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JP2015073215

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DESCRIPTION JP2015073215
An acoustic electrostatic converter using a surface potential of an electret dielectric film as a
polarization voltage, and partial suction of a diaphragm due to variations in the surface potential
of the electret dielectric film is reliably prevented by a simple method. An acoustic electrostatic
transducer including a diaphragm and a fixed electrode disposed opposite to each other at a
predetermined distance, the electret dielectric film having an electret dielectric film on one of the
opposing surfaces thereof. The surface of the film 22 is divided into a plurality of segment areas
22B, and a predetermined surface potential is applied to each of the segment areas 22B by
polarization processing means. [Selected figure] Figure 6
Electrostatic transducer for sound, method of manufacturing fixed pole thereof, condenser
microphone, condenser headphone
[0001]
The present invention relates to an acoustic electrostatic transducer that uses the surface
potential of an electret dielectric film as a polarization voltage, and more specifically, a technique
for preventing partial adsorption of a diaphragm due to variations in surface potential of the
electret dielectric film It is about
[0002]
An electret dielectric film (hereinafter simply referred to as "electret").
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There are a condenser microphone and a condenser headphone as an acoustic-electric / electricacoustic converter using. According to these converters, since the surface potential of the electret
is used as a polarization voltage, a polarization power supply such as a DC / DC converter is not
necessary, and there is an advantage that the circuit configuration of the converter can be
simplified.
[0003]
Electret materials include FEP (copolymer of tetrafluoroethylene and propylene hexafluoride),
SiO 2 and the like. The electret material is polarized when a direct current high voltage is applied,
and has a characteristic that the polarization remains even after removing the voltage.
[0004]
For the polarization treatment (electretization treatment), a negative corona discharge is usually
used. Although a scorotron or the like having a control grid is used to reduce partial variation of
the surface potential of the electret, the surface potential of the electret partially varies
depending on the surface state of the electret material, charging condition, and the like.
[0005]
As for directional condenser microphones, the tension of the diaphragm is designed to be low in
order to secure the frequency response in the low frequency range. Therefore, the diaphragm is
easily attracted to the fixed electrode side, and when the adsorption stability is exceeded, the
diaphragm is attracted to the fixed electrode by electrostatic attraction.
[0006]
In consideration of this point, in the design of the microphone, the adsorption stability μ is set
so that the adsorption of the diaphragm due to the electrostatic attraction does not occur due to
the storage environment or the use environment. The adsorption stability μ is expressed by the
following equation.
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[0007]
μ = A × s 0 × (db <3> / Eb <2>) (where, A is a permittivity of vacuum, effective area of
diaphragm, proportional constant including distance between diaphragm and fixed electrode, s 0
Is the stiffness of the diaphragm, db is the distance between the diaphragm and the fixed
electrode, and Eb is the surface voltage (polarization voltage) of the fixed electrode. )
[0008]
The stiffness s 0 of the diaphragm and the interelectrode distance db are determined according
to the directional frequency response required for the product. Therefore, by maximizing the
polarization voltage Eb within the range in which the diaphragm is not attracted to the fixed
electrode, it is possible to fabricate a directional condenser microphone with high sensitivity with
a good directional frequency response.
[0009]
In a condenser microphone that applies a polarization voltage to a fixed pole by a DC / DC
converter etc., when the polarization voltage reaches the adsorption limit, in the case of a
diaphragm having a circular film, its center part is first adsorbed to the fixed pole, As the
polarization voltage is increased, the adsorption area expands toward the peripheral portion.
[0010]
However, in the case of using an electret, as described above, there is a variation in the surface
potential, so that the portion to be adsorbed first is not limited to the center.
When the electret has a surface potential near the adsorption limit, variations in the surface
potential cause local partial adsorption.
[0011]
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When the surface potential of the electret is raised and exceeds the adsorption limit, the
adsorption area gradually spreads so that the adsorption occurs at the portion next to it, with the
first adsorbed portion being the center and the next higher surface potential. Take on. If this first
partial adsorption is prevented, the effective surface potential of the electret is maintained at a
high potential state, whereby the sensitivity can be further increased.
[0012]
In order to prevent adsorption of the diaphragm to the fixed electrode, Patent Document 1
proposes that the surface potential of the central portion of the fixed electrode be lower than that
of the peripheral portion. Moreover, in patent document 2, making the center part of a fixed pole
into an electret nonexistence area is proposed.
[0013]
However, in any of Patent Literatures 1 and 2, it is not effective for local partial adsorption other
than the central portion due to the variation of the surface potential of the electret. Further, in
Patent Document 1, since the surface potential is made different between the central portion and
the peripheral portion of the fixed electrode, the polarization treatment of the electret is
performed by using the voltage of the corona discharge in the central portion and the peripheral
portion of the fixed electrode. The process is divided into two steps and the productivity is not
good.
[0014]
JP 2003-153395 JP JP 2005-045410 JP
[0015]
Therefore, it is an object of the present invention to provide a simple and easy method for partial
adsorption of a diaphragm due to variations in surface potential of an electret dielectric film in
an acoustic electrostatic converter using the surface potential of an electret dielectric film as a
polarization voltage. It is to prevent reliably.
[0016]
In order to solve the above problems, the present invention includes an electrostatic electrostatic
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type having an electret dielectric film on the opposite surface side of a diaphragm and a fixed
electrode, which are disposed opposite to each other at a predetermined interval. In the
converter, the surface of the electret dielectric film is divided into a plurality of segment areas,
and a predetermined surface potential is applied to each of the segment areas by polarization
processing means.
[0017]
Further, the present invention is characterized in that Sb <Sa, where the area of the maximum
surface potential area where the surface potential of the electret dielectric film is the highest is
Sa and the area of the segment area is Sb.
[0018]
The electret dielectric film may be provided on the diaphragm side, but the acoustic electrostatic
transducer of the present invention is a back electret type in which the electret dielectric film is
disposed on the fixed electrode side. Is preferred.
[0019]
The present invention includes a condenser microphone provided with the above-described
acoustic electrostatic transducer and a condenser headphone.
[0020]
Furthermore, the present invention includes first and second two manufacturing methods for
manufacturing a fixed electrode having an electret dielectric film on the surface facing the
diaphragm, and the first manufacturing method includes the above electret dielectric A metal
mask for dividing the surface into a plurality of segment regions is disposed on the body film,
and the electret dielectric film is subjected to polarization treatment by corona discharge in a
state where the metal mask and the fixed electrode are both grounded. A predetermined surface
potential is applied to each of the segment areas.
[0021]
The second manufacturing method is characterized in that the electret dielectric film is subjected
to polarization treatment by corona discharge over the entire surface, and then the charged
surface is divided into a plurality of segment regions by alcohol elimination.
[0022]
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According to the present invention, the electret dielectric film is divided into a plurality of
segment regions, so that the amount of displacement is small with a small area pseudopartitioned opposite to each segment region on the diaphragm side. Since a plurality of sub
vibration regions are included and electrostatic attraction force acts on each of the sub vibration
regions, even if there is variation in the surface potential of the electret dielectric film, toward the
high potential portion thereof The diaphragm is not distorted and drawn, and the diaphragm as a
whole maintains a substantially flat shape in a macro view, so there is almost no possibility that
the diaphragm is partially adsorbed to the fixed electrode side.
[0023]
Further, by setting the area Sb of each segment area to be smaller than the area Sa of the
maximum surface potential area (Sb <Sa), the area Sc (≒ Sb) of each sub vibration area of the
diaphragm is also maximized accordingly. Since it is smaller than the area Sa of the surface
potential area, the first partial adsorption of the diaphragm in the maximum surface potential
area is prevented.
Therefore, the effective surface potential of the electret is maintained at the high potential state,
and in the case of the condenser microphone, the sensitivity can be further increased.
[0024]
Further, according to the manufacturing method of the present invention, polarization processing
by corona discharge may be simultaneously applied to each segment region of the electret
dielectric film, and it is not necessary to intentionally apply different surface potentials to each
segment region. Therefore, the fixed pole having the electret dielectric film can be efficiently
manufactured.
[0025]
BRIEF DESCRIPTION OF THE DRAWINGS Sectional drawing which shows the electrostatic
transducer for sound which concerns on one Embodiment of this invention.
The (a) top view of the fixed pole contained in the above-mentioned electrostatic transducer for
sound, (b) The partial expanded sectional view which met the AA line.
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FIG. 6 is a schematic view showing a maximum surface potential region generated by the
variation of the surface potential of the electret dielectric film.
The top view which shows an example of the metal mask for segment area | region formation.
Schematic which shows the state which polarizes the electret dielectric film by a corona
discharge using the said metal mask.
(A) The top view which shows the fixed pole which has the segment area | region which carried
out the polarization process, (b) The expanded sectional view which shows the orientation state
of the dipole in a segment area | region.
The typical sectional view for making the fixed pole and the diaphragm counter, and explaining
the operation of the present invention.
[0026]
Next, embodiments of the present invention will be described with reference to FIGS. 1 to 7, but
the present invention is not limited thereto.
[0027]
First, referring to FIG. 1, the acoustic electrostatic transducer 10 according to this embodiment is
an electrostatic transducer designed for use in a bi-directional capacitor microphone, and is made
of synthetic resin and formed in a disc shape. A seat 12 and a pair of capacitor units 1L and 11R
disposed on both sides of the seat 12 are provided.
In FIG. 1, the capacitor unit 11R on the right side is disassembled, but the capacitor units 11L
and 11R have the same configuration.
[0028]
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Each of the capacitor units 11L and 11R includes a fixed electrode 20 and a diaphragm 30
disposed opposite to the fixed electrode 20 via an electrically insulating spacer ring 40.
The diaphragm 30 is made of, for example, a thin film of a synthetic resin such as PET (Poly
Ethylene Terephthalate), and is stretched on the diaphragm ring 31 in a state where a
predetermined tension is applied.
[0029]
A communication hole 11a for acoustically connecting between the capacitor units 11L and 11R
is bored in the insulating seat 12, and a sound hole (a hole for passing sound) 20a is bored in the
fixed pole 20 as well. It is done.
The numbers of the communication holes 11a and the sound holes 20a may be arbitrarily
determined.
[0030]
The acoustic electrostatic converter 10 according to this embodiment is a back electret type, and
as shown in FIGS. 2A and 2B, the fixed electrode 20 is a surface facing the diaphragm 30 of the
metal electrode plate 21. The electret dielectric film 22 is integrally provided on the side.
[0031]
For the electret dielectric film 22, FEP (copolymer of tetrafluoride ethylene and propylene
hexafluoride), SiO 2 or the like is used, and usually, corona discharge is applied to its polarization
treatment (electret treatment). However, the surface potential of the electret partially varies
depending on the surface state of the electret material, the charging condition, and the like.
[0032]
When the surface potential of the electret is locally high and exceeds the adsorption limit to the
diaphragm, first, adsorption starts from the high potential portion, and the adsorption area is
generated next to the next higher surface potential. Exhibit a phenomenon that gradually spreads
in a beaded manner.
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[0033]
Therefore, it is important to maintain the performance of this acoustic electrostatic transducer
10, although it is important to prevent partial adsorption that first occurs in the portion where
the surface potential is the highest (maximum surface potential region). As described above, the
maximum surface potential area 22A is not necessarily generated at the central portion of the
fixed pole 20.
[0034]
As an example, in FIG. 3, among the surface potentials applied to the electret dielectric film 22 by
the polarization process, the maximum surface potential region 22A having the highest surface
potential is biased from the central portion of the fixed electrode 20 to the peripheral portion
side. Show the condition that occurred.
[0035]
The partial charge of the electret dielectric film and the area thereof can be measured by the
surface voltage measurement method of the electret dielectric film described in, for example,
Japanese Patent No. 4663532 (Japanese Patent Laid-Open No. 2007-194774).
[0036]
As described above, since it is not known in which part of the fixed electrode 20 the maximum
surface potential region 22A having the highest surface potential is generated, in the present
invention, the electret dielectric film 22 is formed as shown in FIG. The surface is divided into a
plurality of segment areas 22B.
[0037]
Therefore, in this embodiment, the surface of the electret dielectric film 22 is divided into the
segment regions 22B using a metal mask 50 as shown in FIG.
[0038]
In this embodiment, the metal mask 50 has a circular blank portion 50a at the center by pressing
a metal plate having a thickness of 0.1 to 0.5 mm such as aluminum, and has a fan-shaped shape
around it. It is formed by punching into the shape of a spoke wheel having eight blank portions
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50b.
The outer diameter of the metal mask 50 is preferably equal to the diameter of the electret
dielectric film 22.
[0039]
In polarizing (electretizing) the electret dielectric film 22, as shown in FIG. 5, the metal mask 50
is disposed in close contact with the electret dielectric film 22, and the metal mask 50 and the
electrode plate 21 are formed. Is grounded, a direct current high voltage is applied by the
negative corona discharge device 60 having the discharge wire 61.
[0040]
As a result, as shown in FIG. 6A, each portion of the electret dielectric film 22 corresponding to
each blank portion 50a, 50b of the metal mask 50 becomes a negatively charged segment region
22B.
[0041]
That is, as shown in FIG. 6A, the negative side of the dipoles present in each segment region 22B
is aligned along the surface of the electret dielectric film 22 by the above polarization treatment.
Thus, a predetermined negative surface potential is applied to each segment region 22B.
[0042]
The respective regions (between the respective segment regions 22B and the peripheral portion
of the electret dielectric film 22) covered by the hub portion 51a, the spoke portion 51b and the
rim portion 51c which the metal mask 50 shown in FIG. It is left as a non-polarized area (nonpolarized area) 22C which is not polarized.
[0043]
A state in which the diaphragm 30 is opposed to the fixed electrode 20 is shown in FIG. 7, but in
the present invention, since the electret dielectric film 22 is divided into a plurality of segment
areas 22B, the diaphragm 30 is The effective vibration area (the entire main vibration area
surrounded by the diaphragm ring 31) includes a plurality of sub vibration areas 30a which are
virtually divided to face each segment area 22B.
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[0044]
Since this sub vibration area 30a has a smaller area than the effective vibration area (the entire
main vibration area) of the diaphragm 30, the entire surface of the diaphragm 30 is
electrostatically attracted as shown by a chain line in FIG. The amount of displacement is smaller
than in the case.
[0045]
Moreover, since electrostatic attraction force acts on the corresponding sub vibration region 30a
from the divided segment region 22, even if the surface potential of the electret dielectric film 22
has variations, the high potential portion thereof The diaphragm 30 is not distorted and
attracted, and the diaphragm as a whole maintains a substantially flat shape in a macro view, so
there is almost no possibility that the diaphragm 30 is partially adsorbed to the fixed electrode
20 side. .
[0046]
In a preferred embodiment of the present invention, the area of the maximum surface potential
area 22A of the electret dielectric film 22 is Sa, and the area of each segment area 22B is Sb, so
that Sb <Sa.
[0047]
The area Sa of the maximum surface potential area 22A in this case may be a value obtained
from an empirical value or a predicted value.
Moreover, although it is preferable that area Sb of each segment area | region 22B is
substantially equal (substantially identical), it may be unequal.
[0048]
According to this, the area Sc (≒ Sb) of each sub vibration area 30a of the diaphragm 30 is also
smaller than the area Sa of the maximum surface potential area 22A, so the first portion of the
diaphragm 30 in the maximum surface potential area 22A It is possible to prevent adsorption.
[0049]
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In the above embodiment, the electret dielectric film 22 is divided into nine segment regions 22B,
but if it is divided into two or more, it is included in the present invention.
Further, the manner of division may be either equal division or unequal division, or may be
divided concentrically.
[0050]
Further, in the above embodiment, the electret dielectric film 22 is divided into a plurality of
segment regions at the time of polarization processing using the metal mask 50, but as another
method, the electret dielectric film 22 is covered over the entire surface After polarization
treatment by corona discharge, the charged surface can be divided into a plurality of segment
areas by alcohol elimination.
[0051]
As an example, the hub 51a, the spoke 51b and the rim 51c of the spoke wheel-shaped metal
mask 50 are provided with a floc having a liquid absorbing property with short fibers, for
example, by electrostatic flocking, and the floc contains alcohol. By disposing the electret
dielectric film 22 on the polarized electret dielectric film 22, the electret dielectric film 22 can be
divided into a plurality of segment regions as shown in FIG. 6A.
[0052]
The acoustic electrostatic converter 10 according to the above embodiment is designed for a bidirectional bi-directional condenser microphone having a pair of capacitor units 11L and 11R,
but the acoustic electrostatic converter according to the present invention The present invention
is not limited to an acoustic-electrical converter such as a unidirectional directivity condenser
microphone in which the condenser unit is only on one side or a nondirectional condenser
microphone without a specific directivity. The present invention is also applicable to condenser
headphones.
Further, it may be a film electret type having an electret dielectric film on the diaphragm side.
[0053]
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DESCRIPTION OF REFERENCE NUMERALS 10 acoustic electrostatic converter 20 fixed electrode
21 electrode plate 22 electret dielectric film 22A maximum surface potential region 22B segment
region 30 diaphragm 30a sub vibration region 50 metal mask 60 corona discharge device 61
discharge wire
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