5.2 Improvement in efficiency for higher frequencies

Measurements as well as simulation results confirm that eddy currents induced in the conductive parts of the moving coil driver cause a reduction of the coil inductance, which influences the electrical input impedance at high frequencies (see Figure 7). Generally, an electrodynamic loudspeaker will be driven from an amplifier representing a voltage source (amplifier with low internal impedance at the output terminals), so that a reduced electrical input impedance will produce higher Lorentz forces and, therefore, higher sound pressure levels in the ambient air.

This effect was modeled by setting up an appropriate finite element model, as shown in Figure (8-b). An additional copper-cylinder was placed in the magnet gap between the aluminum former and the pole. Figure 10 shows the influence on the frequency response of the electrical input impedance. Due to the additional induced eddy currents the impedance magnitude can be halved at about 20 kHz, which results in an increase of the sound pressure level. Therefore, this effect can be used to improve the efficiency of Mid- and High-Frequency-Loudspeakers.

On the other hand, simulation results reveal that eddy currents induced in the former have to be prevented. Because of Lenz's law, the resulting Lorentz force acting on the former tends to reduce the excursion of the moving parts and, therewith, the sound pressure level, too. The loudspeaker manufacturers take this aspect into consideration by using axially slotted aluminum formers.

  

Abbildung 10: Electrical input impedance with the proposed metallic cylinder