Electrodynamic transducer and use thereof in loudspeakers and geophones

Electrodynamic transducer, comprising:

• a yoke with at least one electrical coil (2) placed in a static magnetic field can move about a rest position in an excursion range of a vertical free space,
• the coil(s) being wound and fixed on a segment of circular or elliptical cross-sectioned vertical straight cylinder forming a mandrel (12),
• a return mean enabling the mandrel bearing the coil(s) to be returned to the rest position in the absence of an external bias,
• the straight cylinder defining an internal volume toward the inside of said cylinder defining an external volume toward the outside of said cylinder, wherein,
• the magnetic field is produced by an external magnetic construction comprising at least one ring-shaped fixed permanent magnet arranged in the external volume as well as an internal magnetic construction comprising at least one ring-shaped or pellet-shaped fixed permanent magnet arranged in the internal volume,
• the external and internal magnetic constructions are substantially in a face to face relation on either side of the vertical free space,
• a motor comprising no ferromagnetic or magnetic part extending between the external volume and the internal volume, the yoke, at least in the part thereof holds the magnets in a fixed position, said motor being made of a non-ferromagnetic and nonmagnetic material, and
• a ratio R of the inductance value Lp of the coil at the rest position and blocked in the transducer to the inductance value L1 of the same coil when free and isolated in the space, namely R=Lp/L1, has a value comprised between 0.5 and 2 in the useful frequency band of the transducer.