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JP2007318327

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DESCRIPTION JP2007318327
An object of the present invention is to relax the restriction on the allowable amplitude while
maintaining the linearity of the force acting on the diaphragm of the electrostatic speaker. An
electrostatic speaker according to the present invention includes electrodes 21 and 22 facing
each other, a diaphragm 10, and elastic members 30 interposed in spaces between the electrodes
21 and 22 and diaphragm 10, respectively. The elastic member 30 is roughly configured, and is
characterized in that it has an elastic characteristic that generates a restoring force
corresponding to the high-order term of the electrostatic force Fm exerted on the diaphragm 10
by the electrodes 21 and 22. [Selected figure] Figure 1
Electrostatic speaker
[0001]
The present invention relates to the structure of an electrostatic speaker.
[0002]
A speaker called an electrostatic speaker (capacitor speaker) is known.
Electrostatic loudspeakers are attracting attention because they are relatively simple in structure,
so they can be designed to be lightweight and compact, and that they can be theoretically
handled easily. An electrostatic speaker is typically a sheet-like member (hereinafter referred to
as "conductive sheet member") in which two parallel flat electrodes facing each other across an
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air gap, and the both ends thereof are fixed to a housing. It is composed of a diaphragm or a
diaphragm (so-called push-pull type). When a predetermined bias voltage is applied to the
diaphragm and the voltage applied to both electrodes is changed, the electrostatic force acting on
the diaphragm changes, and thereby the diaphragm is displaced. Since the diaphragm is usually
fixed to the case at its ridges and sides, the displacement becomes larger toward the central
portion, and the diaphragm as a whole is in a bent state. When the voltage applied to the
electrodes is changed according to the input musical tone signal, the diaphragm repeats
displacement (i.e. vibrates) accordingly, and an acoustic wave corresponding to the input musical
tone signal is generated from the diaphragm. The generated musical tone is emitted to the
outside of the speaker through, for example, a hole or the like formed in a metal plate electrode
which is an electrode (see Non-Patent Document 1). Seiji Sakamoto, "Speaker and Speaker
System", Nikkan Kogyo
[0003]
Thus, although the electrostatic force generated by the input signal and the elastic stress
(restoring force) due to the displacement act on the diaphragm, the characteristics of these two
forces limit the allowable amplitude to the diaphragm. There is a problem that is imposed. The
problem will be described in detail below. FIG. 6 is a view schematically showing a cross section
of a general push-pull electrostatic speaker 100. As shown in FIG. Note that, for convenience of
explanation, only the flat counter electrodes 101 and 102 and the diaphragm 103 which are
main components are shown. Further, in the same drawing, the x-axis is defined in the direction
perpendicular to the facing surfaces of the electrodes 102 and 103 and the surface of the
diaphragm 103. When no signal is input, the diaphragm 103 is positioned at x = 0, which is the
middle position between both electrodes. That is, at this time, the distances to both electrodes are
d, and in the diaphragm 103, the electrostatic force in the + x direction and the electrostatic force
in the -x direction are balanced, and no elastic stress is applied. .
[0004]
Now, when a musical tone signal is inputted and an electrostatic force corresponding to the
musical tone signal acts on the diaphragm 103, the diaphragm 103 is drawn to the side of one of
the electrodes. Assuming that the diaphragm 103 (to be exact, the central part of the diaphragm)
is displaced to the position x, the electrostatic force Fm acting on the diaphragm at this position
is expressed by the following equation using B as a positive constant. Be done.
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[0005]
[0006]
When this is expanded into a power series, it is expressed as the following equation.
[0007]
[0008]
As described above, elastic stress acts on the diaphragm 103 with the displacement.
Generally, the elastic stress Fs acting on the diaphragm 103 at the position x (that is, the
displacement amount is x) is generally expressed by the following equation, with an elastic
coefficient specific to the material and structure of the diaphragm as A (positive constant). Can be
represented.
[0009]
[0010]
Therefore, the force Ftotal acting on the diaphragm 103 is as follows.
[0011]
[0012]
FIG. 7 shows the relationship between the electrostatic force Fm acting on the diaphragm 103
and the elastic stress Fs and the displacement.
In the same figure, in order to compare the magnitude of the electrostatic force Fm with the
magnitude of the elastic stress Fs, the positive and negative of the elastic stress Fs are reversed.
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As can be seen from the figure, since the relationship of Fm> Fs always holds when the
displacement amount becomes larger than xc (that is, the amplitude of the diaphragm 103
becomes 2xc or more), theoretically, the diaphragm 103 is finally It can be seen that contacts
with either electrode.
Alternatively, when the displacement exceeds the elastic limit of the diaphragm 103 before
contact, the diaphragm 103 may be broken.
[0013]
In order to prevent such a situation, it is necessary to suppress the amplitude of the diaphragm
103 within a certain range.
This will be described with reference to FIG.
FIG. 8 shows all the forces Ftotal (ie, the sum of the electrostatic force Fm and the elastic stress
Fs) acting on the diaphragm 103 according to the amount of displacement. As can be seen from
FIG. 8, Ftotal has a maximum at x = ± x2, but the slope of the curve is positive in the region
outside ± xc. This is nothing but indicating that the force in the same direction as the direction
of displacement is acting, and when the diaphragm 103 enters this area, the problems of contact
and breakage described above occur. Therefore, the amplitude of the diaphragm 103 should be
limited to at least 2 × x1, that is, the displacement of the diaphragm 103 falls within the range
of −x1 to x1. Specifically, it is conceivable to set an upper limit value to the power of the input
signal.
[0014]
However, even if the risk of contact and breakage is eliminated, problems remain in acoustic
characteristics. This can be easily understood in consideration of the time change of Ftotal acting
on the diaphragm 103. In terms of acoustic characteristics, as shown in FIG. 10, it is considered
ideal that the sum of the forces acting on the diaphragm 10 is a linear restoring force. FIG. 10
shows the total temporal change of the force acting on the diaphragm performing an ideal
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vibration with an amplitude of 2Fmax. Therefore, when a force as shown in FIG. 9 is acting on the
diaphragm 103, the vibration of the diaphragm 10 is separated from the ideal state, and the
acoustic characteristics are degraded. Therefore, in consideration of acoustic characteristics,
usually, the power of the input signal is limited so that the displacement shown in FIG. 8 falls
within the range of -x2 to x2. When aiming to further improve the acoustic characteristics, in
order to bring the vibration of the diaphragm closer to an ideal one, a region where the
relationship between displacement and force can be regarded as substantially linear (from -x3 to
x3 in FIG. 8) The need to limit the amplitude to the range shown) is also conceivable.
[0015]
It should be noted that if the distance between the two electrodes is increased, it is clear that the
allowable range of the amplitude of the diaphragm 10 becomes wider with respect to the
problem of contact mentioned above, but in this case, the electrostatic force acting on the
diaphragm is reduced. There is also the problem that the pressure also decreases, and the
problem that the voltage applied to the electrode has to be increased in order to secure a
predetermined output sound pressure. As described above, in the conventional electrostatic
speaker, it is difficult to achieve both the expansion of the amplitude (displacement allowable
range) of the diaphragm and the linearity of the force applied to the diaphragm, which is the
electrostatic type. It has been a factor that impedes the improvement of the performance of the
speaker. Therefore, the present invention aims to alleviate the restriction on the allowable
amplitude while maintaining the linearity of the force acting on the diaphragm of the
electrostatic speaker.
[0016]
In order to solve the above problems, the electrostatic loudspeaker according to the present
invention comprises a pair of opposing electrodes and a diaphragm provided between the
opposing electrodes and displaceable by electrostatic force, and is a cube of distortion amount in
a predetermined direction An elastic member having a linear elasticity characteristic to generate
a restoring force proportional to the distance between the diaphragm and the counter electrode
such that the predetermined direction is the displacement direction of the diaphragm I assume.
According to the present invention, since a restoring force that cancels third-order distortion is
exerted on the diaphragm from the inserted elastic body, even if the amplitude (displacement
allowable range) of the diaphragm becomes large, the diaphragm can be used. The linearity of
the force acting can be maintained.
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[0017]
In a preferred embodiment, the linear elastic property further includes a contribution
proportional to the first power of the strain amount.
[0018]
In another preferred embodiment, the distance from the diaphragm to the counter electrode in
the non-displacement state is d, the displacement of the diaphragm is x, and B is a positive
constant, and the electrostatic force Fm acting on the diaphragm is When represented by (A), the
elastic member generates a restoring force Fs expressed by the equation (B).
(Number A) Fm = B (1 / (d-x) ^ 2) -B (1 / (d + x) ^ 2) (A) (Number B) Fs = B (x ^ 3) / (d ^ 5) (B)
[0019]
In still another preferred embodiment, the elastic member is fixed in a predetermined pressurized
state to achieve the elastic property.
[0020]
Hereinafter, preferred embodiments of the present invention will be described with reference to
the drawings.
FIG. 1 is a perspective view of the general structure of an electrostatic speaker 1 according to an
embodiment of the present invention. As shown in the figure, the electrostatic loudspeaker 1
comprises a diaphragm 10, two flat electrodes (hereinafter simply referred to as electrodes) 21
and 22 facing the diaphragm 10, and a space between the diaphragm 10 and the electrodes 21
and 22. It comprises the elastic member 30 provided in the space.
[0021]
The diaphragm 10 has a metal film deposited on a film such as PET (polyethylene terephthalate,
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polyethylene terephthalate), PP (polypropylene, polypropylene) or the like, or a conductive paint
applied, for example, having a thickness of several microns to several tens of microns. Plate-like
(film-like) member. The diaphragm 10 is supported by pressure (elastic force) from the elastic
member 30 from both sides. Alternatively, the diaphragm 10 is a fixing means (not shown)
formed of an insulating material such as vinyl chloride, acrylic (methyl methacrylate), rubber,
etc., in a state where a predetermined tension is acting on the diaphragm 10, For example, one
side of the edge may be fixed to the housing (not shown) of the electrostatic speaker 1.
[0022]
The electrodes 21 and 22 are made of a material having conductivity and a high sound wave
permeability, such as a punching metal having holes (not shown) in a metal plate, a sputtered
non-woven fabric, a non-woven fabric coated with a conductive paint, etc. , And fixed to a
housing (not shown) of the electrostatic speaker 1. At this time, the distance d from the
diaphragm 10 to both electrodes 21 and 22 is arranged to be equal. In other words, the exactly
middle position of the opposing electrodes is the fixed position of the diaphragm 10 (precisely,
the diaphragm 10 in the non-displacement state, which is a state when no signal is input).
[0023]
Further, the electrostatic speaker 1 is provided with a power supply (not shown) so that voltages
of opposite polarities are applied to the respective electrodes 21 (22), and a bias voltage can be
applied to the diaphragm 10. ing. In addition, the electrostatic speaker 1 includes an input unit
for inputting an audio signal from the outside, and causes the diaphragm 10 to vibrate according
to the audio signal by changing the value of the applied voltage according to the audio signal. It
is possible to The sound wave generated by the vibration of the diaphragm 10 passes through
the electrode 21 or 22 and is emitted to the outside of the speaker.
[0024]
The elastic member 30 is made of a non-conductive material capable of being deformed against
an external force, such as non-woven fabric, cotton or sponge, having predetermined elastic
properties. The elastic member 30 applies, for example, an adhesive layer on its surface, and is
fixed to the electrodes 21 and 22 through the adhesive layer. The elastic member 30 is not
limited to one composed of a single material, but may be a composite structure in which a
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plurality of springs are covered with a covering material. When the diaphragm 10 is displaced
(vibrated), the elastic member 30 is deformed according to its elastic modulus, and a force
(restoring force) in the reverse direction to the displacement is applied to the diaphragm 10.
Incidentally, the elastic property as referred to below means how to strain with respect to a force
in a predetermined direction given from the outside (in the case of the present invention, a force
in a direction perpendicular to the electrodes 21 and 22 exerted by the diaphragm 10) As a
result, it is an elastic property in a broad sense that defines whether or not a restoring force is
generated outside, and using a strain-stress curve, a linear elastic coefficient (Young's modulus)
in the thickness direction, a nonlinear elastic coefficient (secant coefficient), etc. It can be defined.
The electrostatic loudspeaker 1 according to the present invention differs from the electrostatic
loudspeaker of the prior art in that the diaphragm 10 receives a restoring force from the inserted
elastic member 30. In the present invention, since the elastic characteristics of the elastic
member 30 are characterized, this point will be described in detail below.
[0025]
In the following description of the present invention, the same parameters as used in the
description with reference to FIGS. 6 to 10 are used for electrostatic loudspeakers in the prior
art. In the present invention, as in the prior art, the electrostatic force Fm acting on the
diaphragm 10 displaced by x is expressed by the equation (1). Note that, to be precise, since the
diaphragm 10 is bent, it is preferable to define the displacement of the center of the diaphragm
as x. Here, when the displacement x of the diaphragm 10 is sufficiently smaller than the distance
d between the electrode 21 (22) and the diaphragm 10, the approximation of the equation (2)
holds. On the other hand, as described above, the restoring force Fs generated in the diaphragm
10 due to the displacement x of the diaphragm 10 and the elastic characteristics unique to the
diaphragm 10 and the fixing method to the housing is expressed by the equation (3) Ru. In the
present invention, when the diaphragm 10 is displaced by x, the elastic member 30 on the side to
which the diaphragm 10 is displaced is also distorted by x in the direction perpendicular to the
both electrodes, thereby attempting to restore this distortion. A force acts on the diaphragm 10.
The force Fse received from the diaphragm 10 is represented by the following equation as a
function of the strain x.
[0026]
[0027]
FIG. 2 shows the sum Fs ′ of the stress to which Fs and Fse are added and the electrostatic force
Fm in comparison.
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[0028]
The sum F'total of forces acting on the diaphragm 10 of the electrostatic speaker 1 is expressed
by the following equation.
[0029]
[0030]
FIG. 3 is a graph showing the relationship between F'total and displacement x.
As easily understood from this figure, the magnitude of the restoring force acting on the
diaphragm 10 is proportional to the amount of displacement.
FIG. 4 is a graph showing the time change of the force F'total acting on the diaphragm 10 when
the diaphragm 10 is vibrating.
As described above, in the present invention, since the restoring force acting on the diaphragm
10 can be regarded as linear, when the position of the diaphragm 10 is sufficiently away from
the origin, in other words, when the amplitude of the diaphragm 10 is sufficiently large. Even if
the linearity of F'total is not substantially lost, this makes it possible to make the diaphragm 10
vibrate ideally.
In other words, the range of displacement in which the linearity of the force acting on the
diaphragm 10 is maintained is expanded as compared with the conventional electrostatic
speaker. Thereby, sound pressure and sound quality can be simultaneously improved.
[0031]
Next, a method of forming the elastic member 30 having the above-described elastic
characteristics will be described. In the present invention, of course, the elastic member 30 may
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be made of a single material having elastic properties represented by the formula (5), but even if
there is no single material having such properties, The elastic member 30 having the elastic
characteristics described above can be configured in various ways. In the present invention, the
method of manufacturing and processing the elastic member 30 is not limited. For example, the
elastic member 30 may be made of a composite material. Specifically, a plurality of elastic
members whose elastic characteristics are known can be joined to be designed to exhibit the
above-described elastic characteristics as a whole. In particular, in the case of a single material,
when there is no material having the above-described elastic characteristics, the elastic member
30 can be fixed by applying a predetermined pressure when inserting the elastic member 30 as
described above. Characteristics may be realized. Hereinafter, this method will be described.
[0032]
FIG. 5 is an example of the elastic characteristics of the elastic member 30 in a state in which no
pressure is applied, expressed by a strain (ε) -stress (σ) curve. According to the figure, in a
normal state, the elastic member 30 has an approximately linear elastic property in a region
where strain is small (0 <x <x1), but non-linearity appears when the strain becomes large I
understand that. As described above, when the elastic member 30 is fixed between the electrode
21 (22) and the diaphragm 10 without applying a pressure or the like in a normal state, the
characteristics as expressed by the equation (5) can be exhibited. It may not be possible.
[0033]
Therefore, in the present invention, an elastic member having elastic properties as shown in FIG.
5 is used in a region that meets the desired conditions. Specifically, the elastic member 30 is
fixed in a state where the applied pressure Pex corresponding to the elastic characteristic
described above is applied. The value of the pressure Pex can be determined, for example, by
calculating the origin of a region satisfying a predetermined degree of approximation when q is a
constant and is approximated as σ (ε) to qε ^ 3. This corresponds to moving the origin of the
coordinate system (ε-σ) from O to O ′ to realize the elastic property of the region
corresponding to the new coordinate system (ε′−σ ′). Specifically, an elastic member having
a thickness d + x2 may be prepared, and this may be interposed in the space (distance d)
between the electrode 21 (22) and the diaphragm 10.
[0034]
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The present invention is characterized by using the elastic member 30 having an elastic property
that cancels at least the third order term of the electrostatic force Fm as expressed by the
equation (5). , It is not restricted to what was expressed by Formula (5). For example, C may be a
constant and may further have a first-order term as expressed by the following equation.
[0035]
[0036]
Even in this case, it is clear that the linearity of the force F'total is not affected.
Alternatively, the elastic member 30 may further have a term that cancels the high-order term
(the fifth or higher-order term) in the equation (2).
[0037]
Furthermore, in the present invention, it is not necessary to satisfy equation (5) mathematically
exactly. The point is that the non-linear term of the electrostatic force Fm represented by the
equation (1) is substantially or substantially canceled, thereby having an elastic property such
that the non-linearity of the restoring force acting on the diaphragm can be substantially ignored.
Thus, the effects of the present invention described above can be achieved. Further, in the above
embodiment, only Fse was considered as the force from the elastic member on the side where the
diaphragm 10 was displaced, but with this displacement, the force on the other side of the elastic
member 30 acting on the diaphragm 10 (restoring force It is possible to cancel the electrostatic
force Fm with higher accuracy, considering the force in the opposite direction). In addition, even
if the linear elastic property of the elastic member 30 does not strictly satisfy the equation (5), at
least in proportion or substantially in proportion to the third power of the distortion amount, the
B or the speaker related to the value of the applied voltage By adjusting the value of the distance
d related to the thickness, it is possible to make the value of the proportionality coefficient 8B / d
^ 5 coincide with or approximate to the proportionality coefficient of the elastic property.
[0038]
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It is a figure showing the appearance structure of electrostatic type speaker 1 concerning the
present invention. 5 is a graph showing a force acting on a diaphragm 10 in the electrostatic
speaker 1. 5 is a graph showing a force acting on a diaphragm 10 in the electrostatic speaker 1.
5 is a graph showing a force acting on a diaphragm 10 in the electrostatic speaker 1. It is a graph
showing the distortion-stress characteristic of elastic member 30. It is a figure showing the
external appearance structure of the conventional electrostatic speaker 100. FIG. 5 is a graph
showing a force acting on a diaphragm 103 in the electrostatic speaker 100. 5 is a graph
showing a force acting on a diaphragm 103 in the electrostatic speaker 100. 5 is a graph
showing a force acting on a diaphragm 103 in the electrostatic speaker 100. 5 is a graph
showing a force acting on a diaphragm 103 in the electrostatic speaker 100.
Explanation of sign
[0039]
1, 100 ... electrostatic speaker, 10, 103 ... diaphragm, 21, 22, 101, 102 ... electrode, 30 ... elastic
member.
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