Thiele Small Parameters
The Thiele Small Parameters (often referred to as T/S Parameters) are provided in specification sheets by most manufacturers – but what are they for? A more detailed explanation of each parameter is provided in the relevant sections, but in simple terms, these parameters define the electromechanical properties of a loudspeaker driver, describing its electrical and mechanical behavior.
Once you understand some of the basics of the Thiele Small parameters, you will know what to look for when it comes to choosing a loudspeaker driver. If you’re not interested in the finer details, it’s enough to know that these parameters are mainly used for simulating loudspeaker behavior to optimize cabinet design. For those keen to learn more about these parameters, this section should cover almost everything you need to know.
The following small signal mechanical parameters describe a driver’s behaviour at low power levels
- BL – Motor strength, pushing force generated by the voice coil
- Mms – Moving mass of the cone, including air load
- Cms – Compliance – Flexibility of the suspension, opposite of stiffness
- Rms – Mechanical resistance of the suspension
- Le – Voice coil inductance
- Re – DC Resistance of the voice coil
- Sd – Surface area of the diaphragm (cone), measured in square meters
- η₀ (eta zero) – Reference efficiency (%) – how effectively electricity is converted into sound
- Z – Nominal impedance (Ω), average electrical resistance in operation
Since these characteristics are difficult to measure directly, it is often easier to derive missing parameters from other measured values. Other parameters, known as small signal parameters, are as follows:
- Fs – Free Air Resonance (also described as Resonant Frequency)
- Qes – Electrical Q at Fs
- Qms – Mechanical Q at Fs
- Qts – Total Q (derived from Qes and Qms)
- Vas – Compliance Equivalent Volume
Large signal parameters, listed below, are used when predicting driver behaviour at high power levels:
- Xmax – Maximum Linear Excursion of the cone and voice coil (one way)
- Xvar – Maximum linear excursion before significant distortion
- Xlim – Maximum physical excursion before damage, also known as Xmech or Xdamage
- Pe – Continuous power handling capacity, usually to AES Standard
- Vd – Peak displacement volume, calculated as Sd × Xmax
Other commonly used parameters:
- EBP (Efficiency Bandwidth Product) – Fs / Qes useful to determine optimal enclosure type
- SPL – Sensitivity at 1W/1m (dB)