JP2001339796A - Condenser microphone - Google Patents
Condenser microphoneInfo
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- JP2001339796A JP2001339796A JP2000158956A JP2000158956A JP2001339796A JP 2001339796 A JP2001339796 A JP 2001339796A JP 2000158956 A JP2000158956 A JP 2000158956A JP 2000158956 A JP2000158956 A JP 2000158956A JP 2001339796 A JP2001339796 A JP 2001339796A
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- microphone
- oscillation
- condenser
- capacitor
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- Electrostatic, Electromagnetic, Magneto- Strictive, And Variable-Resistance Transducers (AREA)
Abstract
Description
【０００１】[0001]
【発明の属する技術分野】この発明は、半導体製造技術
を駆使して作製するコンデンサ型マイクロフォンに関す
る。BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a condenser microphone manufactured by making full use of semiconductor manufacturing technology.
【０００２】[0002]
【従来の技術】半導体製造技術を駆使したマイクロマシ
ン加工技術を利用して作製するコンデンサ型小型マイク
ロフォン（以下、ＩＣマイクロフォン）はセンシング部
分、初段増幅回路を一体製造できるため品質安定性が高
く、小型、軽量、量産に適している。2. Description of the Related Art Capacitor-type small microphones (hereinafter referred to as IC microphones), which are manufactured using a micromachining technology utilizing semiconductor manufacturing technology, are capable of integrally manufacturing a sensing part and a first-stage amplifier circuit. Light weight, suitable for mass production.
【０００３】図１にＩＣマイクロフォンの構造と信号の
取り出し方の例を示した（参考文献１）。FIG. 1 shows an example of a structure of an IC microphone and a method of extracting a signal (Reference Document 1).
【０００４】ＩＣマイクロフォンにおいて音を検知する
部分は、薄く平坦な振動電極１と背電極２で構成される
コンデンサである。３は背電極２と電気的に接続する接
続配線（コンタクト）、４はケースである。The part of the IC microphone that detects sound is a capacitor composed of a thin and flat vibrating electrode 1 and a back electrode 2. Reference numeral 3 denotes a connection wiring (contact) electrically connected to the back electrode 2, and 4 denotes a case.
【０００５】音波による音圧変化で生じる振動電極１の
変位を、コンデンサの容量変化として検出する。このマ
イクから取り出せる信号電圧ΔVは、バイアス電圧を
Vb、蓄えられる電荷をq、コンデンサの容量をC、振動電
極−背電極間の距離をs、音圧によって生じるCの変化、
sの変化をそれぞれΔC、Δs、とすると、 ΔV＝−q（ΔC／C2）＝（Δs／s）Vb……（１） で与えられる（参考文献２）。The displacement of the vibrating electrode 1 caused by a change in sound pressure due to a sound wave is detected as a change in the capacitance of a capacitor. The signal voltage ΔV that can be extracted from this microphone
V b , the stored charge is q, the capacitance of the capacitor is C, the distance between the vibrating electrode and the back electrode is s, the change in C caused by sound pressure,
Assuming that changes in s are ΔC and Δs, respectively, ΔV = −q (ΔC / C 2 ) = (Δs / s) V b (1) is given (reference document 2).
【０００６】この電圧変化を電極から読み出していた。This voltage change is read from the electrode.
【０００７】[0007]
【発明が解決しようとする課題】ＩＣマイクロフォンの
感度を向上させるために従来行われてきた方策は、
（１）式より、 Vbを大きくする。 sを小さくする。 Δsが大きくなるように振動電極の材料に柔らかいも
のを使用するなどし、音圧に対する変形を大きくする。
すなわち振動電極のスチフネスを小さくする。 ΔCを大きくするために、電極の面積を大きくする。 というものであった。The measures that have been taken to improve the sensitivity of an IC microphone are as follows.
According to equation (1), V b is increased. Make s smaller. Use a soft material for the vibrating electrode so as to increase Δs, and increase the deformation with respect to the sound pressure.
That is, the stiffness of the vibrating electrode is reduced. In order to increase ΔC, the area of the electrode is increased. It was that.
【０００８】しかし、コンデンサ型マイクロフォンにお
いては、振動電極と背電極が静電引力で貼り付かないよ
うにする必要がある。両極を貼り付かずに保持するため
の指標として、次式の安定度μが定義されている（参考
文献３）。However, in the condenser microphone, it is necessary to prevent the vibrating electrode and the back electrode from sticking due to electrostatic attraction. As an index for holding both poles without sticking, the stability μ in the following equation is defined (Reference Document 3).
【０００９】[0009]
【数１】 (Equation 1)
【００１０】式（２）で、sm：振動電極のスチフネス、
εa：空気の誘電率、S：背電極の面積であり、通常μ＞
７となるように設計される。[0010] In formula (2), s m: the vibrating electrode stiffness,
ε a : dielectric constant of air, S: area of back electrode, usually μ>
7 is designed.
【００１１】式（２）から、感度を向上させるための
〜の方策は安定度の向上と相反するものである。特に
従来のＩＣマイクロフォンにおいては、振動電極と背電
極との間隔sが数μｍと極めて小さいため、高感度化に
は限界があった。From the equation (2), the measure (1) for improving the sensitivity is contrary to the improvement of the stability. In particular, in the conventional IC microphone, since the interval s between the vibrating electrode and the back electrode is as small as several μm, there is a limit in increasing the sensitivity.
【００１２】本発明の目的は以上のような問題を解消し
たコンデンサ型マイクロフォンを提供することにある。An object of the present invention is to provide a condenser microphone which has solved the above problems.
【００１３】[0013]
【課題を解決するための手段】請求項１の発明は、音圧
変化に応答して容量が変化するコンデンサと、前記コン
デンサを容量の一部として持つＬＣ回路と、前記コンデ
ンサの容量変化を前記ＬＣ回路の発振周波数の変化とし
て読み出す出力手段とを具えたことを特徴とする。According to a first aspect of the present invention, there is provided a capacitor having a capacitance that changes in response to a change in sound pressure, an LC circuit having the capacitor as a part of the capacitance, and a capacitance change of the capacitor. Output means for reading as a change in the oscillation frequency of the LC circuit.
【００１４】請求項２の発明は、請求項１において、前
記出力手段は、前記コンデンサの音圧による容量変化に
応答して、搬送波をＦＭ変調して送信する手段を有する
ことを特徴とする。According to a second aspect of the present invention, in the first aspect, the output means has a means for FM-modulating and transmitting a carrier in response to a change in capacitance due to the sound pressure of the capacitor.
【００１５】請求項３の発明は、請求項２において、前
記コンデンサは、前記出力手段の少なくとも一部と同一
基板上に形成したことを特徴とする。According to a third aspect of the present invention, in the second aspect, the capacitor is formed on the same substrate as at least a part of the output means.
【００１６】[0016]
【発明の実施の形態】本発明では、コンデンサ型マイク
ロフォンにおける音圧によるコンデンサの容量の変化
を、このマイクをC（容量）として持つ、ＬＣ発振回路
（コイル及びコンデンサによって発振周波数が決定され
る発振回路）の発振周波数変化として検出した。この方
法によれば、コンデンサ部にバイアス電圧を印加する必
要がないため、高感度化の方策を選ぶ自由度が増す。さ
らに、検出した発振周波数変化を搬送周波数のＦＭ変調
とみなして送信し、それを受信した側で復調して音声信
号を得れば、ワイアレスマイクを構成することができ
る。この場合、コンデンサ部には、発振回路が動作する
のに必要な電圧だけが印加されることになり、通常のＩ
Ｃマイクロフォンのバイアス電圧に比べて非常に低い電
圧ですむため、式（２）の安定度を大きく取ることが可
能で、振動電極と背電極とが貼り付くという問題が生じ
にくくなる。DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, a change in capacitance of a capacitor due to sound pressure in a condenser microphone is determined by using an LC oscillation circuit (oscillation whose oscillation frequency is determined by a coil and a capacitor) having this microphone as C (capacity). Circuit). According to this method, since there is no need to apply a bias voltage to the capacitor section, the degree of freedom in selecting a measure for increasing the sensitivity is increased. Further, if the detected oscillation frequency change is regarded as FM modulation of the carrier frequency and transmitted, and the received signal is demodulated to obtain an audio signal, a wireless microphone can be configured. In this case, only the voltage necessary for the operation of the oscillation circuit is applied to the capacitor section, and the normal I
Since only a very low voltage is required as compared with the bias voltage of the C microphone, the stability of equation (2) can be increased, and the problem that the vibrating electrode and the back electrode stick to each other is less likely to occur.
【００１７】一般的なＬＣ発振回路の発振周波数fは、
コイルのインダクタンスをLとすると次式で定まる。The oscillation frequency f of a general LC oscillation circuit is
If the inductance of the coil is L, it is determined by the following equation.
【００１８】[0018]
【数２】 (Equation 2)
【００１９】音圧によって生じるfの変化をΔfとする
と、Assuming that the change in f caused by the sound pressure is Δf,
【００２０】[0020]
【数３】 (Equation 3)
【００２１】であるから、感度を向上させる為に周波数
変位Δfを大きくする方法として、前述のからの方
策が有効である。Therefore, as a method of increasing the frequency displacement Δf in order to improve the sensitivity, the above-described measures are effective.
【００２２】従来のコンデンサ型ＩＣマイクロフォンで
は、少なくとも数Ｖ、実用的な感度を得るのに数Ｖのバ
イアス電圧Vbを必要としている（参考文献４）。The conventional condenser type IC microphone requires a bias voltage Vb of at least several volts, and several volts to obtain practical sensitivity (Reference 4).
【００２３】一方、本発明のＩＣマイクロホンでは、１
〜２Ｖで動作する発振回路を用いれば、式（２）から、
従来のＩＣマイクロフォンと比べ安定度を数倍から数百
倍にできる。これにより、高感度化の方策を選ぶ自由度
も大きく増す。On the other hand, in the IC microphone of the present invention, 1
If an oscillation circuit operating at up to 2 V is used, from equation (2),
The stability can be increased several times to several hundred times as compared with the conventional IC microphone. This greatly increases the degree of freedom in selecting a high-sensitivity measure.
【００２４】図２に、コンデンサの容量変化を周波数変
化として読み出すようにした本発明の１実施形態の構成
を示した。１は振動電極、２は背電極であって、これら
は、ＩＣマイクを構成する（詳細な構造は図３を参
照）。振動電極１が音圧に応答して振動することによっ
て、振動電極１と背電極２との間の距離が変化し、振動
電極１と背電極２との間の容量が変化する。５は発振／
変調回路であって、接続配線３を介して振動電極１およ
び背電極２と接続することによって構成されるＬＣ発振
回路を有する。このＬＣ発振回路の発振出力信号はアン
テナ（コイルアンテナ）６に供給され、そこから送信さ
れる。このＬＣ発振回路の発振周波数は式（３）によっ
て決定される。FIG. 2 shows a configuration of an embodiment of the present invention in which a change in capacitance of a capacitor is read out as a change in frequency. 1 is a vibrating electrode, 2 is a back electrode, and these constitute an IC microphone (refer to FIG. 3 for a detailed structure). When the vibration electrode 1 vibrates in response to the sound pressure, the distance between the vibration electrode 1 and the back electrode 2 changes, and the capacitance between the vibration electrode 1 and the back electrode 2 changes. 5 is oscillation /
The modulation circuit includes an LC oscillation circuit configured to be connected to the vibration electrode 1 and the back electrode 2 via the connection wiring 3. The oscillation output signal of this LC oscillation circuit is supplied to an antenna (coil antenna) 6 and transmitted therefrom. The oscillation frequency of this LC oscillation circuit is determined by equation (3).
【００２５】したがって、音圧の変化に応答して振動電
極１と背電極２との間の容量が変化することによって、
発振／変調回路５のＬＣ発振回路の発振周波数が変化す
る。すなわち、ＬＣ発振回路の発振出力信号は音圧の変
化に応答してＦＭ変調され、アンテナ６から送信され
る。Therefore, the capacitance between the vibration electrode 1 and the back electrode 2 changes in response to the change in the sound pressure.
The oscillation frequency of the LC oscillation circuit of the oscillation / modulation circuit 5 changes. That is, the oscillation output signal of the LC oscillation circuit is FM-modulated in response to a change in sound pressure, and transmitted from the antenna 6.
【００２６】図３は、ＩＣマイクロフォンの具体的な構
造を示すものであって、（ａ）は、斜視図、（ｂ）は断
面図である。図示されるＩＣマイクロフォンは公知の技
術（参考文献１）を用いて作製した。基板７上には、振
動電極１、背電極２、接続配線３、発振／変調回路５お
よびコイルアンテナ６が形成され、背電極２はケース４
によってカバーされている。FIGS. 3A and 3B show a specific structure of the IC microphone. FIG. 3A is a perspective view, and FIG. 3B is a sectional view. The illustrated IC microphone was manufactured using a known technique (Reference Document 1). A vibrating electrode 1, a back electrode 2, a connection wiring 3, an oscillation / modulation circuit 5 and a coil antenna 6 are formed on a substrate 7.
Covered by
【００２７】以上のように、コンデンサを構成する振動
電極１、背電極２間にバイアス電圧を印加する必要がな
いため、高感度化の方策を選ぶ自由度が増す。さらに、
振動電極１、背電極２間の容量変化によって検出したＬ
Ｃ発振回路の発振周波数変化を搬送周波数のＦＭ変調と
みなして送信し、それを受信した側で復調して音声信号
を得るワイアレスマイクを構成することができ、このよ
うなワイアレスマイクにおいては、コンデンサ部には、
発振／変調回路が動作するのに必要な電圧だけが印加さ
れることになり、通常のＩＣマイクロフォンのように大
きなバイアス電圧を印加する必要がないため、振動電極
と背電極とが貼り付くという問題が生じにくくなる。As described above, since there is no need to apply a bias voltage between the vibrating electrode 1 and the back electrode 2 constituting the capacitor, the degree of freedom in selecting a measure for increasing the sensitivity is increased. further,
L detected by a change in capacitance between the vibrating electrode 1 and the back electrode 2
A change in the oscillation frequency of the C oscillation circuit is regarded as FM modulation of the carrier frequency, the signal is transmitted, and a wireless microphone that obtains an audio signal by demodulating the signal on the receiving side can be configured. In such a wireless microphone, a capacitor is provided. In the department,
Since only the voltage necessary for the operation of the oscillation / modulation circuit is applied, and there is no need to apply a large bias voltage as in a normal IC microphone, there is a problem that the vibration electrode and the back electrode are stuck. Is less likely to occur.
【００２８】（参考文献） 参考文献１：Ｊ．Ｂｅｒｇｖｉｓｔ．Ｆ．Ｒｕｄｏｌ
ｆ，“Ａ Ｓｉｌｉｃｏｎｃｏｎｄｅｎｓｅｒ ｍｉｃ
ｒｏｐｈｏｎｅ ｕｓｉｎｇ ｂｏｎｄ ａｎｄｅｔｃ
ｈ−ｂａｃｋ ｔｅｃｈｎｏｌｏｇｙ” Ｓｅｎｓｏｒ
ｓ ａｎｄ Ａｃｔｕａｔｏｒｓ Ａ４５，ｐｐ．１１
５−１２４（１９９４）． 参考文献２：Ｄｅｎｎｉｓ Ｓ． Ｇｒｅｙｗａｌｌ，
“Ｍｉｃｒｏｍａｃｈｉｎｅｄ ｏｐｔｉｃａｌ−ｉｎ
ｔｅｒｆｅｒｅｎｃｅ ｍｉｃｒｏｐｈｏｎｅ”，Ｓｅ
ｎｓｏｒｓ ａｎｄ Ａｃｔｕａｔｏｒｓ Ａ７５，ｐ
ｐ２５７−２６８（１９９９）． 参考文献３：溝口章夫，「指向性コンデンサ・マイクロ
ホンの小型化の設計」，日本音響学会誌 ３１巻１０
号，ｐｐ５９３−６０１（１９７５）． 参考文献４：Ｇｅｒｈａｒｄ Ｍ． Ｓｅｓｓｌｅｒ，
“ＮＥＷ ＡＣＯＵＳＴＩＣ ＳＥＮＳＯＲＳ”，Ｐｒ
ｏｃ．１５ｔｈ ＩｎｔｅｒｎａｔｉｏｎａｌＣｏｎｇ
ｒｅｓｓ ｏｎ Ａｃｏｕｓｔｉｃｓ，ｐｐ．２５３−
２６０（１９９５）．(References) Reference 1: J.I. Bergvist. F. Rudol
f, "A Siliconcondenser mic
ropone using bond andtec
h-back technology ”Sensor
s and Actors A45, pp. 1-35. 11
5-124 (1994). Reference 2: Dennis S. et al. Graywall,
“Micromachined optical-in
terreference microphone ”, Se
nors and Actuators A75, p
p257-268 (1999). Reference 3: Akio Mizoguchi, "Design of miniaturization of directional condenser microphone", Journal of the Acoustical Society of Japan, Vol. 31, 10
No., pp 593-601 (1975). Reference 4: Gerhard M. Sessler,
“NEW ACOUSTIC SENSORS”, Pr
oc. 15th InternationalCong
less on Acoustics, pp. 253-
260 (1995).
【００２９】[0029]
【発明の効果】以上説明したように本発明によれば、高
感度なコンデンサ型マイクロフォンを提供することがで
きる。また、ワイヤレス化も容易である。As described above, according to the present invention, a highly sensitive condenser microphone can be provided. In addition, wireless communication is easy.
【図１】従来のＩＣマイクロフォンの構造と信号の取り
出し方を示す図である。FIG. 1 is a diagram showing the structure of a conventional IC microphone and how to extract a signal.
【図２】コンデンサの容量変化を周波数変化として読み
出すようにした本発明の１実施形態の構成を示図であ
る。FIG. 2 is a diagram showing a configuration of an embodiment of the present invention in which a change in capacitance of a capacitor is read out as a change in frequency.
【図３】ＩＣマイクロフォンの具体的な構造を示すもの
であって、（ａ）は本発明によるワイアレスマイクの斜
視図、（ｂ）は同断面図である。3A and 3B show a specific structure of an IC microphone, wherein FIG. 3A is a perspective view of a wireless microphone according to the present invention, and FIG. 3B is a sectional view of the same.
１ 振動電極 ２ 背電極 ３ 接続配線 ４ ケース ５ 発振／変調回路 ６ コイルアンテナ ７ 基板 DESCRIPTION OF SYMBOLS 1 Vibration electrode 2 Back electrode 3 Connection wiring 4 Case 5 Oscillation / modulation circuit 6 Coil antenna 7 Substrate
───────────────────────────────────────────────────── フロントページの続き (72)発明者 田島 利文 東京都世田谷区砧一丁目10番11号 日本放 送協会 放送技術研究所内 (72)発明者 斎藤 信雄 東京都世田谷区砧一丁目10番11号 日本放 送協会 放送技術研究所内 (72)発明者 庄田 清武 東京都世田谷区砧一丁目10番11号 日本放 送協会 放送技術研究所内 (72)発明者 江刺 正喜 宮城県仙台市太白区八木山南一丁目11−９ Ｆターム(参考） 5D021 CC15 CC19 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Toshifumi Tajima 1-10-11 Kinuta, Setagaya-ku, Tokyo Japan Broadcasting Corporation Research Institute (72) Inventor Nobuo Saito 1-10-11 Kinuta, Setagaya-ku, Tokyo No. Japan Broadcasting Corporation Broadcasting Research Institute (72) Inventor Kiyotake Shoda 1-10-11 Kinuta, Setagaya-ku, Tokyo Inside Japan Broadcasting Corporation Broadcasting Research Institute (72) Inventor Masaki Esashi Yagiyama Minami, Taihaku-ku, Sendai City, Miyagi Prefecture 1-Chome 11-9 F-term (reference) 5D021 CC15 CC19
Claims (3)
デンサと、前記コンデンサを容量の一部として持つＬＣ
回路と、前記コンデンサの容量変化を前記ＬＣ回路の発
振周波数の変化として読み出す出力手段とを具えたこと
を特徴とするコンデンサ型マイクロフォン。1. A capacitor having a capacitance that changes in response to a change in sound pressure, and an LC having the capacitor as a part of the capacitance.
A capacitor microphone comprising: a circuit; and output means for reading a change in the capacitance of the capacitor as a change in the oscillation frequency of the LC circuit.
に応答して、搬送波をＦＭ変調して送信する手段を有す
ることを特徴とするコンデンサ型マイクロフォン。2. The condenser microphone according to claim 1, wherein the output unit includes a unit that modulates and transmits a carrier wave in response to a change in capacitance due to a sound pressure of the capacitor.
一基板上に形成したことを特徴とするコンデンサ型マイ
クロフォン。3. The condenser microphone according to claim 2, wherein the capacitor is formed on the same substrate as at least a part of the output unit.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2000158956A JP2001339796A (en) | 2000-05-29 | 2000-05-29 | Condenser microphone |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2000158956A JP2001339796A (en) | 2000-05-29 | 2000-05-29 | Condenser microphone |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| JP2001339796A true JP2001339796A (en) | 2001-12-07 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP2000158956A Pending JP2001339796A (en) | 2000-05-29 | 2000-05-29 | Condenser microphone |
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Cited By (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2008510378A (en) * | 2004-08-11 | 2008-04-03 | クゥアルコム・インコーポレイテッド | Integrated audio codec with silicon audio transducer |
| EP1974832A2 (en) | 2007-03-29 | 2008-10-01 | Olympus Medical Systems Corp. | Capacitive micromachined ultrasonic transducer (cMUT) device and in-body-cavity diagnostic ultrasound system |
| JP2009022055A (en) * | 2008-10-27 | 2009-01-29 | Panasonic Corp | Electret capacitor |
| JP2009515443A (en) * | 2005-11-10 | 2009-04-09 | エプコス アクチエンゲゼルシャフト | MEMS microphone, method for manufacturing MEMS microphone, and method for incorporating MEMS microphone |
| JP2009182755A (en) * | 2008-01-31 | 2009-08-13 | Audio Technica Corp | Condenser microphone unit and fixed pole thereof |
| US8169041B2 (en) | 2005-11-10 | 2012-05-01 | Epcos Ag | MEMS package and method for the production thereof |
| US8184845B2 (en) | 2005-02-24 | 2012-05-22 | Epcos Ag | Electrical module comprising a MEMS microphone |
| US8582788B2 (en) | 2005-02-24 | 2013-11-12 | Epcos Ag | MEMS microphone |
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2000
- 2000-05-29 JP JP2000158956A patent/JP2001339796A/en active Pending
Cited By (11)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2008510378A (en) * | 2004-08-11 | 2008-04-03 | クゥアルコム・インコーポレイテッド | Integrated audio codec with silicon audio transducer |
| US8184845B2 (en) | 2005-02-24 | 2012-05-22 | Epcos Ag | Electrical module comprising a MEMS microphone |
| US8582788B2 (en) | 2005-02-24 | 2013-11-12 | Epcos Ag | MEMS microphone |
| JP2009515443A (en) * | 2005-11-10 | 2009-04-09 | エプコス アクチエンゲゼルシャフト | MEMS microphone, method for manufacturing MEMS microphone, and method for incorporating MEMS microphone |
| US8169041B2 (en) | 2005-11-10 | 2012-05-01 | Epcos Ag | MEMS package and method for the production thereof |
| US8229139B2 (en) | 2005-11-10 | 2012-07-24 | Epcos Ag | MEMS microphone, production method and method for installing |
| US8432007B2 (en) | 2005-11-10 | 2013-04-30 | Epcos Ag | MEMS package and method for the production thereof |
| EP1974832A2 (en) | 2007-03-29 | 2008-10-01 | Olympus Medical Systems Corp. | Capacitive micromachined ultrasonic transducer (cMUT) device and in-body-cavity diagnostic ultrasound system |
| US8157740B2 (en) | 2007-03-29 | 2012-04-17 | Olympus Medical Systems Corp. | Capacitive micromachined ultrasonic transducer (cMUT) device and in-body-cavity diagnostic ultrasound system |
| JP2009182755A (en) * | 2008-01-31 | 2009-08-13 | Audio Technica Corp | Condenser microphone unit and fixed pole thereof |
| JP2009022055A (en) * | 2008-10-27 | 2009-01-29 | Panasonic Corp | Electret capacitor |
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