| 摘要 |
PURPOSE:To improve frequency characteristics by forming an annular projection part at the part of a case which contacts a diaphragm holding ring oppositely and pressing the holding ring, and improving the airtightness between the case and the holding ring. CONSTITUTION:A fixed electrode 22 which uses electret fixed to a supporting member 21 is provided in the case 20 which is made of conductive material such as aluminum and has a bottom part 19. A diaphragm 33 which oscillates freely is spaced oppositely to the fixed electrode and the holding ring 24 is interposed at a part facing the bottom part 19 between the circumferential edge part of the diaphragm 23 and case 20. The annular projection part 30 is provided to the bottom part 19 facing the holding ring 24 oppositely to press the holding ring 24 against the projection. Consequently, the projection part absorbs the variance of the parts enclosed in the case regardless the variance, and the airtightness between the case and holding ring is improved to obtain superior frequency characteristics. |
