finished loudspeaker driver, and as such the small signal parameters are often more practical when using an “off the shelf” loudspeaker driver.

ParameterNameDescription
SdSurface areaThe effective projected area of the diaphragm. Very difficult to measure accurately because roughly half of the surround moves with the cone.
BlForce factorThe product of magnetic flux density B produced by the magnet and the length of the voice coil l, in Tesla-metres (T·m). It is the mechanical force developed by the current flowing through the voice coil.

## Small Signal Parameters

These parameters are measured at small signal levels.