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Scanning the Magnetic Field of Electro-Dynamic Transducers

The magnetic flux density in the magnetic gap and the geometry of the moving coil determine the force factor Bl which is an important parameter of the electro-dynamical transducer. The magnitude of the. The paper presents a new measurement technique for scanning the flux density B(z, ö) on a cylindrical surface using a Hall sensor and robotics changing the position of the sensor versus vertical position z and angle ö. The results derived from the scanning process reveal the real B field in the gap considering the fringe field and irregularities in the magnetization which may initiate a rocking mode and rubbing of the voice coil at higher amplitudes. Using the geometry of the coil the static force factor Bl(x, i=0) can be calculated as a function of voice coil displacement x and compared with the dynamic force factor B(x,i) measured by using a voice coil current i. Discrepancies between dynamic and static force factor characteristics are discussed and conclusions for loudspeaker design and manufacturing are derived.

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