Electrical Equivalent Circuit Model for Dynamic Moving-Coil Transducers Incorporating a Semi Inductor

In the J. Audio Eng. Soc. (Vol. 56 no.9. 2008 September, p.696 to 709), is a paper by Knud Thorborg and Andrew D. Unruh “Electrical Equivalent Circuit Model for Dynamic Moving-Coil Transducers Incorporating a Semi Inductor”. At the presentation in Vienna 122nd Convention of the Audio Engineering Society, May 2007 was shown animation in colours visualizing the eddy currents in the iron parts of a loudspeaker motor caused by the voice-coil current (30 Hz, 1 Amp).

In the Journal it was only possible in a black and white (Fig. 4 p.700) to show a momentary snapshot of this animation. In a footnote it was promised that the full animation would be shown on the Tymphany home page. So here it is.

What we see is the right half of a loudspeaker motor cut through, showing simulated eddy current densities in the iron parts as a result of 1 A at 30 Hz excitation current in the voice-coil. (The simulation is the result of a finite elements analysis (using the software MagNet from Infolytica Corporation).

The animation shows us how the eddy current density spreads from the surface into the material as “rings in the water” (following Maxwell’s equations). As the current changes direction it can be observed how a blue line of zero current is moving into the iron and disappearing. Inside this line the current has not yet changed direction, and “skin depth” has to be understood as a value representing this picture with opposite currents inside and outside the zero line.

What was shown at the Vienna conference was a simulation with the voice-coil blocked at the rest position. In the following we show a simulation including voice-coil movement and this time is included an aluminium short-circuiting ring and an aluminium pole extension (see Fig. 11 p. 704). The excitation is 50V at 50Hz and based upon actual equivalent mass MMS of the moving system and resulting compliance of the suspension CMS and damping factor RMS. This time blue and red indicate current in opposite directions and zero current is green.