Speaker device with a magnetic gap filled with magnetic fluid and changing magnetic flux density in axial and circumferential direction

A speaker device comprising:

• a magnet formed in a ring shape;
• a yoke having a center pole portion inserted in a center of the magnet;
• a plate formed in a ring shape and arranged on an outer circumferential surface of the center pole portion of the yoke while being attached to the magnet;
• a coil bobbin formed in a cylindrical shape and movable in an axial direction of the center pole portion while being partially fitted on the center pole portion of the yoke;
• a voice coil wrapped around an outer circumferential surface of the coil bobbin, at least part of the voice coil being arranged in a magnetic gap formed between the plate and the center pole portion of the yoke;
• a diaphragm having its inner circumferential portion connected to the coil bobbin, the diaphragm being vibrated as the coil bobbin moves;
• and
• a magnetic fluid filled in the magnetic gap, wherein
• a magnetic gradient is formed that is adapted to change a magnetic force acting on the magnetic fluid by changing a magnetic flux density in a circumferential direction of the center pole portion and changing the magnetic flux density in the axial direction of the center pole portion, and
• the lowest magnetic flux density in the circumferential direction is greater than half the highest magnetic flux density in the axial direction.