Thiele/Small commonly refers to a set of standard parameters that define how a loudspeaker driver performs. Developed by A. N. Thiele of the Australian Broadcasting Commission, and Richard H. Small from the University of Sydney. These serve as useful quantities for designing speakers because they are more easily determined experimentally than the fundamental mechanical parameters.

Fundamental small signal mechanical parameters

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These are the linearized physical parameters of a loudspeaker driver, as defined at small signal levels and modeled in the equivalent circuit. Some of these are not convenient to measure in a finished loudspeaker, so when designing speakers with off the shelf drive units, the more easily measured parameters below are more useful.

• S_d - Projected area of the driver diaphragm, in square metres.
• M_ms - Mass of the diaphragm, including acoustic load, in kilograms.
• C_ms - Compliance of the driver's suspension, in metres per newton (the reciprocal of its stiffness).
• R_ms - The mechanical resistance of a driver's suspension (lossiness) in N·s/m
• L_e - Voice coil inductance measured in millihenries (mH).
• R_e - DC resistance of the voice coil, measured in ohms.
• Bl - The product of magnet strength and the length of wire in the magnetic field, in T·m (tesla·metres).

Small signal parameters

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These parameters are determined by measuring the input impedance of the loudspeaker (especially near the resonance frequency) at small input levels where the mechanical behavior of the driver is largely linear - or proportional to the input.

• F_s - Resonant frequency of the driver

$F\_s\; =\; \backslash frac\{1\}\{2.\backslash pi.\backslash sqrt\{C\_\{ms\}.M\_\{ms\}\}\}$

• Q_es - Electrical Q of the driver at F_s

$Q\_\{es\}\; =\; \backslash frac\{2.\backslash pi.F\_s.M\_\{ms\}.R\_e\}\{(Bl)^2\}$

• Q_ms - Mechanical Q of the driver at F_s

$Q\_\{ms\}\; =\; \backslash frac\{2.\backslash pi.F\_s.M\_\{ms\}\}\{R\_\{ms\}\}$

• Q_ts - Total Q of the driver at F_s

$Q\_\{ts\}\; =\; \backslash frac\{Q\_\{ms\}.Q\_\{es\}\}\{Q\_\{ms\}\; +\; Q\_\{es\}\}$

• V_as - Volume of air in cubic metres which, when acted upon by a piston of area S_d, has the same compliance as the driver's suspension. To get V_as in litres, multiply the result of the equation below by 1000.

$V\_\{as\}\; =\; \backslash rho.c^2.S\_d^2.C\_\{ms\}$

Where ρ is the density of air (1.184 kg/m³ at 25°C), and c is the speed of sound (346.3 m/s at 25°C).

Large signal parameters

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These parameters are useful for predicting the approximate output capability of a driver in a particular configuration.

• X_max - Maximum linear peak (or sometimes peak-to-peak) excursion (in mm) of the cone
• X_mech - Maximum physical excursion of the driver before damage
• P_e - Thermal capacity of the driver, in watts
• V_d - Peak displacement volume, calculated by V_d = S_d·X_max

Other parameters

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• Z_max - The impedance of the loudspeaker at F_s, used when measuring Q_es and Q_ms.

$Z\_\{max\}\; =\; R\_e\backslash left(1+\backslash frac\{Q\_\{ms\}\}\{Q\_\{es\}\}\backslash right)$

• EBP - The Efficiency Bandwidth Product, an indicator of whether a driver should be in a vented or sealed enclosure.

$EBP\; =\; \backslash frac\{F\_s\}\{Q\_\{es\}\}$

• Z_nom - The nominal impedance of the loudspeaker, typically 4, 8 or 16 ohms.
• η₀ - The reference or "power available" efficiency of the driver, in percent.

$\backslash eta\_0\; =\; \backslash left(\backslash frac\{9.614\backslash times10^\{-7\}\; .\; F\_s^3\; .\; V\_\{as\}\}\{Q\_\{es\}\}\backslash right)\backslash times100\backslash \; \%$

See also

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• Electrical characteristics of a dynamic loudspeaker

External links

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• measuring Thiele Small Parameters (method 1 - the classic method)
• measuring parameters (method 2)

Audio engineering | Speakers

Thiele-Small