An accurate means for measuring the magnetic flux density of moving coil loudspeakers is critically important. The paper presents a new measurement technique by scanning flux density on a cylindrical surface within and outside the magnetic gap using a Hall sensor combined with robotics to move the sensor. Scanning the magnet field on the circumference of the gap seems to be the only way for detecting axial asymmetries and other magnetic field irregularities caused by material inhomogeneity and manufacturing failures, such as partial magnetization and incorrect adjustment of the parts. Variation of magnetic field in the gap reveals an asymmetrical driving force that causes rocking modes and voice coil rubbing.
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https://aes2.org/publications/elibrary-page/?id=16359
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