Acoustic sensor
A vibrating electrode plate that senses a sound pressure faces a counter electrode plate to constitute a capacitance type acoustic sensor. In the counter electrode plate, acoustic perforations are opened in order to pass vibration, and plural projections are provided on a surface facing the vibrating electrode plate. An interval between the projections is decreased in a region where the vibrating electrode plate has high flexibility to easily generate local sticking with the counter electrode plate. The interval between the projections is increased in a region where the vibrating electrode plate has low flexibility to hardly generate local sticking with the counter electrode plate. The projections thus arranged prevent firm fixing of the vibrating electrode plate to the counter electrode plate and interruption of vibration of the vibrating electrode plate.
1. An acoustic sensor comprising:
a substrate;
a vibrating electrode plate that is fixed to the substrate to sense a sound pressure; and
a counter electrode plate that is fixed to the substrate to face the vibrating electrode plate with an air gap interposed therebetween,
wherein a plurality of projections are provided on a surface on the air gap side of one of the vibrating electrode plate and the counter electrode plate, and
wherein, in a perpendicular section in a diagonal direction of one of the vibrating electrode plate and the counter electrode plate, each projection comprises a leading end surface and two side surfaces,
wherein the two side surfaces are perpendicular to the leading end surface and are connected to a same one of the vibrating electrode plate and the counter electrode plate,
wherein an interval between the adjacent projections is changed according to a projection forming region in one of the vibrating electrode plate and the counter electrode plate, and
wherein, in one of the vibrating electrode plate and the counter electrode plate on which the projections are provided, the interval between the adjacent projections in a high-flexibility region of the vibrating electrode plate or a counter region of the counter electrode plate facing the high-flexibility region is smaller than the interval between the adjacent projections in a low-flexibility region of the vibrating electrode plate or a counter region of the counter electrode plate facing the low-flexibility region.
2. The acoustic sensor according to claim 1 ,
wherein the vibrating electrode plate is fixed to the substrate along an outer circumferential edge of a movable portion of the vibrating electrode plate, and
wherein the interval between the adjacent projections in a central portion of the movable portion or a region facing the central portion on the counter electrode plate is smaller than the interval between the adjacent projections in an outer circumferential portion of the movable portion or a region facing the outer circumferential portion on the counter electrode plate.
3. The acoustic sensor according to claim 2 ,
wherein the movable portion of the vibrating electrode plate is formed into a circular disc shape, and
wherein the interval between the adjacent projections in a region having a radius from R/8 to R/2 (R being a radius of the movable portion) around a center of the vibrating electrode plate or a region having a radius from R/8 to R/2 around a position facing the center on the counter electrode plate is smaller than the interval between the adjacent projections in a region located outside the region.
4. The acoustic sensor according to claim 1 ,
wherein an outer circumferential portion of the movable portion of the vibrating electrode plate is partially fixed to the substrate at a plurality of points, and
wherein the interval between the adjacent projections in a region located between the fixed points on the vibrating electrode plate or a region located between points on the counter electrode plate facing the fixed points is smaller than the interval between the adjacent projections in the remaining projection forming region.
5. The acoustic sensor according to claim 1 , wherein the projections are arranged along a plurality of concentric circles or a plurality of polygons.
6. The acoustic sensor according to claim 1 ,
wherein the counter electrode plate includes a plurality of acoustic perforations in order to pass the sound pressure, and
wherein each of the projections is disposed in a central portion of a region surrounded by the acoustic perforations.
7. The acoustic sensor according to claim 1 ,
wherein the counter electrode plate includes a plurality of acoustic perforations in order to pass the sound pressure, and
wherein each of the projections is disposed out of a center in a region surrounded by the acoustic perforations.
8. The acoustic sensor according to claim 1 ,
wherein the counter electrode plate includes a plurality of acoustic perforations in order to pass the sound pressure, and
wherein each of the projections is disposed in a position in contact with the acoustic perforation.