FR2530911A1 - ELECTROSTATIC TRANSDUCER, ESPECIALLY A CONDENSER MICROPHONE - Google Patents

Abstract

AN ELECTROSTATIC TRANSDUCER ACCORDING TO THE INVENTION, CONSTITUTING IN PARTICULAR A CONDENSER MICROPHONE, HAS A DIAPHRAGM AND A COUNTER-ELECTRODE, ALSO CALLED BASE OR PLATE. THE COUNTER-ELECTRODE 11 AFFECTS THE SHAPE OF A THIN PLATE, THE SURROUND OF WHICH SHOWS SLOTS 11A, 17, 19A, 20A, 21A, 22A. WITH THIS FORM OF COUNTER-ELECTRODE, THE USEFUL EFFICIENCY CAPACITY IS ABOUT 70 AND THE PARASITIC CAPACITY IS ABOUT 30 OF THE TOTAL CAPACITY.

Description

The subject of the invention is an electro-transducer

  state, in particular a condenser microphone, having a

  electrode diaphragm and a counter-electrode, also called

base or plate.

  In a condenser microphone of this type, the counter-electrode, placed at a distance of 30 to 50 μm behind the diaphragm, constitutes the rigid and immobile electrode of the microphone and is most often in the form of a conductive flat disk. This disc is provided with a relatively large number of 2 holes or slots to couple the very narrow and small air chamber between the diaphragm and the counter electrode, or to other

  acoustically active means, inside the housing of the micro-

  as is necessary for directional microphones, to the external acoustic field through acoustic means coupled thereto. Such cross-electrodes are described, for example,

  in the German patent applications DE-AS 14 37 420 and DE-OS 24 30 068.

  In electrostatic transducers, particularly

  in condenser microphones, the capacitance of condensa-

  is essentially determined by the surface area of the counterelectrode and the electrically conductive diaphragm in front of it and by the distance between them. However, when the diaphragm receives a sound, only a part of the total capac- ity is useful for the signaling transmission. remaining effective capacity, called parasitic capacity, which is mainly due to fixed elements specific to the construction, leads to the degradation of the transmission factor and the signal-to-noise ratio, as well as the aggravation of the non-linear distortions. It is therefore necessary that the ratio of useful capacity to parasitic capacity should be kept as high

as possible.

  The counter-electrodes known until now, with a diameter of more than 15 mm, still make it possible to maintain this ratio sufficiently high so that the negative effects such as those indicated in FIG.

  above remain within acceptable limits.

  However, miniaturization, which is also progressing

  in the field of condenser microphones, is now

  Whereas the diaphragm and the counter electrode have a diameter

  less than 7 am This results in the problem that the capacity to

  The unit becomes very large in relation to the effective capacity.

  thus with a challenge of the electrodes 3 e 58 mu -c: no distance

  between diaphragm and counteractivity of 4 '9, for example, 1

  of the center-electrode of a caracteristic capacity

  of 3 4 p F and a useful capillary capazite of 3 p F.

  The purpose of the intention is to find for the counter-

  electrode a form with which the effective effective capacity repre-

  It is approximately 70% and the parasite capacity is approximately 0 to 30% of the total capacity. According to the invention, this result is obtained by providing peripheral recesses or crenellations on a platelet-shaped counter electrode. As a result of the sustenance at its perimeter protruding portions, located between the crenellations and having a certeine red semblance with the branches

  of a star, such a counter electrode has a capacitance

  1.3 pF and a useful capacity of 3 pF for a total diameter of 5.8 mani The useful capacity / pest capacity ratio is thus brought back to a favorable value. Above all, thanks to this new

  shape of the counter-electrode, the sensitivity of the condenser microphone

  It increases by 30% compared to a counter-electrode

  of conventional shape with a total diameter of less than 7 minutes.

  The slot-shaped recesses on the periphery of the counter-electrode form a rearward sweep for the air contained in the narrow and small air chamber between the diaphragm and the counter-electrode. The function of the piercings or slots in a conventional counter electrode is provided by the slots in cooperation with the spacer ring between the counter electrode and the diaphragm, so that the coupling with other means of activity

  acoustic, such as friction, elasticity or iner-

  acoustics, as well as the connection with the acoustic field n necessary for directional microphones, are established through

  slots formed between the spacer ring and the crenellations.

  The manufacture by cutting of a platelet-shaped counter-electrode according to the invention, with the slots on the

  around, especially if its shape resembles a star, is particularly

  relatively simple and economical compared to the manufacture of a

2 530911

  counter-electrode of conventional type Indeed, when the diameter of the counter-electrode becomes smaller than 7 mm, the known execution

  poses great difficulties as regards the choice of a di-

  hole meter having a suitable ratio to the thickness of the

  In many cases, it is necessary to resort to methods of attack which are much more expensive than cutting. The very difficult cutting of holes or cracks in particular synthetic materials, for example

  in "Teflon" 'for the realization of transverse counter-electrodes

  formable in electrets, is no longer necessary with the new form of the counter electrode, because the slots around the periphery of the

  wafer can be formed easily by cutting.

  As the parasitic capacity only acts in the

  edge of the electrode, it is advantageous to give aax

  a surface area so large as to reduce the para-

  According to another characteristic of the invention, the total area of the recesses or crenellations represents up to% of the remaining surface area of the electrode. Thus, the useful capacity / parasitic capacitance ratio actually takes a value

  necessary for the realization of a condenser microphone of high-

  thanks to its high sensitivity, its good transmission factor

  its good signal-to-noise ratio and its low nonlinear distortions. The surface of the counter-electrode-delimited by the slots may have the shape of not only a circle but also that of a polygon, an oval or an ellipse However, for practical reasons, we will generally use the circular shape for the manufacture of a condenser microphone There are nevertheless special applications, such as hearing aids for the hearing impaired, where other forms are advantageous, so that the counter-electrode, so also the diaphragm of a condenser microphone according to the invention -, - may also have an outline

polygonal, oval or elliptical.

  For the different forms of current

  electrostatic transducer, for example with fine polarization voltage from outside the transducer or with polarization voltage produced inside the same

  transducer using an electret, the invention provides a counter-

  electrode which can be formed in a known manner of metal, of material

  conductive synthetic material or coated with metal or

  such as an electrolyte or metal covered with such material.

  Other features and advantages of the invention

  will become clearer in the following description of

  several nonlimiting exemplary embodiments, as well as the appended drawing, in which: FIGS. 1a to 1c are plan views of some conventional counter-electrodes

  Figure 2 is an axial section of a microphone.

  capacitor comprising a counter-electrode according to the invention

  Figure 3 is a plan view of a counter

  electrode according to the invention, circular in shape; and Figs. 4a to 4d are plan views showing various other embodiments of the counter-electrode according to the invention. The counter-electrodes 1, 2, 3 of FIGS. 1a-1c, having perforations, in particular holes, represent

  the known state of the art The holes 4 are generally dis-

  placed in a geometric pattern, while the slots 5 are arranged in a circle The air chamber of low height between the diaphragm and the counter-electrode can be coupled through the holes 4 as through the slots 5 to other means The dotted marginal area 6 of the counter-electrode contributes a large part to the parasitic capacitance The ring-shaped capacitor formed by this annular zone and the corresponding marginal zone of the diaphragm, situated in front and acting as an electrode, has a capacity that is greater than half of the total capacity of the capacitor formed by

  the diaphragm and the counter-electrode.

  FIG. 2 shows a condenser microphone comprising a counterelectrode according to the invention. It has a casing 7 whose front side has openings 7a of sound input. A distance behind these openings, the casing 7 contains a

  8 ring on which is stretched a diaphragm 9, a ring between-

  electrically insulating measuring board 10 and a counter electrode 11.

  The stack thus formed is pressed by a retaining spring 12 against an inner shoulder 7 bi the spring 12 bearing on a plate -5 bottom 13 The counter-electrode 11 in the form of a wafer has slots lla along its periphery Through the slots 14a, these slots allow the coupling of the air chamber 14 to

  acoustic means behind the electrode 1.

  Figure 3 shows the execution that will be

  the most important of a counter electrode according to the invantion.

  This counter-electrode is designated 15 and has a circular shape with radially projecting portions 16 which have a

  certain resemblance to the branches of a star and which are deli-

  The electrode areas determining the formation of the parasitic capacitance are located on the projecting portions 16, where they form dashed areas. These zones are so small, with respect to the surface area. of the electrode, that the parasitic capacity represents only one third of the total capacity, which gives a favorable

  useful effective capacity and parasitic capacity.

  FIGS. 4 to 4 d show other possible embodiments of a counter-electrode according to the invention in the form of a wafer The peripheries 19 and 20 of the embodiments according to FIGS. 4 a and 4 b correspond roughly to a polygon, while the embodiment according to FIG. 4c has an oval periphery 21 and that of FIG. 4 an elliptical periphery 22. The crenellated recesses made along the electrode edge are designated

  respectively by 19a, 20a, 21a and 22a.

Claims (4)

REVENDICATIIONS   1 Electrostatic transducer, in particular micro-   condenser phone, having an electrode diaphragm and a counterelectrode, also called a base or plate, characterized in that the counter-electrode (11) has the shape of a wafer whose periphery has recesses or crenellations (1a, 17, l9 a, a, 21a, 22a).   2 transducer according to claim 1, characterized in that the total area of the slots represents up to 70% of   the remaining surface area of the counter-electrode.   Transducer according to claim 1 or 2, characterized   characterized in that the surface of the counter-electrode delimited by the crenels (la, 17, 19a, 20a, 21a, 22a) has essentially the shape of a circle, an ova, a ellipse or a regular polygon, in particular a square, a rectangle or a hexagon (FIGS. 4).   4 Transducer according to any one of the claims   1 to 3, characterized in that the counterelectrode (11) is formed in a manner known per se of metal, a metal-coated synthetic material, an electrically conductive synthetic material, a material capable of be transformed into electret or a metal on which was applied a material capable of being transformed into electret.