Patent ID: 7149323

Claim:
A speaker comprising: a magnetic circuit provided with a magnetic gap, a through hole and a recess surrounding said through hole, said recess being formed with a diameter larger than a diameter of said through hole and being disposed radially outwardly of said through hole; a frame fixed on said magnetic circuit; a diaphragm having an outer periphery fixed to said frame; a voice coil supported in said magnetic gap; a center cap fixed on a top end of a voice coil bobbin fixed to said diaphragm; a bearing provided in said through hole; and a shaft supported by said bearing so as to be movable up and down, said shaft being fixed to said center cap; wherein said magnetic circuit comprises a yoke having a magnet mounting surface, a magnet mounted on said magnet mounting surface of said yoke, and a top plate having a magnet mounting surface mounted on said magnet; wherein said recess is formed adjacent to said through hole in said magnet mounting surface of one of said yoke and said top plate so as to extend only part way through said one of said yoke and said top plate; wherein a magnetic fluid is retained in said recess and in said through hole around said shaft; and wherein said recess is contiguous with said through hole, so as to be in fluid communication with said through hole.