Reducing Harmonic Distortion in Loudspeakers
Investigations were carried out on the mechanism by which sound distortion in loudspeakers is caused by nonlinear magnetic characteristics in the speaker magnetic circuit, and concrete measures to reduce this distortion. 2nd and 3rd harmonics, not in the original signal, may be produced by either the non-uniformity of the magnetic field in the air gap that the voice coil moves, or nonlinear magnetic characteristics of magnetic materials magnetized by voice-coil current. The article clarifies analytically the reason why the distortion due to the latter cause is brought about. The magnetic material in the vicinity of the voice coil is magnetized in both the AC signal current and the permanent magnet. A minor hysteresis loop induced by this magnetization is the cause of the sound distortion. The 2nd harmonic distortion arises from the unsymmetry of the minor loop, while the 3rd one is related to the magnitude o minor-hysteresis loss. In the speaker systems of Mitsubishi Electric, the FN rings-the iron nickel alloys which have been developed by special metallurgical means-are inserted around the voice coil. The considerable reduction of distortion has been attained by making use of the magnetic material of excellent linearity.
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