Electret microphone

Details Electret microphone Electret microphone

2002-10-07

2004-06-01

Tran, Sinh (Department: 2643)

Electrical audio signal processing systems and devices

Electro-acoustic audio transducer

Having electrostatic element

C381S174000

Reexamination Certificate

active

06744896

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a condenser microphone, and more particularly to an electret microphone used for a portable telephone, video camera and others.
FIG. 4
is a sectional view showing a first conventional electret microphone. The electret microphone comprises a substrate
410
mounted on a bottom of a case
500
, an annular frame
400
, a back electrode
330
mounted on the frame
400
, and a vibration film unit
360
.
The back electrode
330
comprises a back electrode
310
made of a metallic plate, and an electret layer
320
formed on the back electrode
310
. The vibration film unit
360
comprises a vibrating film
340
made of conductive and mounted on the back electrode
330
interposing a spacer
370
, and a holding frame
350
. The vibrating film
340
and the back electrode
310
compose a condenser.
The vibrating film
340
is vibrated by air entering passing through an opening
260
a
of the case
500
. The capacitance of the condenser changes with the vibration of the vibrating film
340
to generate an electric signal. The electric signal is transmitted to an integrated circuit
420
on the substrate
410
to produce an output signal through output electrodes
430
and
440
.
FIG. 5
is a sectional view showing a second conventional microphone which is disclosed in Japanese Patent Application Laid Open 2000-50393.
A case part
200
comprises a substrate
210
made of insulation material, a first frame
220
, second frame
230
, third frame
240
, fourth frame
250
, and a cover
260
, which frames and cover are stacked on the substrate
210
and adhered to each other. The first, second and third frames
220
,
230
and
240
are made of ceramic, and the fourth frame
250
is made of metal.
As shown in
FIG. 6
, each of the frames
220
,
230
,
240
and
250
has a square shape. On the substrate
210
and first to third frames
220
-
240
, connecting electrodes
210
b
,
220
b
,
230
b
and
240
b
are provided by conductive films, and these electrodes are contacted with each other. Outside sizes of the frames are same, but inside size of the third frame
240
is larger than that of the first and second frames
220
and
230
, and the inside size of the fourth frame
250
is larger than that of the third frame
240
. Thus, a first shoulder
230
a
and a second shoulder
240
a
are formed on the second frame
230
and on the third frame
240
.
Referring to
FIG. 5
, a microphone part
100
comprises a back electrode
110
made of metal and secured to the first shoulder
230
a
, an electret layer
120
formed on the back electrode
110
, a diaphragm electrode
140
mounted on a second shoulder
240
a
of the third frame
240
interposing a lower spacer
150
, and an upper spacer
160
between the diaphragm electrode
140
and the cover
260
. There is formed vents
111
perforating the back electrode and electret layer
120
.
The diaphragm electrode
140
and the back electrode
110
compose a condenser. The diaphragm electrode
140
is vibrated by air entering passing through a sound collecting hole
260
a
of the cover
260
. The capacitance of the condenser changes with the vibration of the diaphragm electrode
140
to generate an electric signal. The electric signal is transmitted to an integrated circuit
170
on the substrate
210
through connecting electrodes
210
b
,
220
b
and
240
b.
In the conventional microphone shown in
FIG. 4
, since the diameter of the back electrode
310
is equal to the diameter of the vibrating film
340
, the capacitance of the capacitor formed by the back electrode and the vibrating film becomes composition of the effective capacitance by the effective area
340
a
of the vibrating film
340
and effective area of the electrode
310
and the stray capacitance by the non-operative portion
340
b
of the vibrating film
340
.
Consequently, the sensitivity of the microphone decreases with the increase of the stray capacitance.
In the conventional microphone shown in
FIGS. 5 and 6
, the entire surface of the back electrode
110
is the effective area and corresponds to the effective area of the diaphragm electrode
140
. Therefore, there is no stray capacitance, so that the microphone can be held at high sensitivity.
However, the back electrode
110
is positioned by the second frame
230
, and the diaphragm electrode
140
is positioned by the third frame
240
. Consequently, it is difficult to hold the accuracy of the distance between the back electrode
110
and the diaphragm electrode
140
, since both the members are supported on separate members.
In addition, the vents
111
formed in the back electrode
120
prevent the capacitance to change in proportion to the amplitude of the diaphragm electrode
140
. This causes the acoustic characteristics to reduce.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an electret microphone in which a diaphragm electrode film is effectively vibrated, there by improving acoustic characteristics.
There is an electret microphone comprising a substrate having a circuit, a back plate having a recess in an underside thereof and a stationary back electrode at a central portion thereof, and secured to the substrate, an electret layer formed on the stationary back electrode, a spacer having an opening and mounted on the back plate so as to form a vacancy between an inside wall of the opening and a periphery of the stationary back electrode, a diaphragm electrode provided on the spacer, a frame mounted on the diaphragm electrode, the back plate having vents in the vacancy so as to communicate a space between the back electrode and the diaphragm electrode to a space in the recess of the back plate.
The substrate, the back plate and the frame are made of same material.
Each of the substrate, back plate, spacer and frame has a square shape in plan view, the stationary back electrode has a circular shape, four vents are provided between the four corners of the back plate and the periphery of the stationary back electrode.
In an aspect of the invention the vents are provided on a border area of the back electrode.
These and other objects and features of the present invention will become more apparent from the following detailed description with reference to the accompanying drawings.


REFERENCES:
patent: 2002/0172385 (2002-11-01), Tanabe et al.
patent: 1259094 (2002-11-01), None

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