DE1639444A1 - Electret and process for its manufacture - Google Patents

Description

DR-INS. O.PL.-INQ.M.SC. D.PU.-PHYS, DR. DiPU.-PHYS. HÖGER - LEGAL RIGHT-GRIESSBACH - HAECKER PATENT LAWYERS IN STUTTGART

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Thermo Electron Engineering

Corporation Waltham, Massachusetts

Electret and process for its manufacture

The invention relates to an electret with a through and through polarized theraoplaetisohen dielectric and a process for its manufacture! distant * deals the invention with the use of dines.eoXeheh

Electrets in electroa & ustisehen transducers * microphones and Loudspeakers «y

In order to achieve a sufficiently linear sensitivity electrostatic converters such as capacitor Mkrqgißnen and elektrostatisohsn Reaching speakers is known to be one

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relatively high Sleiohapannuisgs bias voltages in the converter lay on. It has long been known that in Prisip for this purpose electret © with a metallized surface all active elements in the transducer can be used «

An electret is a dielectric material that is given to one bo has been exposed to a strong electrostatic field that a residual, internal polarization is still after Switching off the field remains »which is relatively permanent. The result is an electrostatic Charge in the materiaJisin that can serve? the required green voltage ~ Yoi * spamrang for one electrostatic converter su supply *

In practice, however, numerous problems have arisen both in manufacture and in use the electret © »so that these are not very common» A fundamental disadvantage of all with the help of the known Method of made electrets is that their polarization was not stable. Extreme values of! Temperature and moisture in the known electretexi a rapid decrease in polarization, However, even under controlled environmental conditions, the internal charge decreases

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the well-known electret © gradually decreases too quickly when ■ that they are generally used for practical use could become" .

^A further problem with the use of electret "" in electrostatic converters arises from the fact that the electret has to be arranged between the metallic areas of the electrodes of the converter. As a result, the thickness of the electret, which has not been below the limit since then, has limited the capacity of the transducer such an electret equipped transducer excluded.

Attempts have now been made to eliminate both problems at the same time by making the electret from a thin Film, which can be made of polyethylene terephthalate? such a film made from the material is as so-called mylar film available in stores «converter with Mylar film electrets actually perform very well Capacities of the converter, so that the stray capacitance & sa

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no longer cause any problems in connected circuits, which is why such a converter can also be used in transistorized circuits with relatively low input resistances. However, it has been established through experiments that the apparent stability of the polarization in such transducers is at least to a certain extent only feigned by the poor puncture of the transducer. In particular, electrostatic transducers were tested in the tests ₉ whose elephant steps were made of Mylar film. The films were metallized on one side to create an electrode that served as the transducer membrane, and on the other side the electret was faced with a rear metal plate which formed the other electrode of the transducer.

To take advantage of the thin mylar film for achieving to be able to use a high capacity in the converter at all, the electret membrane must necessarily except ** neatly placed close to the rear metal plate will. It has now been found in the tests that the diaphragm in conventionally constructed transducers of this type has a pronounced tendency to stick to the metal plate

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stick, especially with high polarization of the Electrets “As a result, such a transducer with a sticky membrane has only an extremely small one Sensitivity.

If the Pest ^ the membrane of the only real disadvantage of such a transducer sticking, this would gradually according to the decrease of the polarization of the electret show an increasing sensitivity, can finally completely free to oscillate until the membrane · then takes .'dann the sensitivity of Converter according to the continuously progressive destruction of the electret * Other effects, however, mask this process and make the sensitivity appear stable »

Initially, the tension relaxes originally under tension standing membrane containing the electret. As a result, the transducer becomes less sensitive to it

original sensitivity thanks to the slacker membrane, provided that the electret is not originally polarized v? ar _a If the membrane relaxes gradually, the lower tensile stress also leads to a lower sensitivity, so that the increase in sensation *

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lipiness as a result of the decreasing inclination of the membrane is covered for sticking and the total sensitivity seems to remain stable. It also compensates for the loss of charge over time in the same way Way: the tendency of the membrane to stick together decreases due to the decrease in electrostatic forces, which in turn reduces the Sensitivity increases, but this is also reduced again by reducing the charge « Chance can only lead to the fact that the different compensating influences lead to stable properties of the converter for one to one and a half years, however, the sensitivity then decreases extraordinarily strong ",

First of all, the invention is based on the object of providing an electret of the type mentioned at the outset, which is particularly suitable for use in electroacoustic Converters particularly well suited, and this object is achieved according to the invention in that that which is opposite the surface of the dielectric which is in particular metallized in a manner known per se Surface of the dielectric carries an electrostatic charge, whereby the electret when used in a electroacoustic »converter advantageously - as on

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known - is formed as a thin film.

"Particularly good stability of the internal charge the following materials, which also have the advantage that one surface is extraordinarily can be highly charged and this charge is also very stable remain; these materials from which the electret is preferably made according to a further feature of the invention are polycarbonates, polyimides, polyhalohydrocarbons, polyphenyls, noxyäe, polysulfones, polyvinylidenes and / or their copolymers. It was found that all of these materials are in the With regard to the production of an electret © Mylar-films are far superior.

It is particularly advantageous if the electret a surface coating made of a material from the following group of materials hatι polymers and copolymers of styrene, halogenated hydrocarbons and vinylidene chloride. It was found that these Wej & stoffe im Compared to others, they have a pronounced ability to accept and increase an electrostatic surface charge keep.

Although it is possible per se, the dielectric material

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to be used alone in some applications with transducers according to the invention, it is nevertheless preferable if one side of the electret is known to be metallized or covered with a metal foil »

According to the invention, a method is now also proposed with the help of which more stable elephant treads with from the start better properties than the known ones Have electrets made

A method in which the dielectric in the ground is polar * in a strong electrical equilibrium voltage field is siert, is to lead according to the invention so that then one surface of the dielectric electrostatically charged. It is known to heat a thermoplastic dielectric until it softens ttnd it is the strong electrical direct voltage field suspend and then cool it down »After a Another feature of the invention, such a method is now modified so that the dielectric in the atmosphere an ionized gas is polarized »The inventive The process thus proceeds as follows: First of all, the dielectric material inside is thereby created

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polarized by exposing it to a high DC voltage field while it is heated to a temperature at which it is relatively soft, ie it should then be in a plastic, amorphous state in which there is no longer any tensile stress in the material * Its metallic electrode is permanently placed on one side of the dielectric material, and an ionized gas, such as air or another inert gas, is in contact with the other side of the dielectric as long as it is exposed to the DC voltage field. The latter is maintained until the dielectric material has slowly cooled down until it reaches the solid state, ie until it reaches a state _f in which it can be loaded again by tensile forces and is flexible. Then one surface of the internally polarized electrolyte is provided with a surface charge, which is generated either by increasing the DC voltage field to extraordinarily high strengths of more than 10 volts per centimeter while maintaining the contact of one surface of the dielectric with the ionized gas or by heating the dielectric surface can be achieved with a material of opposite triboelectric polarity. In this way

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'' result in extraordinarily strong and stable elephant steps *

According to the invention, a specified electroacoustic, electrostatic transducer, which sees the aforementioned ITaehteile has a simple structure and consequently cheaper Can be produced · Am such a transducer, which in a known manner one of an electret as one of two electrodes has generated sleioh tension bias is through characterized the use of an electret according to the invention, and about gluing an electret membrane to it an opposite plate or the like from the front * exclude in, spacers or the like are provided between these two elements, which the Determine the minimum distance between them.

According to a further feature of the invention are Spacers or support elements in one piece with one of the both electrodes. With such support elements can in the simplest way, an extremely small, but not undershot, minimum distance between the two electrodes adjust, which leads to a high sensitivity of the transducer. Therefore, in such a converter

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Use extra strong electrets without the sensitivity of the transducer #uroh the inclination of the Eiektrets from sticking to the other electrode at risk will.

Further »advantageous developments of the invention Characteristic features emerge from the claims and / or from the following description, which the Explanation of some embodiments shown in the drawing of the invention transducers and one Plant for carrying out the method according to the invention serves; show it*

3? Ig. 1 is a visual representation, partly in section, an installation for carrying out a step of the manufacturing method according to the invention of an electretsf

3? Ig.2 a section through a first according to the invention Converter}

3 shows a plan view of a guide piece of a base plate for an electret as used in the transducer according to Pig, 2, Jjedooh on a larger scale as in? ig, 2y

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.4 a perspective and schematic representation with pieces of a further exemplary embodiment of a transducer according to the invention, some of which have crawled away;

5 a section in the manner of an exploded view with elements omitted from the rope according to line 5-5 in Fig. 4? ..

6 shows a schematic and perspective bar position with partially weggebroohenen pieces of a further embodiment of a fiction geniäs sen Converter

Fig. 7 is a section through this transducer along the line 7-7 in Fig. 6, with some parts in proportion were drawn too large for others and also a different scale than in Fig. 6 was used;

8 shows an asrial section through a further exemplary embodiment of a transducer according to the invention and an associated component

In the vegetable of the invention, the dielectric material from which the electret is made is selected on the basis of two properties - the first property is the ability of this material to accept and store an internal electrostatic charge, while the other

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Sigensohaft should be the ability of the material accept and hold a surface charge. Am dielectric material that works best for a Polarization in the Masae is suitable, has a polar molecular structure, a low charge carrier mobility and a low diffusion, but a high concentration deeper Pallen for ions and electrons as well as a high glass or Crystallization transition temperature, so that the wide If the property is given, the material should have a high specific resistance, the correct absorption capacity for charges of a certain sign, i.e. usually the correct frictional electrical polarity, and have a low absorption capacity for vapors «. From the large number of dielectrics available, the following have been found to be particularly suitable, and all of them are commercially available polycarbonates, polyimides, polyhalohydrocarbons, Polyphenylene oxides, polysulfones and polyvinylidene chloride layers and films and layers and ropes made from copolymers of these materials * All of these materials are particularly suitable for the production of electret © according to the invention, since they are all have to an unexpectedly high degree the desired properties for the polarization in the interior as well as the

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Charge acceptance on the surface are required. The surface properties of films of polystyrene and its derivatives, Polyhalogenkohlenwasserstoffen and polyvinylidene chloride are particularly well · If this throwing materials as a coating or copolymer at the surface of a material body are present, the imrde selected with regard to the internal polarization properties of Blektrets leave so aicfc optimal results with regard to the internal polarization and the surface properties.

The inventive experience for the production of a Electrets take into account both internal and surface polarization. The 3? Ig.l explains an attachment to carry out the first step of the invention Process, namely the polarization inside the selected material

The dielectric 1, which is in the form of a film or a Plate has, is suspended or supported within a suitable furnace 3 and it has on its according to Pig.l

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lower surface a metallic coating, which in the direct mechanical and electrical contact with a first electrode 5 is. tJ is above the dielectric 1 a second electrode 7 arranged at a distance, which has the lorin of a grid.

A conventional high-voltage direct current source, denoted by 9, is connected to the two electrodes 5 and 7 is. It should have a field strength of approximately 10 fcV / om in the area between the two electrodes, and in this electric field there is then the Dielectric 1.

Although the furnace 3 itself is heated in any way can be, the heating is done with the help of a

Induction heating coil 11 preferred, which only needs to be arranged so that they the dielectric 1 and the two electrodes 5 and 7 encloses. It can be peiet with the help of a common white voltage source in the HP area and this alternating voltage heating current source is denoted by 12. The Weohselfeld generated by the induction heating coil 11 should be so strong that with its help not just the dielectric 1 to the desired one

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I temperature can be heated, but that the gas contained in the furnace 3, for example air, is ionized to form a considerable rope. This ionized gas forms a plasma electrode with a very low impedance compared to the impedance of a non-ionized gas, and this plasma connects the electrode 7 to the upper surface of the dielectric 1. It has been found that in this way a substantially produce more uniform surface charge on the dielectric than is possible with a continuous and indirect contact with the dielectric surface electrode, and furthermore, with the method according to the invention, the local breakthroughs in the dielectric that occur with the last-mentioned arrangement can be avoided.

If an induced field is not used for heating, the Weohsel field required for ionization can also be used With the aid of a grid, which is arranged above the electrode 5 in the furnace 3, as a modification of the one described In the exemplary embodiment, the AC voltage source can also be placed between the electrodes 5 and 7, if by usual means the same - from the alternate

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voltage source is isolated * It could be assumed that the alternating field has no influence on the effect of the direct voltage field or even impaired it} However, it was surprisingly found that a higher polarization degree can be achieved by superimposing the two fields. The dielectric 1 is now brought to a temperature at which it is relatively soft, for example to 250 ⁰ O in the case of a polycarbonate. Then the dielectric is kept at this temperature for 10-100 minutes, while the two superimposed ones are unfortunately maintained at the same time. Finally the heating is turned off while the equilibrium voltage field remains switched on for an hour or two while the dielectric cools down to room temperature.

After the dielectric as a whole (i.e. also inside) has been polarized, it will dissipate with a surface charge. The dielectric surface that covers the metal of the dielectric t is opposite, can be charged with a charge of the polarity that the frictional electrical polarity of the dielectric material corresponds, either by »that the dielectric is exposed to an ionized gas while it is exposed to a

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high bias, or by rubbing the dielectric on another material selected in a known manner is, namely so that it is an opposite has frictional electrical polarity. The net charge that as the difference between the surface charge and the

«■ 8 polarization is defined in the mass, can easily 10 to 10 "* 'coulombs per square centimeter, as is has shown through experiments.

The I? Ig "2 and J show an exemplary embodiment of a" according to the invention electroacoustic electrostatic

Converter which contains an electret according to the invention, which has preferably been produced with the aid of the method just described The converter shown in Fig. 2 Sar is symmetrical to the axis A _t , apart from minor exceptions, which emerge from Pig. 2 and *

The transducer contains a base plate 13 made of a plastic or the like, which has an upwardly projecting peripheral flange 15 with a shoulder 17 on which a rear metal plate 19 rests. The latter is perforated, namely it has a series of openings 21, which have preferably been incorporated with a chemical process "a diameter of". , .WW ** - * - ^ - *****

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preferably 1.3 mm and at intervals of 0.38 mm are arranged from center to centerj would of course be possible but also other dimensions. The Durohbrbruch form channels for the rear with acoustic vibrations Metal plate 19 air flowing through so that the tree

between this metal plate and the base plate 13 serves as an acoustic pressure chamber «

On the upper surface of the metal plate 19, a grid of fine metal wires is formed, and this metal wire grid shall be designated in the following with 23} it can consist of electroformed nickel or the like, and the metal wires are preferably arranged at intervals of 1.0 to 2.5 mm from each other and more commonly Way attached to the metal plate 19, for example by welding. Is is also possible, the wire mesh 23 in one piece with the metal plate 19, and finally the wires of the wire mesh 23 be replaced by a row of protruding pillars, which are also integral with the metal plate 19 · These pillars then preferably have a height between 0.003 and 0.013 mm and a diameter between 0.05 and 0.13 mm, while the distances between the pillars lie between 0.5 and 1.5 mm,

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A membrane 25 lies over the metal plate 19. an electret having an exposed dielectric surface 27 and a top metallized surface 29. The membrane 25 can have a small distance from the wire mesh 23, but it can also rest against this; in each pail this wire grid determines the smallest Distance between the diaphragm and the rear metal plate 19 and prevents the diaphragm from being electrostatic Attractive forces on the metal plate 19 stuck.

The membrane is clamped between the annular edge 31 of a metallic protective grille 33 and an outer, likewise metallic retaining ring 35, the latter pressing the membrane against the peripheral flange 15. This) retaining ring 35 may be secured to the base plate 13 in any desired manner, for example by a thread or by means of an adhesive or the like.

The electrical leads to the converter according to Pig »2 can be carried out differently. One is shown Supply line 37, which is connected to the metallic retaining ring 35 and via the and the protective grille 33 with the

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metallized surface 29 of the membrane 25 in electrical conductive connection is established * A second feed line 39 can be connected directly to the rear metal plate 19 and through a suitable opening in the base plate and the retaining ring 35 led out, namely using conventional insulating means, which are not shown in detail.

The]? Ig. 4 and 5 lines show an unusual exemplary embodiment a converter according to the invention with two electret membranes. In its preferred embodiment, this transducer is a pressure gradient responsive microphone with a sensitivity curve showing eight runners » 3? Ig.4 shows the schematic structure of the converter the assembly, while Fig.5 the details naoh kind of a Represents exploded view.

The transducer contains a number of plastic retaining rings 45, 47, 49, 51 and 53. Figure 4 shows the rear side 55 of the retaining ring 45, but the corresponding cables of the retaining rings 47 to 53 have been omitted in order to simplify the drawing.

The electrode of this transducer comprises a double »back ·· spicy plate, the two chemically processed © metal grids

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57 and 59, which with their back sides against each other and are provided with breakthroughs (breakthroughs 61 of the metal grid 57), the diameter of which is 0.15 mm and the distances from center to center can be 0.38 mm. They form air flow through the rear Plate through and serve essentially the same purpose as the openings 21 of the illustrated in Fig.2 Converter.

At the top of the metal grid 59 and at the bottom of the metal grid gate 57, wire grids 63 made of, for example, electroformed Kicke1 are welded on, the wires being 0.006 mm thick and just as wide. These wire grids 63 carry an upper and a lower leakage membrane 65 and 67, respectively.

The two membranes 65 and 67 can in the same way be prepared as described above. Its two external surfaces are metallized, while the

dielectric surfaces contact the wire grids 63.

There are circumferential flanges on the two metal grids 57 and 59 69 and 71 tightly formed which form clamping surfaces around the two To keep diaphragms 65 and 67 flat and stretched. The two

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letateren are associated with retaining rings 73 and 75 made of metal against the peripheral flange ^ 9 and 71 pressed

The metallic retaining rings 73 and 75 are electrically connected to one another by a connecting line 77 and a connecting line 79 leads from the retaining ring 73 to the outside. Sciaiesjdlieh performs a connection line 80 from the Meta11gittern to a äusaeren Contact »The terminals IS and 80 of a suitable pre-amplifier can be integrally Schlos sen in" any desired manner to the terminals, if the inventive transducer is used as a microphone while ^ech to an amplifier attached *. ^l08 ^? rden when the converter is operated as a loudspeaker.

Property of the converter according to the Pig. 4 and 5 is that it - based on a certain area twice is as sensitive as a transducer with a single one Membrane ,, In known transducers, a similar sensitivity was achieved by using a membrane placed between two rigid plates. In contrast, the converter according to FIGS. 4 and 5 has the advantage a small and fixed distance between the Diaphragms 65 and 67 and the upper

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surfaces of the plates designed as metal grids 57 and 59. Another advantage is that the outer metallized surfaces of the membranes 65 and form an electrostatic shield for the input electrode, which includes the hotall grids 57 and 59. Finally, the inner, charged surfaces of the two electret diaphragms 65 and 67 are protected Vov atmospheric ions and impurities. As a result, the stability of the converter according to FIGS. 4 and 5 is extraordinarily great.

The fig.6 and.7 show a further embodiment a converter according to the invention, which has the advantage / one Extremely simple structure and a long life of the polarization on-.veist · The base plate f & - the WfenftleX'B consists of a non-porous and electric conductive yter fabric,

This base plate carries one made of plastic Spacer ring 85 attached to it. A retaining ring 85 made of plastic is attached to it

has an inner flange 87 that is offset upward.

The base plate 81 also carries a plastic masonry grid 89 which is formed in the warm state, the one downwards

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has recessed central zone 91, which has a rear Plate in the form of an electret 9? absorbs and deceives. In this version, the electret serves as a lever stationary electrode of the transducer.

Ss has a metallized sole on its underside 95, which via a connecting line 97 and a contact plate 99 with the underside of the base plate 31 electrically conductively connected isü. Part of the latter may consist of paper pulp. The remainder of the electret 93 consists of a

relatively thick plastic film with a diameter of approx 0.23mm. .

The film is preferably embossed so that a raised pattern, preferably in the form of a grid, is formed from protective elements 101 which can be approximately 0.006 mm high. The film also has openings 103 for the production of an acoustic connection, a chamber formed by the spaces between the elements of the plastic mesh 89 and the space above the electret 93. nrietoen "icünnen the openings 103 ·" Durohmeeser 0 _p 13 m 'and clementines distance of Mitt "zn middle of ur ^ cfJah? 0.38 m

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to have. Because 'electret $ 9 can be used in the mentioned alert and provided with a fire flue charge

The moving membrane of the transducer of the Pig. 6 and 7 a metallized plastic roll 105, the switches the inner flange 87 and the upper surface of the de · üunotstoffmasohengltters 89 with the help of an intermediate, metallic support ring 107 is mounted · Like the Pig · 6 and 7 show, the metallized surface is the upper surface of the film 105 »which serves as a membrane. But it is also not necessary to produce the whole pollen 105 from metal.

Line connecting line 111 connects the film 105 with a outer connection contact 109, which is also connected to the upper surface of the conductive base plate "1 is in electrical contact" so that one shown in FIG A circuit is formed in which a resistor 113 divides the resistance of the conductive base plate 81. Is the Converter mounted so that the base plate 81 is in contact with a netaiiiuohen ohaesle tteht "so the contact plate 99 serves" le ilaeoe "and the active" output terminal is the Anaohluaokontakt 109.

j) he in the? ig, 6 and 7 shown WgHdIe? leaves easily duroh übliohft Vuvtomungsr & vt & fcvzn for

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manufacture · Also allows the use of the 93 as a stationary EHctrode a somewhat masstrez » With such a construction, the Haltbasfetit Polarization increased, because internal course flows at thinner elements occur more frequently · Tests have shown For example, that with the specified dimensions, the polarization durability is roughly 25 times greater than an electret with a dloke of about 0.006 mm *

Measurement 8 shows an exemplary embodiment of an invention Converter »which is particularly insensitive to fluctuations in temperature and moisture is · Both influences normally worsen the egg: step and foot their destruction.

The converter of? Ig.8 has a two-part cylindrical housing _f · da · can be connected via a Gewindtstüok 117 with a likewise a Gewindestüok 119 *-looking upper housing 121 with a Oehäuseunterteil 115 ·

Both housing parts ·, there should be an accident · »·

Dft · housing lower part 115 can
Preamplifier 123 contained _? the schematic is fcgo split

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and one input terminal of which with the lower housing part himself and its other input terminal with a common one Ansohlusebuohee 125 is connected »via the one electric Contact with a contact pin 127 of the upper housing part 121 can be produced »

The contact pin 127 has two puddles 129 'one' Wipe the sealing membrane 151 made of rubber or the like

calibrate which on your hand with the help of a support «· rings 133 and the lower edge of a mounting frame 135 a tight seal with the lower end of the housing Upper part J.21 forms.

Pas mounting frame 135 has a base 137 in which a. Extension piece 139 of the contact pin 127 attached A capillary tube 141 also leads through the bottom

hinduoh, over which a possible pressure difference between can level the two sides of the bottom 137 * Instead of using a capillary tube, the bottom 137 but also be made of sintered metal »so that the pressure equalization takes place through its pores · On the The bottom is also an Irooknungsmttelpaokung 143 attached to a suitable socket, dfe ». Feuohtig · fceit from the air in the space above the Versciilueemembran 131 ent

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far away

A back is attached to a metal plate 142 attached, which with · play in a bolting in the contact pin « extension piece 139 engages · The latter will of a screw spring 143, which the metal plate 142 upwards and from the bottom 137 of the honing frame pushes away

The rear metal plate 142 can be closed with the help of an oheml * see procedures with breakthroughs 149 vej7 "be helsett, and support ribs are integral with it on its upper side 151 »which have a slektret membrane · 153 · The leit is metallized on its top, while its triache underside is supported on the supporting ribs 151 *

The membrane 153 is between the upper edge wall piece: of the assembly device 135 and a metallic distance ^ · ring 157 held, and the latter also serves to hold a metallic protective grille 159, the other- » on the other hand against a flange 161 at the upper end of the upper Gehliuseobteils 121.

In operation, the rubber closure membrane forms 131 an expandable closure, with the help of which

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Allow the distances to compensate for the external pressure, while pressure equalization between the spaces above and below the base 137 of the honing frame 135 takes place via the capillary tube 14-1. The electret membrane 15? and the electrically conductive metal plate 142 interact in such a way that this has already been described on the basis of the other exemplary embodiments »

By closing the bottom of the mounting frame 135 «· Apart from the capillary tube 141 - there are two advantages First of all, the space within the assembly frame 135 and the Eiefctret membrane 153 is reached a pressure chamber with essentially unchangeable Volume> into which the membrane can work «Second the capillary tube prevents a static pressure difference from gradually developing on both sides of the membrane, which the property of the inventive converter according to * Partly would affect and sum rupture of the membrane could lead »

- 31 9 # 17/0351

Claims (12)

Claims I. Electroacoustic transducer with one of an electret as a DC voltage bias generated by two electrodes, the first electrode of which has a membrane and the electret of which has a metallized surface, thereby characterized in that the latter faces the outside of the pair of electrodes and that of the membrane facing side of the second electrode a pattern, in particular a grid of support elements is provided, which the membrane at least at a fixed distance from the hold the second electrode and over which the membrane is stretched. 2. Converter according to claim 1, characterized in that the electret forms the membrane and the second electrode is rigid is. 3. Converter according to claim 2, characterized in that the second electrode is a metal plate which has openings to a pressure chamber, the volume of which is large compared to the volume swept by the membrane during operation. 4-. Converter with two arrangements according to Claim 2, characterized in that that the second electrodes have breakthroughs and with their surfaces facing away from the membranes lie against each other (Fig. 4). 5. Converter according to claim 1, characterized in that a metal grid on both sides of a fine metal telephoto wire is surrounded, and that over this mesh wire on each of the two sides of the metal grid - 2 1088.17 / 0361 ^{BAD 0RI6IN} «- an electret membrane is stretched, the metallized surfaces of which are turned outwards. 6. Converter according to claim 1, characterized in that the electret is rigid and has openings and on its one, a metallic or metallized membrane facing surface a pattern, in particular a grid \ of support elements over which the membrane is stretched, and that the other surface of the electret is provided with a metal coating (Fig. 6). 7 · Converter according to claim 6, characterized in that on the side of the electret facing away from the membrane a electrically conductive and non-porous base plate is arranged, which is electrically non-conductive second via a grid Supporting elements the electret carries, and that the membrane and the base plate the end-face terminations of a Electret receiving first chamber and the spaces between the second support elements within the first Chamber form a pressure chamber (Fig. 6). 8. Converter according to claim 7, characterized in that one side of the base plate is electrically conductive with the metal coating of the electret and the other side of the base plate is connected to the membrane (Fig. 6). 9 · Converter according to claim 1, characterized by a metallic base plate with a series of openings and a fine metal wire mesh on a surface (Fig.1). 1098 17/035 1 10. Converter according to claim 1, characterized by two opposing, plate-shaped components of one of which has the electret with its metallized surface facing outwards, and of which the other Component has a metallic conductor, one component being rigid and provided with openings, while the other component forms a continuous membrane, and furthermore on the one facing the membrane Surface of the rigid component a pattern, in particular a grid of support elements is formed against which the Membrane and which are made of a material whose electrical conductivity is at least comparable to that on the membrane facing surface of the rigid component. 11. A transducer according to claim 1, characterized in that it contains a first and a second impermeable membrane, which together with housing parts form a closed chamber, the first membrane being relatively strong is deflectable and the second membrane has a flexible, impermeable first electrode and a second, perforated Has electrode which are opposite to each other and of which one has the electret, its dielectric Surface facing the other electrode, and that on the surface facing the first electrode second electrode a pattern, preferably a grid of support elements is provided that the first electrode wear (Pig. 7). 12. Converter according to claim 11, characterized by a second chamber formed in the first chamber substantially constant volume by the second membrane - 4 _ 109817/0351 is complete and picks up the perforated electrode, the second communicating with the first chamber through at least one relatively narrow passage. 109817/0351 as. Lee r side