A flat casing is formed by joining a hollow cylindrical cover to a hollow cylindrical frame, and a d.c. magnetic field generating magnet in the form of a disk is fixed to the inner surface of the cover. Disposed inside the casing is a disklike diaphragm spaced apart from the lower surface of the magnet by a gap and having its outer peripheral portion fixedly held between the cover and the frame. A hollow cylindrical drive coil coaxial with the magnet is fixed to the lower surface of the diaphragm and has an axis perpendicular to the diaphragm. Stated specifically, the drive coil has an outside diameter which is at least 80% to not greater than 116% of the outside diameter of the magnet and an inside diameter which is at least 66% to not greater than 94% of the magnet outside diameter. The transducer is diminished in power consumption, has a reduced thickness and is yet highly efficient.