A new linear voice-coil motor for ironless loudspeakers: Analytical study

This paper presents a new linear voice-coil structure which is dedicated to ironless loudspeaker applications: the motor is made of ring permanent magnets with unconventional magnetization directions. In such motors, the magnetic flux density must be uniform in the air gap along the axial direction. This characteristic is difficult to reach with only one or two ring permanent magnets with radial or axial polarizations. Thus, this paper shows how to combine several ring permanent magnets of various polarizations so as to obtain the intended flux density uniformity. Moreover, the calculations are carried out thanks to exact analytical expressions of the magnetic field created by arc-shaped and ring permanent magnets, which allow easy optimizations.