FIGS. 12A and 12B show a plan view and a cross-sectional view, respectively, of a conventional electroacoustic transducer 200 of an electromagnetic type (hereinafter referred to as an “electromagnetic transducer”). The conventional electromagnetic transducer 200 includes a cylindrical housing 107 and a disk-shaped yoke 106 disposed so as to cover the bottom face of the housing 107. A center pole 103, which forms an integral part of the yoke 106, is provided in a central portion of the yoke 106. A coil 104 is wound around the center pole 103. Spaced from the outer periphery of the coil 104 is provided an annular magnet 105, with an appropriate interspace maintained between the coil 104 and the inner periphery of the annular magnet 105 around the entire circumference thereof. The outer peripheral surface of the magnet 105 is abutted to the inner peripheral surface of the housing 107. An upper end of the housing 107 supports a first diaphragm 100 so that an appropriate interspace exists between the first diaphragm 100 and the magnet 105, the coil 104, and the center pole 103. In a central portion of the first diaphragm 100, a second diaphragm 101 which is made of a magnetic member is provided so as to be concentric with the first diaphragm 100.
Now, the operation and effects of the above-described conventional electromagnetic transducer 200 will be described. In an initial state where no current flows through the coil 104, a magnetic path is formed by the magnet 105, the second diaphragm 101, the center pole 103, and the yoke 106. As a result, the second diaphragm 101 is attracted toward the magnet 105 and the center pole 103, up to a point of equilibrium with the elastic force of the first diaphragm 100. If an alternating current flows through the coil 104 in this state, an alternating magnetic field is generated in the aforementioned magnetic path, so that a driving force is generated on the second diaphragm 101. Such a driving force generated on the second diaphragm 101 causes the second diaphragm 101 to be displaced from its initial state, along with the fixed first diaphragm 100, due to an interaction with an attraction force which is generated by the magnet 105 and the driving force. The vibration caused by such displacement transmits sound.
FIG. 13 illustrates a characteristic curve of the driving force generated on the second diaphragm 101 of the electromagnetic transducer 200. The vertical axis of the graph represents driving force, whereas the horizontal axis of the graph represents a distance from the center pole 103 to the second diaphragm 101 (i.e., a “magnetic gap value”). As seen from FIG. 13, once the magnetic gap value has reached a certain value (i.e., about 0.4 mm in this exemplary case), the driving force thereafter decreases in inverse proportion to the magnetic gap value. In other words, although there is a need to secure a large amplitude (and therefore a large magnetic gap value) for obtaining a high sound pressure level and enabling reproduction of low-frequency ranges, such a large magnetic gap value inevitably leads to a reduced driving force, which defeats the purpose of obtaining a high sound pressure level. On the other hand, in FIG. 13, the reduced driving force in the neighborhood of the center pole 103 is accounted for by the second diaphragm 101 experiencing magnetic saturation.