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JP2007274341

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DESCRIPTION JP2007274341
An electrostatic loudspeaker having a lower lowest resonance frequency. According to the
present invention, a vibrating electrode (30) having a surface and vibrating according to an input
signal, a fixed electrode (10) having a surface facing the surface of the vibrating electrode (30)
and spaced from the vibrating electrode (30) A cushion insulating layer formed between the
surface of the vibrating electrode 30 and the surface of the fixed electrode 10 and having a
predetermined stiffness and insulation property, and having a gap penetrating the gap between
the vibrating electrode 30 and the fixed electrode 10 An electrostatic speaker having a cushion
insulating layer 20 is provided. [Selected figure] Figure 1
Electrostatic speaker
[0001]
The present invention relates to an electrostatic speaker.
[0002]
BACKGROUND Conventionally, a speaker called an electrostatic speaker (capacitor speaker) is
known.
The electrostatic loudspeaker has, as a basic structure, two parallel flat electrodes facing each
other across an air gap. One of the two parallel flat electrodes is fixed (referred to as a fixed
electrode or fixed plate), and the other is often movable (referred to as a movable electrode, a
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vibrating electrode, or a diaphragm). When the input signal is superimposed on the bias voltage
applied between the parallel flat electrodes, the attraction between the parallel flat electrodes
changes. The electrostatic speaker outputs a sound by vibrating the diaphragm by this suction
force.
[0003]
In general, many electrostatic speakers have a total of two fixed electrodes, one on each side of
the movable electrode. The outer peripheral portion of the movable electrode is fixed between
two fixed electrodes. In the electrostatic loudspeaker of this structure, the amplitude of the
movable electrode depends on the tension applied to the movable electrode and the strength of
the input signal. Since the amplitude of the movable electrode is smaller than that of an ordinary
electrodynamic speaker, the area of the movable electrode needs to be increased in order to
obtain the same sound pressure as that of the electrodynamic speaker. That is, there is a problem
that the electrostatic speaker is increased in size. Furthermore, since the outer periphery of the
movable electrode is fixed, there is a problem that when the amplitude of the movable electrode
is increased to increase the sound pressure, the central portion of the movable electrode comes
in contact with the fixed electrode. As a countermeasure against this problem, it is conceivable to
widen the distance between the fixed electrodes. However, if the distance between the fixed
electrodes is increased, it is necessary to increase the bias voltage applied to the fixed electrodes
and the voltage of the input signal in order to obtain the same sound pressure, that is, the
efficiency of the speaker decreases. The
[0004]
As a technique for solving the above problems, there are techniques described in Patent
Documents 1 and 2, for example. Patent Document 1 discloses a technique for reducing tension
related to a movable electrode by providing an elastic spacer in a portion where the movable
electrode is fixed to the fixed electrode. Patent Document 2 discloses a technique in which a
movable electrode is vibrated in a form close to a flat plate by using a plurality of plate-like
electrodes which are mutually insulated as a fixed electrode. In addition, in order to avoid the
problems unique to the electrostatic speaker as described above, an electrodynamic planar
speaker has also been developed (see, for example, Patent Document 3). However, the
electrodynamic flat loudspeaker has a problem in that the efficiency and responsiveness are
worse as compared with the electrostatic loudspeaker.
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[0005]
Here, as a technique for solving the problem of sound pressure in an electrostatic speaker, there
is a technique described in Non-Patent Document 1, for example. Non-Patent Document 1
discloses an electrostatic loudspeaker having a so-called edgeless structure in which the outer
periphery of the movable electrode is not fixed (supported).
[0006]
Patent No. 3353031 Patent No. 3277498 Patent No. 7-0358758 Masanori Okazaki, 4 others,
"Condenser speaker with diaphragm vibrating with piston all over the band and its application",
Japan Acoustics Society Fall 2004 Proceedings of the Conference on Acoustics, The Acoustical
Society of Japan, September 2004, p. 563-564
[0007]
By adopting the edgeless structure, the sound pressure of the electrostatic speaker can be
improved.
However, there is a problem that the minimum resonance frequency of the speaker can not be
reduced simply by adopting the edgeless structure. The present invention has been made in view
of the above-described circumstances, and provides an electrostatic loudspeaker that can reduce
the lowest resonance frequency.
[0008]
In order to solve the problems described above, the present invention has a vibrating electrode
that has a surface and that vibrates according to an input signal, and a surface facing the surface
of the vibrating electrode, and is fixed to be spaced apart from the vibrating electrode. A cushion
insulating layer formed between an electrode, a surface of the vibrating electrode, and a surface
of the fixed electrode and having a predetermined stiffness and insulation property, and a gap
penetrating the gap between the vibrating electrode and the fixed electrode And a cushion
insulating layer.
[0009]
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In a preferred embodiment, in the electrostatic speaker, the cushion insulating layer may be
formed of a plurality of cushioning materials disposed with a gap between each other, and a gap
between each cushioning material may be the air gap. .
In this aspect, at least the individual stiffness of each cushioning material, the shape of each
cushioning material, and the arrangement of each cushioning material are determined such that
each of the plurality of cushioning materials has a predetermined value of stiffness for the entire
assembly. May be
[0010]
In another preferred embodiment, in the electrostatic loudspeaker, the cushion insulating layer is
formed of one or more cushioning materials, and the one or more cushioning materials penetrate
the gap between the vibrating electrode and the fixed electrode. A hole may be formed.
[0011]
Hereinafter, an embodiment of the present invention will be described with reference to the
drawings.
FIG. 1 is a view showing the configuration of a speaker unit 1 according to an embodiment of the
present invention. The speaker unit 1 is a so-called push-pull type electrostatic speaker having
two fixed electrodes of a fixed electrode 10 and a fixed electrode 50. The speaker unit 1 has a
vibrating membrane 30 sandwiched between the fixed electrode 10 and the fixed electrode 50.
The vibrating film 30 functions as a vibrating electrode in the electrostatic speaker. A cushion
layer 20 is provided between the fixed electrode 10 and the vibrating membrane 30. Similarly, a
cushion layer 40 is provided between the fixed electrode 50 and the vibrating membrane 30. A
spacer 60 is provided between the fixed electrode 10 and the fixed electrode 50.
[0012]
The fixed electrode 10 and the fixed electrode 50 are flat electrodes. The fixed electrode 10 and
the fixed electrode 50 are formed of, for example, a conductive material having a structure that
transmits an acoustic wave, such as a punching metal. Alternatively, the fixed electrode 10 and
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the fixed electrode 50 may be formed of a non-woven fabric in which a conductive material such
as metal is deposited by sputtering. Further alternatively, the fixed electrode 10 and the fixed
electrode 50 may be formed of a non-woven fabric coated with a conductive dye. In short, the
fixed electrode 10 and the fixed electrode 50 may be formed using any material that has both
conductivity and sound wave transmission.
[0013]
The vibrating film 30 is, for example, a material obtained by depositing a conductive material
such as metal on a film (thin film or sheet) using a polymer material such as PET (polyethylene
terephthalate, polyethylene terephthalate), PP (polypropylene, polypropylene) or polyester. It is
formed by Alternatively, the vibrating membrane 30 may be formed of a material obtained by
applying a conductive dye to a film. The vibrating membrane 30 is sandwiched between the
cushioning layer 20 and the cushioning layer 40. The vibrating membrane 30 is supported by
the cushioning layer 20 and the cushioning layer 40. In the prior art, the outer periphery of the
vibrating membrane is fixed relative to the fixed electrode, ie, the vibrating membrane is
tensioned. However, in the present embodiment, the outer peripheral portion of the vibrating
membrane 30 is not fixed to the fixed electrode 10 (fixed electrode 50). That is, no tension is
applied to the vibrating membrane 30.
[0014]
A bias voltage is supplied to the fixed electrode 10 from a power supply (not shown).
Furthermore, the speaker unit 1 has an input unit for inputting an audio signal (input signal)
from a signal source (not shown). The input signal is superimposed on the bias voltage and
supplied to the fixed electrode 10. Thus, a voltage corresponding to the input signal is applied
between the fixed electrode 10 and the vibrating film 30. When a potential difference is
generated between the fixed electrode 10 and the vibrating membrane 30 by the applied voltage,
an electrostatic force acts on the vibrating membrane 30. That is, the diaphragm 30 is displaced
or vibrated according to the input signal. A sound corresponding to this vibration state
(frequency, amplitude, phase, etc.) is generated from the vibrating film 30. In the present
embodiment, since the speaker unit 1 is a push-pull type electrostatic speaker, the fixed electrode
50 is supplied with an applied voltage in reverse phase. By supplying the signal of the opposite
phase, an electrostatic force having the same direction and magnitude as the electrostatic force
acting between the fixed electrode 10 and the vibrating film 30 acts between the fixed electrode
50 and the vibrating film 30. That is, in the push-pull type electrostatic speaker, twice as much
electrostatic force acts on the vibrating film 30 as compared with the single type.
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[0015]
The spacer 60 functions to fix and insulate the fixed electrode 10 and the fixed electrode 50
from each other. The spacer 60 is formed of, for example, an insulating material such as vinyl
chloride, acrylic (methyl methacrylate), rubber or the like. The spacer 60 is fixed to the fixed
electrode 10 and the fixed electrode 50 using an adhesive or the like.
[0016]
The cushion layer 20 and the cushion layer 40 are for making the frequency characteristic of the
speaker unit 1 a desired characteristic. The cushion layer 20 and the cushion layer 40 are
formed using an elastic body or an elastic material having a predetermined elastic modulus (for
example, can be represented by a linear elastic modulus (Young's modulus) or the like in the
thickness direction). The elastic modulus of the cushioning layer 20 and the cushioning layer 40
is determined as follows. For example, as described in Seiji Sakamoto, "Speaker and Speaker
System", Nikkan Kogyo Shimbun, 1967, the lowest resonance frequency f0 of the system is
expressed by the following equation (1). Here, C0 is a capacitance of a capacitor formed by the
fixed electrode 10 (fixed electrode 50) and the vibrating film 30. The distance between the fixed
electrode 10 (fixed electrode 50) and the vibrating film 30 in a state in which no DC electric field
and input signal are applied between the fixed electrode 10 (fixed electrode 50) and the vibrating
film 30 d, DC electric field and input signal Assuming that the displacement of the vibrating
membrane 30 when x is added is x0, C0 is represented by the following equation (2).
[0017]
Further, E0 is a bias voltage applied to the fixed electrode 10 (fixed electrode 50), that is, the
potential of the common ground to the fixed electrode 10 (fixed electrode 50). ε is the dielectric
constant of the cushion layer 20 (the cushion layer 40). S is the area of the surface of the
diaphragm 30 facing the fixed electrode 10 (fixed electrode 50). MMD is the mass of the
vibrating membrane 30. MMA is an air added mass (radiated mass) on one side of the vibrating
membrane 30.
[0018]
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Also, SM is the equivalent stiffness of the system. As in the prior art, in the electrostatic
loudspeaker having a structure in which the outer peripheral portion of the diaphragm is fixed
(supported) to the fixed electrode by applying tension to the diaphragm (diaphragm), SM is the
diaphragm Represents the tension applied to the However, in the structure in which the vibrating
membrane is not tensioned as in the present embodiment, the tension can not be used as the SM.
Therefore, in the present embodiment, the stiffness of the cushion layer 20 (the cushion layer
40) (a proportional constant between applied load and displacement and having a dimension of N
/ m) is used as SM. Once the stiffness is determined, the elastic modulus of the cushion layer 20
(the cushion layer 40) can be determined by considering the shape and thickness of the cushion
layer 20 (the cushion layer 40). As the elastic modulus, for example, Young's modulus can be
used in the range in which the stress-strain characteristics are linear, and secant coefficient can
be used in the range in which the stress-strain characteristics are nonlinear.
[0019]
If Equation (1) is solved for SM, then Equation (3) is obtained. Here, SME = 2εSE0 <2> / (d + x0)
<3>, which represents the negative stiffness of the capacitor. Thus, the stiffness SM of the
cushioning material is a function of the lowest resonance frequency f0 of the system. Substituting
the desired lowest resonance frequency, that is, the design value of the lowest resonance
frequency into Equation (3), the stiffness of the cushion layer 20 (the cushion layer 40) is
obtained. By using the cushioning material having the stiffness thus calculated, an electrostatic
speaker having a desired lowest resonance frequency can be manufactured. The stiffness S of the
cushion layer 20 (the cushion layer 40) may be within a predetermined error range with respect
to the stiffness SM calculated by the equation (3). For example, the speaker unit 1 may be
manufactured using a cushioning material having a stiffness S that satisfies 0.8 ≦ (S / SM) ≦
1.2.
[0020]
FIG. 2 is a view showing a first embodiment of the cushioning layer. FIG. 2 is a plan view of the
fixed electrode 10 as viewed from the front. In FIG. 2, only the fixed electrode 10 and the cushion
layer 20 are illustrated in order to avoid complication of the drawing. In the present embodiment,
the cushion layer 20 has a plurality of cushion members 20-1. The shape of each cushion
member 20-1 is a rectangular parallelepiped. The plurality of cushion members 20-1 are
arranged on the fixed electrode 10 at equal intervals. The fixed electrode 10 and the cushion
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member 20-1 are fixed, for example, using an adhesive. Assuming that the stiffness of each
cushion member 20-1 is SM <i>, the stiffness S of the cushion layer 20, that is, the entire
plurality of cushion members 20-1 is S = と な る SM <i>. In the present embodiment, S = ΣSM
<i> satisfies 0.8 ≦ (S / SM) ≦ 1.2. Thus, the cushion layer 20 is formed between the surface of
the fixed electrode 10 and the surface of the vibrating membrane 30. Also, the cushioning layer
20 has predetermined stiffness and insulation. Furthermore, the cushion layer 20 has an air gap
that penetrates the gap between the vibrating membrane 30 and the fixed electrode 10. In the
present embodiment, the cushioning layer 20 is formed by a plurality of cushioning members 201 arranged with a gap therebetween. The gap between the cushion members 20-1 is a gap
passing through the gap between the vibrating membrane 30 and the fixed electrode 10.
Furthermore, at least the individual stiffness of each cushioning material, the shape of each
cushioning material, and the arrangement of each cushioning material are determined such that
the stiffness of each of the cushioning members 20-1 is a predetermined value for the entire
assembly. .
[0021]
Thus, the reason for using a plurality of cushion members as the cushion layer is as follows. The
stiffness is obtained, for example, by dividing the Young's modulus of the cushioning layer by the
thickness of the cushioning layer and further multiplying the area of the cushioning layer. That
is, even when the same material is used, the stiffness of the cushion layer can be reduced by
reducing the area. Thereby, even in the case of producing a large-sized electrostatic speaker unit,
it is possible to suppress an increase in the stiffness of the cushion layer. That is, an electrostatic
speaker unit having characteristics of low f0 can be manufactured.
[0022]
FIG. 3 is a view showing a second embodiment of the cushioning layer. In the first embodiment,
the cushion members 20-1 are arranged at equal intervals, but in the second embodiment, the
plurality of cushion members 20-1 are arranged at random intervals. In the first and second
embodiments, in order to increase the distance between the cushion members 20-1, it is
desirable to use a vibrating film 30 made of a harder material.
[0023]
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FIG. 4 is a view showing a third embodiment of the cushioning layer. In the present embodiment,
the cushion layer 20 has a plurality of cushion members 20-2. The shape of the cushion member
20-2 is a cube. The plurality of cushion members 20-2 are regularly arranged on the fixed
electrode 10 at equal intervals. The shape of the cushion member 20-2 is not limited to a cube. It
may be a solid having any shape such as a polygonal cylinder, a cylinder, a sphere, a cone and
the like. Also, the cushion members 20-2 may be arranged at random intervals.
[0024]
FIG. 5 is a view showing a fourth embodiment of the cushioning layer. In the present
embodiment, the cushioning layer 20 is formed of a cushioning material 20-3 having a plurality
of holes H. The holes H penetrate from the front surface to the back surface of the cushioning
material 20-3. In addition, the cross-sectional shape of the hole H is not limited to a tetragon. It
may be a triangle or a polygon having five or more sides. Alternatively, it may be circular or oval.
Also, the holes H may be regularly arranged at equal intervals or randomly. The point is that the
cushioning material 20-3 may have any shape as long as a hole penetrating the gap between the
vibrating membrane 30 and the fixed electrode 10 is formed. In addition, although the aspect by
which the cushion layer 20 is formed of the single cushion material is shown in FIG. 5, the
cushion layer 20 may be formed of the several cushion material which has the hole H,
respectively. In this case, the holes H and the gaps between the cushioning materials form the
gaps of the cushioning layer 20. Moreover, the void part which connects each hole H may be
installed.
[0025]
FIGS. 2 to 5 are merely examples, and the shape of the cushion layer is not limited to this. The
point is that the cushion layer may have any shape as long as it has a stiffness that falls within a
predetermined error range from the stiffness calculated by equation (3). Moreover, although only
the cushioning layer 20 was demonstrated in FIGS. 2-5, the same may be said of the cushioning
layer 40. FIG.
[0026]
FIG. 6 is a diagram illustrating frequency characteristics of the speaker unit 1. In the example of
FIG. 6, as the vibrating film 30, a PET film having a thickness of 3 μm (micrometer) in which Al
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is vapor-deposited at a thickness of 400 Å (angstrom) is used. Also, as the fixed electrode 10 and
the fixed electrode 50, a punching metal having an aperture ratio of 41.9% is used. A voltage of
5.25 kV is used as a bias voltage, and a sine wave of 4000 Vpp is used as an input signal. In FIG.
6, the solid line indicates the measured value of the frequency characteristic when using a
material of high stiffness (specifically, 133291 N / m) as the cushion layer, and the dotted line
indicates the low stiffness of the cushion layer (specifically, 28920 N / m). The measured value
of the frequency characteristic at the time of using the material of is shown. By using the low
stiffness cushioning layer, the lowest resonance frequency f0 of the speaker unit 1 can be
reduced.
[0027]
In order to lower the lowest resonance frequency f0 of the speaker unit 1, merely using a
cushioning material with low stiffness (low Young's modulus) causes problems in workability and
manufacture, such as the cushioning material easily collapsing. However, according to the
present embodiment, cushion materials having a somewhat high Young's modulus are discretely
arranged. Therefore, the stiffness of the entire cushion layer can be reduced while securing the
processability. As a result, it is possible to obtain a speaker with a low lowest resonance
frequency f0.
[0028]
FIG. 7 is a diagram comparing the calculated value and the measured value of the frequency
characteristic of the speaker unit 1. FIG. 7 (a) shows the calculated value by simulation, and FIG.
7 (b) shows the actually measured value by experiment. In particular, in the frequency range of 2
kHz or less, the calculated value and the measured value agree well. Thus, according to the
present embodiment, an electrostatic loudspeaker having a desired lowest resonance frequency
f0 can be manufactured.
[0029]
In the above embodiment, the aspect in which the speaker unit 1 is a push-pull electrostatic
speaker has been described. However, the speaker unit 1 may be a so-called single type
electrostatic speaker having only one fixed electrode. In the case of a single type electrostatic
speaker, the coefficient of the second term of the numerator within the square root of the
04-05-2019
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equation (1) is not “2” but “1”. That is, assuming that the number of fixed electrodes is n,
the negative stiffness of the capacitor can be expressed as SME = nεSE0 <2> / (d + x0) <3>. In
the case of a push-pull type electrostatic speaker, SME = 2εSE0 <2> / (d + x0) <3>, and in the
case of a single type electrostatic speaker, SME = εSE0 <2> / (d + x0) <3> It becomes.
[0030]
In the above-mentioned embodiment, the example which controls the frequency characteristic of
a speaker by designing the stiffness of a cushion material to a suitable value was explained.
However, the frequency characteristic of the speaker may be controlled by designing the
dielectric constant of the cushion material to an appropriate value. In general, the dielectric
constant of the cushioning material is higher than air. Therefore, by using a cushion material
having a higher dielectric constant, it is possible to reduce f0 and to improve the electrostatic
drive force.
[0031]
It is a figure showing composition of speaker unit 1 concerning one embodiment of the present
invention. It is a figure which shows 1st Embodiment of a cushion layer. It is a figure which
shows 2nd Embodiment of a cushion layer. It is a figure which shows 3rd Embodiment of a
cushion layer. It is a figure which shows 4th Embodiment of a cushion layer. FIG. 5 is a diagram
illustrating frequency characteristics of the speaker unit 1; It is a figure which compares the
calculated value of the frequency characteristic of the speaker unit 1, and actual value.
Explanation of sign
[0032]
DESCRIPTION OF SYMBOLS 1 ... Speaker unit, 20-1 * 20-2 ... Cushion member, 10 ... Fixed
electrode, 20 ... Cushion layer, 30 ... Vibrating film, 40 ... Cushion layer, 50 ... Fixed electrode, 60
... Spacer
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