JP4899590B2 - Electrostatic speaker - Google Patents

Description

  The present invention relates to an electrostatic speaker.

  Conventionally, a speaker called an electrostatic speaker (condenser speaker) is known. The electrostatic speaker has two parallel flat electrodes facing each other with a gap as a basic structure. In many cases, two parallel flat electrodes have a structure in which one is fixed (referred to as a fixed electrode or a fixed plate) and the other is movable (referred to as a movable electrode, a vibration electrode, or a vibration plate). When an input signal is superimposed on the bias voltage applied between the parallel plane electrodes, the attractive force between the parallel plane electrodes changes. The electrostatic speaker outputs sound by vibrating the diaphragm with this suction force.

  In general, many electrostatic speakers have two fixed electrodes, one on each side of the movable electrode. The outer peripheral part of the movable electrode is fixed between two fixed electrodes. In the electrostatic speaker having this structure, the amplitude of the movable electrode depends on the tension applied to the movable electrode and the intensity of the input signal. Since the amplitude of the movable electrode is smaller than that of a normal electrodynamic speaker, it is necessary to increase the area of the movable electrode in order to obtain a sound pressure equivalent to that of the electrodynamic speaker. In other words, there is a problem that the electrostatic speaker becomes large. 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 into contact with the fixed electrode. As a countermeasure against this problem, it is conceivable to increase the interval between the fixed electrodes. However, if the interval between the fixed electrodes is increased, in order to obtain the same sound pressure, it is necessary to increase the bias voltage and the input signal voltage applied to the fixed electrodes, that is, the efficiency of the speaker decreases. It was.

As a technique for solving the above problem, there are techniques described in Patent Documents 1 and 2, for example. Patent Document 1 discloses a technique for reducing the tension associated with the 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 for vibrating a movable electrode in a form close to a flat plate by using a plurality of plate-like electrodes insulated from each other as a fixed electrode.
Moreover, in order to avoid the problem peculiar to the above-mentioned electrostatic type speaker, the electrodynamic type flat speaker is also developed (for example, refer patent document 3). However, the electrodynamic flat speaker has a problem that efficiency and responsiveness are poor as compared with the electrostatic speaker.

  Here, as a technique for solving the problem of sound pressure in an electrostatic speaker, for example, there is a technique described in Non-Patent Document 1. Non-Patent Document 1 discloses an electrostatic speaker having a so-called edgeless structure in which the outer periphery of a movable electrode is not fixed (supported).

Japanese Patent No. 3353531 Japanese Patent No. 3277498 Japanese Patent Publication No. 7-038758 Masanori Okazaki, 4 others, “Condenser speaker with diaphragm that vibrates piston in all bands and its application”, Acoustical Society of Japan 2004 Fall Meeting Presentation, Acoustical Society of Japan, September 2004, p. 563-564

By adopting the edgeless structure, the sound pressure of the electrostatic speaker can be improved. However, simply adopting the edgeless structure has a problem that the minimum resonance frequency of the speaker cannot be lowered.
The present invention has been made in view of the above circumstances, and provides an electrostatic speaker capable of lowering the lowest resonance frequency.

  In order to solve the above-described problem, the present invention has a vibration electrode that has a surface and vibrates in response to an input signal, a surface that faces the surface of the vibration electrode, and is fixedly spaced from the vibration electrode. A cushion insulating layer formed between an electrode and a surface of the vibration electrode and the surface of the fixed electrode, and having a predetermined stiffness and insulation, and a gap portion passing through a gap between the vibration electrode and the fixed electrode There is provided an electrostatic speaker having a cushion insulating layer having

In a preferred aspect, in the electrostatic speaker, the cushion insulating layer may be formed by a plurality of cushion materials arranged with a gap therebetween, and the gap between the cushion materials may be the gap portion. .
In this aspect, at least the individual stiffness of each cushion material, the shape of each cushion material, and the arrangement of each cushion material are determined so that the stiffness of each of the plurality of cushion materials becomes a predetermined value. May be.

  In another preferable aspect, in the electrostatic speaker, the cushion insulating layer is formed of one or a plurality of cushion materials, and the one or the plurality of cushion materials penetrates a gap between the vibration electrode and the fixed electrode. A hole may be formed.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a diagram showing a 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, 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 vibration film 30 functions as a vibration 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.

  The fixed electrode 10 and the fixed electrode 50 are planar electrodes. The fixed electrode 10 and the fixed electrode 50 are formed of a conductive material having a structure that transmits sound waves, such as punching metal, for example. Alternatively, the fixed electrode 10 and the fixed electrode 50 may be formed of a nonwoven fabric obtained by depositing a conductive material such as metal by sputtering. Further alternatively, the fixed electrode 10 and the fixed electrode 50 may be formed of a nonwoven fabric coated with a conductive dye. In short, the fixed electrode 10 and the fixed electrode 50 may be formed using any material as long as the material has both conductivity and sound wave transmission.

  The vibration film 30 is 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), PP (polypropylene, polypropylene), or polyester. It is formed by. Alternatively, the vibration film 30 may be formed of a material obtained by applying a conductive dye to a film. The vibration film 30 is sandwiched between the cushion layer 20 and the cushion layer 40. The vibration film 30 is supported by the cushion layer 20 and the cushion layer 40. In the prior art, the outer peripheral portion of the diaphragm is fixed to the fixed electrode, that is, tension is applied to the diaphragm. 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.

  A bias voltage is supplied to the fixed electrode 10 from a power source (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 membrane 30. When a potential difference is generated between the fixed electrode 10 and the vibrating membrane 30 due to the applied voltage, an electrostatic force acts on the vibrating membrane 30. That is, the vibration film 30 is displaced, that is, vibrates according to the input signal. Sound corresponding to the vibration state (frequency, amplitude, phase, etc.) is generated from the vibration film 30. In the present embodiment, since the speaker unit 1 is a push-pull type electrostatic speaker, an application voltage having an opposite phase is supplied to the fixed electrode 50. By supplying a signal having an antiphase, an electrostatic force having the same direction and magnitude as the electrostatic force acting between the fixed electrode 10 and the vibrating membrane 30 acts between the fixed electrode 50 and the vibrating membrane 30. That is, in the push-pull type electrostatic speaker, the electrostatic force acts twice on the vibration film 30 as compared with the single type.

  The spacer 60 has a function of fixing the fixed electrode 10 and the fixed electrode 50 to each other and insulating them. The spacer 60 is formed of an insulating material such as vinyl chloride, acrylic (methyl methacrylate), or rubber. The spacer 60 is fixed to the fixed electrode 10 and the fixed electrode 50 using an adhesive or the like.

ここで、Ｃ_０は固定電極１０（固定電極５０）および振動膜３０により形成されるコンデンサの静電容量である。固定電極１０（固定電極５０）と振動膜３０との間に直流電界および入力信号が加えられない状態における固定電極１０（固定電極５０）と振動膜３０との距離をｄ、直流電界および入力信号が加えられたときの振動膜３０の変位をｘ_０とすると、Ｃ_０は次式（２）のように表される。

The cushion layer 20 and the cushion layer 40 are for making the frequency characteristics of the speaker unit 1 desired characteristics. 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, a linear elastic modulus (Young's modulus) in the thickness direction). The elastic modulus of the cushion layer 20 and the cushion layer 40 is determined as follows. For example, as described in Shinji Sakamoto, “Speaker and Speaker System”, Nikkan Kogyo Shimbun, 1967, the minimum resonance frequency f ₀ of the system is expressed by the following equation (1).

Here, C ₀ is the capacitance of the capacitor formed by the fixed electrode 10 (fixed electrode 50) and the vibration film 30. The distance between the fixed electrode 10 (fixed electrode 50) and the vibrating membrane 30 in a state where no DC electric field and input signal are applied between the fixed electrode 10 (fixed electrode 50) and the vibrating membrane 30 is d, the DC electric field and the input signal. when x ₀ the displacement of the vibrating film 30 at the time when the added, C ₀ is expressed by the following equation (2).

E ₀ is a bias voltage applied to the fixed electrode 10 (fixed electrode 50), that is, a potential of the fixed electrode 10 (fixed electrode 50) from the common ground. ε is the dielectric constant of the cushion layer 20 (cushion layer 40). S is the area of the surface of the vibrating membrane 30 facing the fixed electrode 10 (fixed electrode 50). M _MD is the mass of the vibrating membrane 30. _MMA is the air additional mass (radiation mass) on one side of the vibrating membrane 30.

In addition, _{S M} is the equivalent stiffness of the system. As in the prior art, the electrostatic speaker of structure for fixing (supporting) the outer peripheral portion with respect to the fixed electrode of the vibrating film by applying a tension (tension) on the vibration film (diaphragm) is, S _M vibration Represents the tension applied to the membrane. However, in the structure not to apply tension to the vibrating film as in the present embodiment, it is not possible to use tension as S _M. Therefore, in the present embodiment, (a proportionality constant between an applied load and displacement, has dimensions of N / m) stiffness of the cushion layer 20 (cushioning layer 40) is used as the S _M. If the stiffness is determined, the elastic modulus of the cushion layer 20 (cushion layer 40) can be determined by considering the shape and thickness of the cushion layer 20 (cushion layer 40). As the elastic modulus, for example, Young's modulus can be used in a range where the stress-strain characteristic is linear, and a secant coefficient can be used in a range where the stress-strain characteristic is non-linear.

ここで、Ｓ_ＭＥ＝２εＳＥ_０ ^２／（ｄ＋ｘ_０）^３であり、コンデンサの負スティフネスを表す。このように、クッション材のスティフネスＳ_Ｍは、系の最低共振周波数ｆ_０の関数となる。式（３）に所望の最低共振周波数、すなわち最低共振周波数の設計値を代入すると、クッション層２０（クッション層４０）のスティフネスが得られる。こうして算出されたスティフネスを有するクッション材を用いることにより、所望の最低共振周波数を有する静電型スピーカを作製することができる。クッション層２０（クッション層４０）のスティフネスＳは、式（３）で算出されるスティフネスＳ_Ｍに対しあらかじめ決められた誤差範囲内に収まっていればよい。例えば、スピーカユニット１は、０．８≦（Ｓ／Ｓ_Ｍ）≦１．２を満たすスティフネスＳを有するクッション材を用いて作製されてもよい。 Solving equation (1) _{S M,} the following equation (3).

Here, S _ME = 2εSE ₀ ² / (d + x ₀ ) ³ , which represents the negative stiffness of the capacitor. Thus, the stiffness S _M of the cushion material is a function of the lowest resonance frequency f ₀ of the system. When the desired minimum resonance frequency, that is, the design value of the minimum resonance frequency, is substituted into Equation (3), the stiffness of the cushion layer 20 (cushion layer 40) can be obtained. By using the cushion material having the stiffness calculated in this way, an electrostatic speaker having a desired minimum resonance frequency can be manufactured. Stiffness S of the cushion layer 20 (cushioning layer 40) need only fall within the error range determined in advance with respect to the stiffness S _M calculated by Equation (3). For example, the speaker unit 1 may be manufactured using a cushion material having a stiffness S that satisfies 0.8 ≦ (S / S _M ) ≦ 1.2.

Drawing 2 is a figure showing a 1st embodiment of a cushion 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 shown in order to avoid complicated drawing. In the present embodiment, the cushion layer 20 includes 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 member 20-1 are fixed using, for example, an adhesive. When the stiffness of each cushion member 20-1 is S _M ⁱ , the stiffness S of the cushion layer 20, that is, the plurality of cushion members 20-1 as a whole, is S = ΣS _M ⁱ . In the present embodiment, S = ΣS _M ⁱ satisfies 0.8 ≦ (S / S _M ) ≦ 1.2. As described above, the cushion layer 20 is formed between the surface of the fixed electrode 10 and the surface of the vibration film 30. The cushion layer 20 has predetermined stiffness and insulation. Further, the cushion layer 20 has a gap that penetrates the gap between the vibration film 30 and the fixed electrode 10. In the present embodiment, the cushion layer 20 is formed by a plurality of cushion members 20-1 that are arranged with a gap therebetween. A gap between the cushion members 20-1 becomes a gap that penetrates the gap between the vibrating membrane 30 and the fixed electrode 10. Further, each of the cushion members 20-1 has at least the individual stiffness of each cushion material, the shape of each cushion material, and the arrangement of each cushion material so that the stiffness of the entire assembly becomes a predetermined value. .

Thus, a plurality of cushion members are used as the cushion layer for the following reason. Stiffness can be obtained, for example, by dividing the Young's modulus of the cushion layer by the thickness of the cushion layer and then multiplying by the area of the cushion 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 when producing a large-sized electrostatic speaker unit, an increase in the stiffness of the cushion layer can be suppressed. That is, an electrostatic speaker unit having a low f ₀ characteristic can be manufactured.

  FIG. 3 is a diagram showing a second embodiment of the cushion 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 interval between the cushion members 20-1, it is desirable to use a vibration film 30 made of a harder material.

  FIG. 4 is a diagram showing a third embodiment of the cushion layer. In the present embodiment, the cushion layer 20 includes 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. Any solid such as a polygonal column, cylinder, sphere, or cone may be used. Further, the cushion members 20-2 may be arranged at random intervals.

  FIG. 5 is a diagram showing a fourth embodiment of the cushion layer. In the present embodiment, the cushion layer 20 is formed of a cushion material 20-3 having a plurality of holes H. The hole H penetrates from the front surface to the back surface of the cushion material 20-3. The cross-sectional shape of the hole H is not limited to a quadrangle. It may be a triangle or a polygon of pentagon or more. Alternatively, it may be circular or elliptical. Further, the holes H may be regularly arranged at regular intervals or randomly. In short, the cushion 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 in which the cushion layer 20 is formed of a single cushion material is shown in FIG. 5, the cushion layer 20 may be formed of a plurality of cushion materials each having a hole H. In this case, the gap between the hole H and each cushion material forms a void portion of the cushion layer 20. Moreover, the space | gap part which connects each hole H may be installed.

  2 to 5 are merely examples, and the shape of the cushion layer is not limited to this. In short, any shape may be used as long as the cushion layer has a stiffness that falls within a predetermined error range from the stiffness calculated by Equation (3). Moreover, although only the cushion layer 20 was demonstrated in FIGS. 2-5, it is the same also about this with respect to the cushion layer 40. FIG.

FIG. 6 is a diagram illustrating the frequency characteristics of the speaker unit 1. In the example of FIG. 6, the vibrating membrane 30 is a PET film having a thickness of 3 μm (micrometers) and deposited with Al having a thickness of 400 Å (angstroms). Further, as the fixed electrode 10 and the fixed electrode 50, punching metal having a hole area ratio of 41.9% is used. A voltage of 5.25 kV is used as the bias voltage, and a sine wave of 4000 Vpp is used as the input signal. In FIG. 6, the solid line shows the measured value of the frequency characteristics when a material having high stiffness (specifically 133329 N / m) is used as the cushion layer, and the dotted line shows low stiffness (specifically 28920 N / m) as the cushion layer. The measured value of the frequency characteristic when using this material is shown. By using the low stiffness cushion layer, the minimum resonance frequency f ₀ of the speaker unit 1 can be lowered.

To reduce the minimum resonance frequency f ₀ of the speaker unit 1, merely using the cushion material of low stiffness (low Young's modulus), intractability problems on workability and manufacturing a cushion material tends collapse occurs. However, according to the present embodiment, cushion materials having a somewhat high Young's modulus are discretely arranged. Therefore, it is possible to reduce the stiffness of the entire cushion layer while ensuring processability. This makes it possible to obtain a low minimum resonance frequency f ₀ speaker.

FIG. 7 is a diagram for comparing the calculated value of the frequency characteristic of the speaker unit 1 and the actually measured value. FIG. 7A shows a calculated value by simulation, and FIG. 7B shows an actually measured value by experiment. In particular, in the frequency region of 2 kHz or less, the calculated value and the actually measured value are in good agreement. Thus, according to the present embodiment, an electrostatic speaker having a desired minimum resonance frequency f ₀ can be manufactured.

In the above-described embodiment, the aspect in which the speaker unit 1 is a push-pull type 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 inside the square root of the equation (1) is not “2” but “1”. That is, when the number of fixed electrodes is n, the negative stiffness of the capacitor can be expressed as S _ME = nεSE ₀ ² / (d + x ₀ ) ³ . In the case of a push-pull type electrostatic speaker, S _ME = 2εSE ₀ ² / (d + x ₀ ) ³ , and in the case of a single type electrostatic speaker, S _ME = εSE ₀ ² / (d + x ₀ ) ³ .

In the above-described embodiment, the example in which the frequency characteristic of the speaker is controlled by designing the stiffness of the cushion material to an appropriate value has been described. 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 cushion material is higher than that of air. Thus, by using a higher dielectric constant cushion, it is possible to improve the electrostatic driving force while reducing the f _0.

It is a figure which shows the structure of the speaker unit 1 which concerns on one Embodiment of this 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. It is a figure which illustrates the frequency characteristic of the speaker unit. It is a figure which compares the calculated value and the measured value of the frequency characteristic of the speaker unit.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Speaker unit, 20-1, 20-2 ... Cushion member, 10 ... Fixed electrode, 20 ... Cushion layer, 30 ... Vibration film, 40 ... Cushion layer, 50 ... Fixed electrode, 60 ... Spacer

Claims (4)

A vibrating electrode having a surface and vibrating in response to an input signal;
A fixed electrode having a surface facing the surface of the vibration electrode and spaced from the vibration electrode;
A cushion insulating layer formed between the surface of the vibrating electrode and the surface of the fixed electrode, having a predetermined stiffness and insulation, and having a gap that penetrates the gap between the vibrating electrode and the fixed electrode possess an insulating layer,
The cross-sectional shape of the gap is constant in a direction perpendicular to the surface of the fixed electrode.
An electrostatic speaker characterized by that .   2. The electrostatic type according to claim 1, wherein the cushion insulating layer is formed of a plurality of cushion materials arranged with a gap therebetween, and a gap between the cushion materials becomes the gap portion. Speaker.   Each of the plurality of cushion materials is determined such that at least the individual stiffness of each cushion material, the shape of each cushion material, and the arrangement of each cushion material are determined so that the stiffness of the entire assembly becomes a predetermined value. The electrostatic speaker according to claim 2, wherein The cushion insulating layer is formed of one or more cushion materials,
The electrostatic speaker according to claim 1, wherein a hole penetrating a gap between the vibration electrode and the fixed electrode is formed in the one or more cushion materials.

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