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Measuring Thiele-Small Parameters

2.0 What it is we are measuring

2.1 Impedance Curves
To give you a better idea of what is going on with a loudspeaker drive unit, here are generic/representative impedance curves for a closed box and a vented box. Note that these figures only show the portion of the impedance curve around the resonance frequency, as that is the main point of interest.

Closed box impedance curve

Figure 1: Closed box impedance curve.

A drive unit in free air will have an impedance curve very similar to that of a closed box. The closed box raises the resonance frequency and increases the Q-factor (the inverse of damping).

Vented box impedance curve

Figure 2: Vented box impedance curve.

The full frequency impedance usually exhibits a rising response a couple of octaves or so after the resonance frequency.


2.2 Getting Good Parameters
Thiele-Small parameters are "small signal" parameters and so must be measured at relatively low levels. This means less than a volt applied to the voice coil. This also helps to keep any non-linearities due to suspension to a minimum.

Some people advocate testing at higher levels as this is more like what level the speaker will actually be used at. However, Thiele and Small designed these equations to be done at low levels, so that's what I recommend. You can always test at low level and again at higher levels to see what differences may occur, you should find that as you increase drive level the resonance frequency drops.

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