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INTRODUCTION

VoiceCoil Impendance paper written by Wolfgang Klippel of Klippel gmbh.

In order to describe the performance of a loudspeaker precisely, one must consider the electrical input impedance at higher frequencies. The voice coil does not operate in free air but close to conducting and magnetically permeable structures: the pole tips, the magnet, the voice coil, former, copper rings, etc. The impedance can be only roughly modelled as a resistor Re and an ideal inductance Le, the occurrence of eddy currents in these structures usually decreases the inductance of the coil and increases losses at higher frequencies [5].

The nature of the voice coil impedance is dependent upon the geometry and physical properties of the magnet and surrounding assemblies. There are three prominent methods of deducing the blocked coil impedance for a particular loudspeaker.

• The blocked impedance may be measured by clamping the voice coil stationary within the magnet assembly to remove the motional part of the impedance. Blocked impedance may then be measured as with conventional impedance measurements [5].
• Various impedance models have been developed in order to describe the electrical behaviour of the coil; these models may be fitted to conventional unblocked impedance measurements [3][4].
• Recently stationary transient FEM has been successfully applied to deduce the blocked impedance [6].

At large amplitudes the impedance also varies significantly with voice coil displacement. Whereas the frequency dependence of the impedance is a linear phenomenon affecting the amplitude response, but not generating distortion, variation of impedance with displacement generates significant harmonic and intermodulation distortion.

The nature of the impedance-generated distortion is dependent upon the coil impedance, or more precisely how the coil impedance varies with frequency & displacement of the coil. The effects of impedance variation with current and magnetic hysteresis are not considered in this paper.

This paper investigates the voice coil impedance as a function of frequency and displacement. Measurements of frequency & displacement dependent impedance have been made using a quasi-static method using the LPM (linear parameter measurement) module of the

Klippel Analyzer [7]. Frequency & displacement dependent impedance has also been predicted using static transient FEM analysis. A new impedance model with displacement dependent parameters is discussed and fitted to the measured data. Finally, distortion induced by the variation of impedance with coil displacement is predicted using a lumped parameter method, these predictions are compared to measurements of the actual distortion. The possibility of using a dynamic transient FE method to predict distortion with a moving voice coil is demonstrated. The nature of this distortion is discussed.

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