FIG. 3 is a sectional view showing a main part of an example of a conventional dynamic microphone. The basic configuration of the microphone comprises a vibration part 10 vibrating with sound waves and a magnetism generating circuit 20. The vibration part 10 comprises a diaphragm 11 and a voice coil (generator coil) 12. In this case, the diaphragm 11 has a center dome 11a and a sub dome 11b provided around the center dome 11a. The voice coil 12 is joined to a boundary between the center dome 11a and the sub dome 11b with an adhesive.
The magnetism generating circuit 20 comprises a permanent magnet 21, a pole piece 22 provided on one pole of the permanent magnet 21, and a ring yoke 24 magnetically connected to the other pole of the permanent magnet 21 via a cup-like yoke 23. The voice coil 12 is supported in a magnetic gap formed between the pole piece 22 and the ring yoke 24 so as to vibrate through the diaphragm 11. A lead 12a of the voice coil 12 is wired along the inner surface of the sub dome 11b and routed to a signal output circuit (not shown).
Incidentally, for picking up sound of musical instruments, dynamic microphones enabling preferred sound quality are used frequently. Particularly when a bass drum is used as a musical instrument, a dynamic microphone is placed in the bass drum to pick up the sound.
Dynamic microphones for base drums pick up heavy bass, and thus in general designs, diaphragms have low stiffness (soft diaphragms) and voice coils are heavy. The bass resonance frequency of dynamic microphones for bass drums is set low, and thus an amplitude is extremely large around a resonance frequency in actual sound pickup. For example, the amplitude sometimes reaches about ±0.5 mm.
When a base drum is strongly hit, an internal pressure rapidly increases, an air current caused by the increased pressure instantly flows from an opening of the drum to the outside, and then the air current strongly presses the diaphragm 11 to the magnetism generating circuit 20 as shown in FIG. 4. Since an amplitude is extremely large at this point, the lead 12a of the voice coil 12 may strongly strike a corner of the ring yoke 24 and break thereon. The lead 12a may be broken also by a drop impact and the like.
As a method for preventing this problem, patent document 1 (Japanese Patent Application Publication No. 2003-1191) discloses a method of elastically supporting a magnetism generating circuit through a suspension.
However, the technique described in patent document 1 requires the suspension and a structure becomes complicated accordingly, resulting in high cost. As shown in FIG. 3, the magnetism generating circuit 20 has an extremely narrow gap, and thus when the magnetism generating circuit is elastically supported through a suspension as in the technique disclosed in patent document 1, the voice coil 12 of FIG. 3 may come into contact with the magnetism generating circuit 20.