Voice Coil Impedance as a Function of Frequency and Displacement
Recent work by Klippel and Voishvillo et al. has shown the significance of voice coil inductance in respect to the non-linear behaviour of loudspeakers. In such work the methods used to derive distortion require the inductance to be represented by an equivalent circuit rather than the frequency domain models of Wright  and Leach . A new technique for measurement of displacement and frequency dependent impedance has been introduced. The complex relationship between coil impedance, frequency and displacement has been both measured and modelled, using stationary transient FEM, with exceptional agreement. Results show that the impedance model requires that its parameters vary independently with displacement to satisfactorily describe all cases. Distortion induced by the variation of impedance with coil displacement is predicted using a lumped parameter method, this prediction is compared to measurements of the actual distortion. The possibility of using a dynamic transient FE method to predict distortion is demonstrated. The nature of the distortion is discussed.
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