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JPH07107597

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This translation is machine-generated. It cannot be guaranteed that it is intelligible, accurate,
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DESCRIPTION JPH07107597
[0001]
FIELD OF THE INVENTION The present invention relates to an electret assembly for an
electroacoustic transducer.
[0002]
The diaphragm of an electret used in a microphone vibrates in response to an acoustic signal
impinging on the diaphragm. This vibration changes the distance between the diaphragm and the
back plate. The diaphragm and the back plate consist of electrodes of the electret. One of the
electrodes is electrostatically charged and a differential voltage is generated between the
electrodes. The vibrations caused by the capacitance between the electrodes formed by the
movement of the diaphragm allow the electret to generate an electrical signal representative of
the impinging acoustic signal. In the case of a sound reproducer, this process works in reverse.
The electrical signal applied between the diaphragm and backplate electrodes of the electret
causes the diaphragm to vibrate and generate an acoustic signal.
[0003]
The size of electret transducers can be very small and, accordingly, have often been used as
hearing aids, in particular as hearing aids for the ears. The largest dimension of a hearing aid, in
particular an electret transducer for a microphone, may be on the order of 0.125 inches.
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1
Obviously the listening aid can incorporate two types of electret transducers. That is, the electret
microphone is used to convert the acoustic signal into an electrical signal, and the electrical
signal is then amplified and reconverted into an acoustic signal that is sent into the user's
auditory tube, the electret acoustic regenerator ( Often referred to as "receiver".
[0004]
However, listening aids provide a distinctly negative environment for any kind of electroacoustic
transducer. Temperature and humidity conditions vary substantially. Aging substantially affects
any transducer. In particular, in the electret, the differential voltage between the diaphragm and
the backplate electrode is stable for over a year, but decays to a low level over a period of time.
The mechanical properties of the parts of the electret, in particular the mechanical dimensions of
the diaphragm, change with time.
[0005]
Usually, the diaphragm of the electret is mechanically pre-machined, as a sensible stiffness is
desired for effective operation in a microphone or sound reproducer. If the diaphragm is too
compliant, this will lie against the other electrode. In the case of most electret assemblies, the
tension can be maintained according to the prior art, as it is necessary to firmly fix the rim or the
periphery of the diaphragm. A few conventional structures have specialized diaphragm
structures, such as those disclosed by Carlson et al. In U.S. Pat. No. 3,740,496 and U.S. Pat. As
disclosed, there is no need to prepressurize the diaphragm. However, these specialized
diaphragm structures typically have one or another corrugated or projecting portion at the rim
or center of the diaphragm.
[0006]
SUMMARY OF THE INVENTION In an electret, the electrostatically charged differential voltage
between the diaphragm and the backplate causes the diaphragm to be pulled towards the
backplate. The resulting very slight deformation of the diaphragm is intended to stiffen it, and
the effect of minimizing or even eliminating the need for mechanical tensioning of the diaphragm
can be used. However, as the differential voltage is further increased, the diaphragm will buckle,
ripple or even collapse, substantially deadly to the performance of the transducer.
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2
[0007]
Thus, the main object of the present invention is in particular a new improvement for hearing
aids that use the differential voltage between the electret electrodes to tension the diaphragm
electrodes but prevent possible buckling and waviness of the diaphragm An electret assembly for
an electroacoustic transducer is provided.
[0008]
Another object of the invention is to provide a new and improved electret assembly which at
least partially compensates for the effects of temperature and humidity fluctuations and aging
and which is relatively easy to manufacture and assemble. is there.
[0009]
Accordingly, an electret assembly for an electroacoustic transducer according to the present
invention comprises a thin, flexible flat diaphragm comprising its first electrode and its second
electrode. The first electrode is charged to a differential voltage with respect to the other
electrode.
The assembly further comprises diaphragm mounting means for separating and securing the
diaphragm generally parallel to the backplate, the differential voltage causing the central portion
of the diaphragm to pull toward the backplate. And thus, the center of the diaphragm is strained
and stiffened.
The diaphragm mounting means allows limited radial movement of the rim of the diaphragm on
the plane of the diaphragm but prevents movement perpendicular to the plane of the diaphragm.
[0010]
DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows the basic components for an
electret assembly 20 constructed in accordance with the present invention. The electret assembly
20 is thin and has a flexible flat diaphragm 21. The diaphragm 21 shown in the preferred
configuration is circular, but rectangular or other diaphragms may also be used. The diaphragm
may consist of a thin flexible plate of metal or other conductive material. The diaphragm is a
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3
synthetic film of a tough resin which is generally available, such as polyethylene terephthalate,
which is generally available under the trade name Meyer and other trade names, that is, a thin
conductive film is one side of the resin film Alternatively, it is often configured as a synthetic film
that is vacuum and / or vapor deposited on both sides. The diaphragm 21 can be permanently
charged electrostatically. In this case, it functions as the charging electrode of the electret, so it is
usually not charged that much.
[0011]
The other electrode of the electret assembly 20 is a back plate 22, which is circular as shown in
FIG. When the diaphragm 21 is a charge electrode, the back plate 22 is simply a metal plate.
However, the back plate 22 is covered with a thin insulating layer or coating on its surface facing
the diaphragm 21. With such a coating, the backplate 22 often becomes the electrostatically
charged electrode of the electret assembly 20. A variety of resins, such as fluoroplastics
commonly available under the tradename Teflon, can maintain the electrostatic charge utilized
for the long term when the backplate 22 is to be the electret charge electrode .
[0012]
In the electret assembly 20 there are two other components 23 and 24. In the illustrated
configuration, the outer diameters of the mounting members 23 and 24 correspond to the outer
diameter of the diaphragm 21. This is not a necessary condition for the operation of the electret.
Preferably, mounting rings 23 and 24 may have larger or smaller outer diameters than other
elements of the assembly. The inner diameters of the rings 23 and 24 are preferably the same.
This defines the outer boundary of the central portion 25 of the diaphragm 21. In the assembled
form of the electret 20, the components are stacked in the order of the back plate 22, the
mounting member 24, the diaphragm 21, and the mounting member 23 from the bottom to the
top as illustrated in each of FIGS.
[0013]
As noted above, there is nothing significant or novel about the electret assembly 20. This works
in the usual way of electret. Thus, due to the acoustic signal impinging on the central portion 25
of the diaphragm 21 in the microphone, the diaphragm is vibrated or moved in a direction
perpendicular to its plane. Such movement of each of the diaphragms changes the volume
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between the two electrodes of the electret, the diaphragm 21 and the back plate 22. An electrical
circuit (not shown) connected to these two electrodes generates an electrical signal
representative of the colliding acoustic signal.
[0014]
The electret 20 can also perform the reverse function. Thus, the electrical circuit can be
connected to the electrode consisting of the diaphragm 21 and the back plate 22. External
circuits are not shown. From this circuit, the diaphragm 21 vibrates back and forth by different
electric signals supplied to the electrodes to generate an acoustic signal. Thus, the electret 20
functions as a sound reproducer or a speaker. This common type of electret is commonly used in
hearing aids. It may have an electret microphone connected to an electret acoustic regenerator or
"receiver" by an amplification circuit.
[0015]
A flat plate, such as an electret diaphragm 21, or a flat disk can be mounted to span a circular
opening as shown in FIG. Furthermore, it is held and stiffened by the electrostatic attraction to
the backplate, which is based on the differential voltage, usually the permanent electrostatic
charge between the diaphragm and the backplate. This general situation is illustrated in FIG.
Here, the central portion 25 of the diaphragm 21 is shown drawn towards the backplate 22.
Deflection of the diaphragm is due to the electrostatic charge difference between the electrodes
21 and 22 of the electret. However, at higher voltages, the peripheral portion or rim 26 of the
diaphragm 21 outside the central opening tends to buckle, as may be represented by the steady
state voltage difference or the signal applied to the electret electrodes. is there. As a result, as
generally shown in FIG. 5, a wave is formed in the diaphragm. The waves of the diaphragm allow
air to pass around the rim. This is critical to the performance of the electret as a microphone or
sound reproducer. Furthermore, due to the increase in the charging difference between the
electrodes, ie, the diaphragm 21 and the back plate 22, the diaphragm eventually falls down and
comes into contact with the back plate. In these states, the electret 20 is no longer functional.
[0016]
The electret configuration 20 limits the movement of the peripheral portion of the diaphragm 21
in the direction perpendicular to its plane and allows movement of the diaphragm parallel to its
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5
plane, so that electrostatic charging between the electret electrodes 21 and 22 is achieved. It is
made to utilize the reinforcement phenomenon which arises by the difference. In the electret 20
this is achieved by external structures not shown. This structure provides sufficient clamping
force to prevent buckling around the rim of the diaphragm 21, thereby allowing movement
perpendicular to the plane of the diaphragm without preventing slight radial movement. It can be
accepted. In this way, even allowing some movement in the plane of the diaphragm does not
cause buckling of the rim portion 26 of the diaphragm 21.
[0017]
Referring to FIG. 6 which shows the diaphragm 21, the dotted line 25A indicates the outer
boundary of the central portion 25. Due to the charging of the electret electrode, the central part
of the diaphragm, as indicated by the arrow T, is directed towards point P and starts to be drawn,
the diaphragm needs it by withdrawing material from the outer or rim portion 26 ( 3 and 4) try
to obtain extra material. The movement of the outer material towards the center P is suppressed.
The reason is that the surrounding fiber 27 is too large for the new surrounding 28 that may be
needed when moving towards the center of the diaphragm. Thus, the rim portion 26 of the
septum is compressed, thereby preventing movement of the septum material towards the center.
On the other hand, the central portion 25 of the diaphragm 21 is generally in tension. The
included force vectors are substantially perpendicular to one another and can co-exist within the
diaphragm 21, varying in relative magnitude along each radius of the diaphragm. If the
compressive force exceeds a limit near the outer edge of the diaphragm, the diaphragm rim will
buckle if not prevented. However, by suppressing and preventing any movement perpendicular
to the outer surface of the support opening 25A, the overall structure of the electret assembly 20
substantially before the central portion 25 of the diaphragm reaches the buckling level. High
compression force can be formed. In this way, a high internal tension is obtained at the central
portion 25 of the diaphragm 21 while maintaining an effective conversion action. By allowing the
movement parallel to the plane of the diaphragm while relating to movement perpendicular to
the plane of the diaphragm, restraining the diaphragm around its rim 26, expansion and
contraction due to environmental factors are not prevented.
[0018]
FIG. 7 shows, on an enlarged scale, the diaphragm 21 and the two clamping members 23 and 24
engaging the rim portion 26 of the diaphragm. When these elements are installed in an electret
assembly with a charging back, the situation shown in FIG. 8 is obtained. The central portion 25
is pulled towards the back plate, which in this example is considered to be under the diaphragm.
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6
The outer edge of the rim portion 26 of the diaphragm can move laterally at a position parallel to
the plane of the diaphragm. (Compare FIG. 7 and FIG. 8). This allows the central portion 25 of the
diaphragm to be reinforced in a shape close to the shallow portion of the sphere. In order to
achieve this result, some clamping pressure should be applied between the members 23 and 24
as indicated by the arrow C in FIGS. 7 and 8. This pressure must be of a sufficient degree to
prevent buckling of the rim portion 26 of the diaphragm without preventing its very limited
movement parallel to the plane of the diaphragm as described above .
[0019]
FIGS. 9 and 10, as noted above, illustrate alternative configurations used for the clamp ring and
diaphragm portions of the electret. The diaphragm 21 has not changed. The clamping members
23 and 24 are the same as in the configuration as shown. However, in this example, a thin outer
ring 31 is interposed between the main clamping rings 23 and 24. The ring 31 is slightly thicker
than the diaphragm 21. For example, if diaphragm 21 has a total thickness of about 0.00006
inches, ring 31 has a total thickness of about 0.00008 inches. This provides some clearance for
the rim 26 of the diaphragm 21 between the mounting members 23 and 24. According to this
configuration, the diaphragm is also prevented from some movement perpendicular to the plane
while allowing limited movement in the direction parallel to the plane of the diaphragm. This
state is shown in FIG. 9 and FIG. In FIG. 9, due to its planar shape, it is assumed that there is no
static electric field to pull the central portion 25 of the diaphragm. FIG. 10, on the other hand,
shows a limited deflection to the partial shape of the sphere. This is formed when this part of the
device is included in a complete electret.
[0020]
11-13 are other arrangements used to achieve the desired effect on diaphragm 21. FIG. In this
example, each corner 129 of the two outer clamping members or rings 123 and 124 has a thin
angular projection 129 on the clamping member 123 and a similar projection 131 on the
clamping member 124. To be plated. The shape for the clamping member 123 is shown in
greater detail in FIGS. In general, the spacer or pad 129 at the corners of the clamp member 123
may have a thickness on the order of 0.00004 inches. A metal layer such as a gold layer of this
thickness can be deposited to form the desired spacer or pad. Accordingly, the two pads 129 and
131 are summed to the desired total thickness of 0.00008 inches as compared to the provisional
thickness of 0.00006 inches for the diaphragm 21. Thus, the arrangement illustrated in FIGS. 1113 provides the same operation as that of the previous arrangement, limiting movement of the
diaphragm 21 in a direction substantially parallel to the plane of the diaphragm, in this plane By
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7
preventing movement of the diaphragm 21 in the vertical direction, buckling is prevented. The
effects of the pads 129 and 131 can also be realized by stamping or embossing the mounting
members 123 and 124.
[0021]
FIG. 14 shows a portion of a microphone 220 containing an electret configured in accordance
with the present invention. The microphone 220 is usually made of metal and has an outer barrel
or housing 210 having an acoustic port 211. In the illustrated microphone, this is a sound inlet.
In a receiver or "speaker" this is an acoustic output. The housing 210 further has a rather large
portion 211 defining an acoustic chamber within the microphone.
[0022]
The configuration of the electret of the microphone 220 generally matches that described above.
The electret is mounted between two support members 223 and 224 and has a diaphragm 221
facing the back plate 222. In the illustrated configuration, the backplate 222 carries an outer
coating or film 227 of electrostatically charged dielectric material. That is, the back plate is a
charging member of the electret in this example. The back plate 222 and its sheath 227 are
attached to the insulator support member 225 and electrically connected to the conductor 228
which forms the actuation circuit for the microphone. Clamping ring 226 and elastomeric spacer
229 complete the internal structure for microphone 220, as shown in FIG. Clamping ring 226 is
used to keep the other elements of the microphone in place as shown, so diaphragm 221 is
restrained with respect to vertical movement as shown. However, using an arrangement
generally similar to that shown in FIGS. 9 and 10, the opening through which the diaphragm is
mounted between members 223 and 224 allows the diaphragm to move in a direction parallel to
its plane .
[0023]
As shown in FIG. 14, the actuation of the electret portion of the microphone 220 substantially
corresponds to that described above with respect to FIGS. 1 to 4 and in particular to the method
shown in FIGS. It corresponds as it is transformed by. Accordingly, it may be unnecessary to
further describe the operational features of the microphone.
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[0024]
In the construction of the electret according to the invention, no electrostatic tensioning means
are usually necessary since the electrostatic charge (differential voltage) between the diaphragm
and the back plate is used to tension the diaphragm. In all instances, the electret contains
mounting means such as mounting members 23, 24 and 123, 124 and 223, 224 which can move
the rim of the diaphragm in its plane in the radial direction. However, at the same time, the
mounting member suppresses or prevents movement of the diaphragm in a direction
perpendicular to the plane of the diaphragm so that buckling and waving are effectively
prevented. The shape of the electret members is not a critical issue. These may be rounded as
shown in FIGS. 1 to 6 or rectangular as shown in FIG. Other shapes, such as hexagons, can be
used if desired for ease of assembly or for other purposes. In any event, the improved electret
assembly configuration of the present invention at least partially compensates for the effects and
aging of temperature and humidity variations.
[0025]
As described above, according to the electret assembly according to the present invention, the
differential voltage between the electret electrodes is used to tension the diaphragm electrodes,
and further, possible buckling and waving of the diaphragm. Can be prevented. Also, the electret
assembly according to the present invention can at least partially compensate for the effects of
temperature and humidity fluctuations and aging, and its manufacture and assembly become
relatively simple and economical.
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