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A Model of Loudspeaker Driver Impedance Incorporating Eddy Currents in the Pole Structure

Measurements of electrodynamic drivers show a blocked-cone impedance which is often dominated by a -jw component. A simple theory shows that eddy currents in the iron pole structure are responsible for this behavior. A more complex theory shows a transition between such semi-inductive behavior at high frequencies to a normal inductive behavior at low frequencies, but a vast reduction of the effective permeability of the iron must be invoked to explain the measured data. This reduction is due to the effective demagnetizing gaps in the pole structure. Other aspects of the eddy currents concern the asymmetric drag force on a slotted aluminum voice coil former, the direct thermal heating of the pole structure by the eddy currents, the electrical compensation of the -jw semi-inductive impedance, and the current waveforms expected when a step or impulse voltage is applied to the driver.

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