Dynamic properties of moving magnet transducers

The equation of motion, the force, and the equivalent circuit of a moving magnet transducer are derived. The system is shown to be a low pass device and the high-frequency cutoff depends only on the properties of the permanent magnet material and the specified maximum displacement. Experiments with two moving magnet designs are described. Problems due to large constant forces are discussed and the importance of adequate centering and spring design is stressed. The development of high coercivity materials is shown to make moving magnet devices more competitive compared to moving coil transducers. The advantages of the former are high efficiency and good low-frequency response. Their use is, however, limited by the existence of the high-frequency cutoff and the more involved mechanical construction required.