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 paper presents a new measurement technique for scanning the flux density B(z, φ) on a cylindrical surface within and outside the magnetic gap using a Hall sensor and robotics which moves the position of the sensor along vertical position z and angle φ. 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 can 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 to the dynamic force factor Bl(x,i>0) measured by dynamic system identification. Discrepancies between dynamic and static force factor characteristics are discussed and conclusions for loudspeaker design and manufacturing are derived.