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Measuring Thiele-Small parameters

Manufacturers often publish Thiele-Small parameters, but production tolerances and test conditions can cause individual samples to differ.

Measuring the actual driver improves enclosure predictions.

What can be found from impedance?

A low-level free-air impedance sweep can be used to determine:

  • \(R_E\)
  • \(f_S\)
  • \(Q_{TS}\)

The resonance appears as the main low-frequency impedance peak.

An anechoic chamber is not required for an electrical impedance measurement, but the driver should be held or mounted so that nearby objects do not interfere mechanically.

Measuring \(R_E\)

Measure voice-coil resistance with a suitable low-resistance meter or impedance fixture.

Keep the test current low. Voice-coil heating increases resistance and changes the calculated parameters.

Measuring \(V_{AS}\)

Two common methods are available.

Added-mass method

Attach a known, nonmagnetic mass evenly to the diaphragm and measure the new resonance frequency. The added mass lowers the resonance, allowing moving mass and compliance to be calculated.

The mass must be secure and distributed so that it does not tilt the cone.

Known-box method

When attaching mass is impractical, mount the driver in a well-sealed enclosure of accurately known volume and measure the new resonance. The air stiffness of the box raises the resonance.

Good measurement practice

  • Use small signals.
  • Keep the driver away from nearby surfaces where possible.
  • Record temperature.
  • Check for air leaks.
  • Repeat the sweep.
  • Use consistent mechanical conditioning before comparing samples.

Important limitation

Thiele-Small parameters are small-signal parameters. Suspension stiffness, BL, inductance, and resistance can change with displacement, current, and temperature.