ELECTRET CAPACITOR MICROPHONE

MICROPHONE ELECTRET A CAPACITANCE

English Abstract

An electret capacitor microphone (ECM) can be
assembled by an automatic assembly machine, and has an
excellent thermal resistance against the influence of
heat during the assembly. The ECM includes a cylindrical
metal gate ring which is to be subjected to a caulking
force, instead of an insulator made of a resin having a
lower thermal resistance. The ECM has an amplifier block
consisting of a first assembly sub-block having a
back-plate and the metal gate ring, and a second assembly
sub-block having an FET and a printed circuit board so as to
be assembled by an automatic assembly machine.

French Abstract

Cette invention concerne un microphone électret capacitif pouvant être assemblé par une machine automatique et présentant une excellente résistance à l'échauffement en cours d'assemblage. L'objet de l'invention comprend une rondelle métallique qui doit être soumise à une force de matage au lieu d'un isolateur en résine à résistance thermique plus faible. Il comporte un bloc amplificateur formé d'un sous-bloc de premier assemblage à plaque arrière et rondelle métallique et d'un sous-bloc de second assemblage à transistor à effet de champ et plaquette de circuit imprimé, qui se prêtent à l'assemblage par une machine automatique.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An electret capacitor microphone comprising:
a bottomed cylindrical metal casing having a
sound hole formed in a bottom thereof;
a diaphragm having a metallized foil on a
surface thereof, said metallized foil facing an inner
surface of said bottom with diaphragm clamping ring
between said diaphragm and said bottom;
an assembly block including a backplate having
a hole formed therein, a cylindrical metal gate ring
electrically connected with said backplate, and an
insulator surrounding said metal gate ring; and
a printed circuit board on which an FET is
mounted.
2. An electret capacitor microphone according to
Claim 1, wherein said FET has an input lead, and said
metal gate ring has a recess formed an end thereof, and
wherein said input lead is interposed between said recess
and said printed circuit board.
3. An electret capacitor microphone according to
Claim 1, said metal gate ring is a metal gate ring shaped
by rounding a single stamped flat sheet.
4. An electret capacitor microphone comprising: a
bottomed cylindrical metal casing having a sound hole
formed in a bottom thereof; a diaphragm clamping metal
ring; a diaphragm having a metallized foil on a surface
thereof; a gap spacer; an assembly block including a
backplate having a hole formed therein, a conductive

metal gate ring, and an insulator surrounding said metal
gate ring; and a printed circuit board on which an FET is
mounted, wherein said metal ring r said diaphragm, said
gap spacer, said assembly block and said printed circuit
board are successively inserted into said metal casing,
and wherein said electret capacitor microphone further
comprises a portion at the opening end of said metal
casing caulked after said insertion.
5. An electret capacitor microphone according to
Claim 4, wherein said FET has an input lead, and said
metal gate ring has a recess formed an end thereof, and
wherein said input lead is interposed between said recess
and said printed circuit board.
6. An electret capacitor microphone according to
Claim 4, said metal gate ring is a metal gate ring shaped
by rounding a single stamped flat sheet.
7. A method for assembling an electret capacitor
microphone, said method comprising the following steps
of:
inserting a diaphragm clamping ring into a
bottomed cylindrical metal casing having a sound hole
formed in a bottom thereof;
disposing a diaphragm having a metallized foil
thereon onto said diaphragm clamping ring;
inserting an assembly block and a gap spacer
into said metal casing so that said gap spacer is
interposed between said diaphragm and said assembly
block, said assembly block including a backplate having a

hole formed therein, a conductive metal gate ring
electrically connected with said backplate, and an
insulator surrounding said gate ring;
disposing a printed circuit board on which an
FET is mounted onto said assembly block; and
caulking an opening end portion of said metal
casing so as to integrate into said electret capacitor
microphone.

Description

2060~2
1 FIELD OF THE 1NV~-~ 11ON AND REL~TED ART STA~
The present invention relates to an electret
capacitor microphone (hereinafter referred to as an
~'ECM~) and, particularly, to an ECM capable of being
assembled by an automatic assembly machine.
A conventional ECM has a bottomed cylindrical
metal casing, and a bottomed cylindrical insulator
telescoped into the metal casing. A diaphragm (electret
member), having a metallized foil on one surface thereof,
and a backplate are disposed between a bottom,of the
metal casing and an opening end of the insulator with a
predete_ ined gap between the diaphragm and the
backplate. A printed circuit board is mounted on an
exposed bottom surface of the cylindrical insulator. An
field effect transistor (FET) for impedance conversion is
disposed in a space within the cylindrical insulator.
The FET has an input lead and an output lead, which are
respectively connected to the backplate and an electrode
on the printed circuit board.
Sound waves from the outside of the ECM cause
the diaphragm to vibrate, thereby causing changes in the
capacitance between the diaphragm and the backplate. The
changes in the capacitance are converted into electrical
changes by the FET.
When assembling the ECM, the backplate, the
. . .
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20~542
1 insulator, the FET and the printed circuit board are pre-
assembled together into an amplifier block. The
diaphragm is disposed inside th0 metal casing. Subse-
quently, the amplifier block is inserted into the metal
casing, and a portion at the opening end of the metal
casing is caulked, thereby completing the assembly.
The construction of the conventional ECM is
complicated throughout the apparatus, in particular, in
the amplifier block. In addition, the amplifier block
requires to be pre-assembled. Thus, the conventional ECM
has a problem in that the apparatus is not suitable for
assembly by an automatic assembly machine.
The conventional ECM also has the following
drawback. The various component parts of the ECM are
fixed in place by caulking a portion of the cylindrical
metal casing, and the component parts include an
insulator which is generally molded from a resin. The
use of a resin product is disadvantageous in terms of
thermal resistance. For instance, where the output lead
of the FET is connected to another printed circuit board
by a solder dip or the like, the resin insulator may be
collapsed or melted due to heat generated during the
solder dipping. This involves the risk of the caulked
portion becoming loose, resulting in the product quality
being deteriorated.
SU~RY OF THE lNV~:NLlON
The present invention has an object to
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20go~
1 el; in~te the above-described disadvantages. The present
invention is particularly directed to providing an ECM
which can be assembled by an automatic assembly machine.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a vertical sectional view
schematically showing an ECM according to a first
embodiment of the present;
Fig. 2 is a perspective view of a metal gate
ring of the ECM shown in Fig. 1;
Fig. 3 is a view for illustrating an example of
the forming of the metal gate ring shown in Fig. 2;
Fig. 4 is a view schematically showing the
procedures for assembling the ECM shown in Fig. 1;
Fig. 5 is a vertical sectional view
schematically showing an ECM according to a second
embo~ t of the present invention;
Fig. 6 is a perspective view of a metal gate
ring of the ECM shown in Fig. S;
Fig. 7 is a vertical sectional view
schematically showing an ECM according to a third
embodiment of the present invention;
Fig. 8 is a vertical sectional view
schematically showing a conventional ECM; and
Fig. 9 is a view schematically showing the
procedures for assembling the conventional ECM.
,~ '

20~0~
1 DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Fig. 1, an ECM includes a bottomed
cylindrical metal casing 1 having a sound hole la formed
in a bottom thereof, a diaphragm clamping metal ring 2
disposed on an inner side of the bottom of the metal
casing 1, a diaphragm 3 (serving as an electret member)
having a metallized foil 3a on one surface thereof
attached to the metal ring 2 by an adhesive or other
suita~le means, a gap spacer 4 and a backplate 5 having a
through-hole Sa located on the other surface of the
diaphragm 3 through the gap spacer 4. An air layer 10 is
defined between the backplate S and the diaphragm 3.
The ECM further includes a cylindrical metal
gate ring 12 supporting the backplate 5. The metal gate
ring 12 is electrically connected with the backplate 5 by
contacting or welding to it. As shown in Fig. 2, a
plurality of recesses 12a are formed at an end portion of
the metal gate ring 12. The metal gate ring 12 can be
easily formed by, for example, rounding stamped flat
sheet, as shown in Fig. 3. An insulator 11 is disposed
to insulate the backplate 5 and the metal gate ring 12
from the surroundings as well as to house therein these
members so as to place them in appropriate positions.
The insulator 11 is held in place by, for example, press-
fitting it onto the backplate 5 or the metal gate ring12.
An FET 7 for impedance conversion is directly
mounted on a printed circuit board 8 and positioned

206~
1 within the metal gate ring 12. An input lead 7a of the
FET 7 is interposed between the recess 12a of the metal
gate ring 12 and a surface of the printed circuit board 8
so as to allow electrical conduction through these
members, and hence to be electrically connected with the
backplate 5. An output lead 7b of the FET 7 is electri-
cally connected with an electrode on the printed circuit
board 8 by a solder 8a provided by dipping. The
component parts of the ECM, described above, are
integrally fixed in place by caulking an opening end
portion of the cylindrical metal casing 1, as indicated
by reference numeral lb. A cloth 9 covers the sound hole
la to prevent penetration of dust. Without the cloth 9,
however, there would be no particular hindrance to the
operation of the ECM.
Descriptions will be given of the assembly
operation of the ECM. First, as shown in Fig. 4, an
amplifier block is prepared in advance, which consists of
a first and a second assembly sub-blocks 21 and 22. The
first sub-block 21 comprises the backplate 5, the metal
gate ring 12 and the insulator 11. The second assembly
sub-block 20 comprises the FET 7 and the printed circuit
board 8. Into the cylindrical metal casing 1 with the
cloth 9 attached thereto, the metal ring 2 to which the
diaphragm 3 is partially fixed, the gap spacer 4, the
first assembly sub-block 21, and the second assembly sub-
block 20 are successively inserted. Thereafter, an end
portion lb of the cylindrical metal casing 1 is caulked,

2~605~2
1 thereby completing the assembly.
The air layer 10 has a capacitance of about
several pF to lOs pF. Sound waves from the outside of
the ECM pass through the sound hole la, and cause the
diaphragm 3 to vibrate, thereby causing slight changes in
the above capacitance. The changes in the capacitance
are transformed by means of the FET 7 to changes in an
electrical output.
This embodiment has an excellent advantage in
that the entire apparatus can be assembled by an
automatic assembly machine, the advantage being helped by
the very simple constructions of the individual assembly
sub-blocks 20 and 21. Another advantage is that, even
when the application of heat during solder dipping or the
like has caused the insulator 11 to partially melt, this
does not cause any adverse influence on the settling
portion or the caulked end portion lb, hence, no adverse
influence on the settling of the components of the
apparatus, thereby making it possible to avoid deteriora-
tion in the product quality, such as loose settling ofthe components.
In contrast, the conventional ECM (shown in
Fig. 8) is assembled in the manner shown in Fig. 9. A
backplate 5, an insulator 6, an FET 7 and a printed
circuit board 8 are previously assembled together into an
amplifier block. Thereafter, a metal ring 2 to which a
diaphragm 3 is attached, a gap spacer 4, and the
amplifier block are successively inserted into a

'' 20~0~2
1 cylindrical metal casing 1 with a cloth 9 attached
thereto. Subsequently, an end portion lb of the cylin-
drical metal casing 1 is caulked, thereby assembling the
components into an integral construction.
In the conventional ECM, the amplifier block is
complicated, and further required to be pre-assembled.
$hus, the construction of the conventional ECM is not
suitable for performing a fully automatized assembly
operation.
In addition, since the components are settled
inside the cylindrical metal casing 1 by the caulked
portion lb in cooperation with the interposed insulator
6, heat caused during solder dipping or the like may
cause the insulator 6 to be collapsed or melted. This
involves the risk of the caulked portion lb becoming
loose, resulting in a deterioration in the product
quality.
Referring to Fig. 5, in a second embodiment of
the present invention, a metal gate ring 13 is, as shown
in Fig. 6, an annular metal gate ring having no recess.
The second embodiment further includes a dual-lead type
FET 14 and a printed circuit board 15 having patterns on
both surfaces thereof.
In the above construction of the second
embodiment, an input lead 14a of the FET 14 is connected
with an inner printed circuit pattern 15c by soldering or
the like, and connected with the metal gate ring 13 via
the inner printed circuit pattern 15c. An output lead
. . : :
' ' , '

2 ~
1 14b of the FET 14 is connected with another inner printed
circuit pattern 15d by soldering, and connected with an
outer printed circuit pattern 15e via the inner printed
circuit pattern 15d, a through hole 15b and a soldered
portion 15a. The soldered portion 15a also serves to
close the through hole 15b. The FET 14 is fixed to the
printed circuit board 15 by an adhesive 16. The adhesive
16 may be also used to close the through hole 15b,
instead of the soldered portion 15a.
With the apparatus according to the second
embodiment, assembly can be performed in a manner similar
to that in the first embodiment, and operation and
advantages similar to those of the first embodiment are
provided.
Referring to Fig. 7, a third embodiment of the
present invention is distinguished in that a metal pin
lead 17 with a flange passes through a through hole 15b
of a printed circuit board 15. The metal pin lead 17 is
connected with an outer printed circuit pattern 15e via a
soldered portion lSa. Thus, the third embodiment
constitutes an ECM known as a legged type ECM.
The apparatus according to the third embodiment
also enables assembly similar to those in the first and
the second embodiments, and provides similar operation
and advantages.

Representative Drawing