PURPOSE: To obtain plural kinds of frequency characteristics by providing a coil within the magnetic circuit of a center pole or a plate.

CONSTITUTION: A magnetic circuit is composed of a magnet 12, a plate 2, a center pole 1 and a yoke 11. At the tip part of this center pole 1, a coil 4 is wound and fixed within the magnetic circuit between this center pole 1 and the plate 2, and each of the winding start and the winding end of this coil 4 is connected with the coil terminal 14 provided on the lower surface of this yoke 11 through the hole formed in the yoke 11. Thus, four kinds of frequency characteristics can be obtained by providing the coil 4 within the magnetic circuit between the center pole 1 and the plate 2 and performing the non- connection of this coil 4, the short of this coil 4, the positive phase connection of this coil 4 and a voice coil 3 and the opposite phase connection of this coil 4 and the voice coil 3 as the connection states of this coil 4.