EP0118356A1 - Electro-acoustic transducer with piezo-electric diaphragm - Google Patents

Abstract

The invention relates to electroacoustic transducers with piezoelectric diaphragm and to its manufacturing process. The object of the invention is to produce transducers in large series with a minimum of elements which make it possible to ensure the functions of embedding the diaphragm (30), of connectors, of shielding, of acoustic filtering and of protection against humidity and dust. The invention is particularly applicable to the production of telephone handsets.

Description

The present invention relates to electroacoustic transducers for converting sound pressure into electrical voltage. It relates more particularly to microphones in which the conversion of an acoustic vibration into an electrical voltage is ensured by a vibrating element made of piezoelectric polymer.

A transducer of this type has been described in a patent application filed by the Applicant on August 11, 1981 and bearing the national number 81.15 506. This transducer uses an elastic structure in the form of a recessed plate having at least one curvature and covered on its two electrode faces connected to an electrical impedance adapter circuit. It is composed of a set of elements arranged according to an original principle which gives it excellent qualities. However, the relatively large number of these elements and their method of assembly do not satisfy a mass production of these transducers, at high speed and at low cost.

In order to overcome these drawbacks, the invention proposes an electroacoustic transducer produced with a minimum of elements and which allow the combination of means ensuring the functions of embedding the vibrating element, internal and external connectors, shielding, filtering. acoustic and protection against humidity and dust.

The invention therefore relates to an electroacoustic transducer whose vibrating element consists of a piezoelectric diaphragm subjected to acoustic pressure on at least one of its faces, the faces of said structure being provided with electrodes forming a capacitor connected to an electrical circuit disposed on a printed circuit, said diaphragm and said electrical circuit being enclosed in a housing, said transducer comprising means for embedding said diaphragm, means for electrical connections of said electrodes to said electrical circuit and at least one pass acoustic filter bottom, characterized in that said housing is constituted by a tubular body whose bottom is a perforated wall corresponding to the front face of the transducer, said body cooperating with a spacer to ensure the embedding of the diaphragm, said body cooperating with said printed circuit to ensure the closing of the transducer and the positioning of the spacer, the electrical connection means being provided by the body and the spacer, said wall and the diaphragm defining a space constituting said filter.

The invention also relates to a method of manufacturing such a transducer, characterized in that its assembly is maintained by the mechanical connection of the body and the printed circuit, this connection ensuring the pressing of the spacer on the diaphragm.

• - la figure 1 est une vue en coupe méridienne d'une capsule microphonique de type connu ;
• - la figure 2 est une vue de principe en coupe méridienne d'une capsule selon l'invention ;
• - la figure 3 est une vue en coupe méridienne d'une capsule microphonique selon l'invention ;
• - la figure 4 est une vue en perspective d'une chemise isolante ;
• - la figure 5 est un diagramme explicatif ;
• - la figure 6 est une vue en coupe d'un emporte-pièce ;
• - la figure 7 est une vue d'un dispositif de bouterollage ;
• - les figures 8 et 9 sont des vues en coupe méridienne de capsules microphoniques selon l'invention ;
• - la figure 10 est un schéma électrique d'un préamplificateur microphonique ;
• - les figures 11 et 12 sont des diagrammes explicatifs.

The invention will be better understood and other advantages will appear from the following description and the appended figures, among which:

• - Figure 1 is a meridian sectional view of a microphonic capsule of known type;
• - Figure 2 is a principle view in meridian section of a capsule according to the invention;
• - Figure 3 is a meridian sectional view of a microphone capsule according to the invention;
• - Figure 4 is a perspective view of an insulating jacket;
• - Figure 5 is an explanatory diagram;
• - Figure 6 is a sectional view of a cookie cutter;
• - Figure 7 is a view of a locking device;
• - Figures 8 and 9 are views in meridian section of microphone capsules according to the invention;
• - Figure 10 is an electrical diagram of a microphone preamplifier;
• - Figures 11 and 12 are explanatory diagrams.

Figure 1 is a meridian sectional view of a microphone capsule with piezoelectric plate according to the prior art. It is formed by a housing comprising an upper metal part 2 which fits into a housing bottom 11 provided with insulated connection terminals 6. A piezoelectric plate 3 provided with metallizations 4 and 5 is embedded frusto-conically between the edge of the upper part 2 of the housing and a metal ring 8 with trapezoidal section. The ring 8 is pressed against the plate 3 by an insulating washer 9 resting on an elastic blocking piece 10 which enters a circular slot in the upper part 2 of the housing. A pad 1 of sound absorbing material is housed in the central recess of the upper part 2 of the housing. This buffer is wedged between the part 9 and a printed circuit board 7 on which are arranged the electronic components of an electrical circuit impedance adapter.

• - le diaphragme piézoélectrique muni d'électrodes sur ses deux faces doit être serré dans un encastrement qui lui impose une forme plane ou bombée :
• - les contacts pris sur ces électrodes doivent être reliés à des circuits électroniques implantés dans la capsule ;
• - la capsule doit être enfermée dans une enveloppe conductrice assurant son blindage ;
• - la capsule doit intégrer les composants acoustiques tels que cavités ou orifices qui contribuent à la mise en forme de la réponse en fréquence ;
• - la capsule doit être pourvue de moyens de liaison électrique avec des circuits extérieurs de traitement du signal ;
• - le procédé de fabrication de la capsule doit être compatible avec les contraintes des techniques d'assemblage automatique pour une cadence d'au moins 1000 capsules à l'heure.

The microphone capsule according to the invention must meet the following requirements:

• - the piezoelectric diaphragm provided with electrodes on its two faces must be clamped in a recess which imposes on it a flat or curved shape:
• - the contacts made on these electrodes must be connected to electronic circuits implanted in the capsule;
• - the capsule must be enclosed in a conductive envelope ensuring its shielding;
• - the capsule must integrate the acoustic components such as cavities or orifices which contribute to the shaping of the frequency response;
• - The capsule must be provided with means of electrical connection with external signal processing circuits;
• - the capsule manufacturing process must be compatible with the constraints of automatic assembly techniques for a rate of at least 1000 capsules per hour.

Figure 2 is a principle view in meridian section of a capsule according to the invention. The piezoelectric diaphragm 20 is shown planar and also recessed planar. It could also be in another form, for example curved, and maintained by a non-planar or pinched recess between knives. In addition to the diaphragm, the capsule has four other parts which combine the various mechanical, acoustic and electrical functions previously mentioned. The diaphragm 20 is clamped between the body 21 and the spacer 22. It is covered on its two faces with metallizations serving as electrodes. The embedding also ensures contact between the electrodes of the diaphragm and the metal parts 21 and 22. The parts 21 and 22, for example of aluminum, in addition to their shielding function, provide the electrical connection between the diaphragm and the double sided printed circuit 23. The annular jacket 24 insulates the body 21 from the spacer 22. The printed circuit 23 can support on its internal face soldered electronic components constituting a preamplifier and on its external face, pins making it possible to connect the capsule to a connection cable. The diaphragm and the internal electronic circuit are thus completely shielded by the equipotential envelope constituted by the external electrode of the diaphragm, the body 21 and the external face of the printed circuit 23.

As shown in FIG. 2, the body 21 and the spacer 22 can be used advantageously to delimit cavities and walls pierced with orifices on either side of the diaphragm so as to synthesize acoustic components capable of regularizing the microphone response curve. These acoustic components are materialized by the wall 25 of the body 21, this wall being pierced with holes 26, and by the wall 27 of the spacer 22, this wall being pierced with the hole 28. In the case where the diaphragm is made of polymer piezoelectric parts 21 and 22 may have a suitable mounting profile to overcome the mechanical effects due to significant temperature variations.

The assembly of the capsule described in FIG. 2 is greatly facilitated by the fact that the symmetry of revolution is everywhere preserved: the relative positioning of the different parts constituting the capsule is thus simply ensured by their stacking and their concentricity. The body 21 initially has, in its lower part, the tubular geometry indicated in dotted lines. The order of assembly operations is as follows: the body first receives the annular insulating jacket 24 which then allows the centering of the diaphragm 20 and the spacer 22. The printed circuit 23 with its soldered components is then put in components inside the capsule. The tightening of the stack and of the embedding is carried out by crimping the body 21 on the external face of the printed circuit.

The metal parts 21 and 22 are suitable for industrial production according to the technique known under the name of impact spinning. This technique is commonly used to manufacture tubes of the pharmaceutical tablet tube type. To this end, it is advantageous to produce these aluminum parts and to subject them to an anti-chemical treatment. corrosion. The insulating jacket 24 will preferably be made of a plastic material with a low dielectric constant so as to minimize the parasitic capacitance between the body and the spacer. It can be obtained by cutting an extruded tube or by injection and molding.

The printed circuit 23 can be of the copper-epoxy or copper on synthetic resin type, the tracks of the circuit being obtained by chemical etching or screen printing. In addition to the holes required to install the components, or pins for external connectors, an additional orifice of small diameter (around 0.5 mm for example) can be provided in order to ensure a static pressure equalizing leak between the two diaphragm faces. It should be noted that the concept of a double-sided printed circuit must be understood here in its broadest sense, and that it can be extended to any structure consisting of an insulating substrate between two electrode systems of geometry adapted to the connectors. wanted. As such, the external electrode may consist of a metal foil plated on the bare face of a single-sided printed circuit and fixed by crimping the body. By micro-cutting followed by a 90 ° folding, this foil can be directly provided with lugs ensuring the connection with the copper-plated and engraved face of the circuit, or with tongues playing the role of pins of external connectors. A flexible printed circuit (of the copper strip type on a polymer film of the trademark Terphane for example) plated on an insulating substrate can be used on the internal face of the printed circuit. The advantage of these variants lies mainly in the fact that they lead to a less expensive sub-assembly than a real double-sided printed circuit.

Figure 3 is an exemplary embodiment of a microphone capsule according to the invention. The capsule uses a piezoelectric diaphragm 30, for example made of polyvinylidene fluoride (PVF ₂ ), the main faces of which are provided with electrodes which may consist either of a layer of polymer charged with conductive particles, or of a metallic deposit (preferably three-layer type, for example chrome-aluminum-chrome). It is also possible to use a diaphragm made of polymer alloys or of PVF ₂ copolymers. The body 31 comprises a wall 32 which comprises the acoustic components contributing to the shaping of the curve capsule response. These are arranged on the front side of the capsule. They play the double role of low-pass filtering and damping of the resonance of the diaphragm, by an appropriate combination of cavities and orifices and implementing an acoustic resistance in the form of a sealed and micro-pierced region 33 of the wall 32. The cavity 37 delimited by this wall and the diaphragm having to be of very small volume, the concavity of the diaphragm is rotated so that an increase in its deflection by thermal expansion does not entail the risk of contact with this wall. A film 34 of polymer, very thin and acoustically transparent (for example polyethylene terephthalate for capacitor, thickness 6 micrometers) protects the microphone against the introduction of dust or droplets and prevents clogging of the microperforated acoustic resistance. This film is pinched at its periphery in a shoulder of the body 31 by a stamped cup 35 forced in. This cup is pierced with orifices 36 and delimits a new cavity 38 which constitutes a second low-pass filter, placed upstream of that linked to the micro-pierced resistance. It also helps to cut off the microphone response beyond 5 kHz. The capsule also includes the metal spacer 40, the double-sided printed circuit 41 and the insulating jacket 39.

Figure 4 is a perspective view of the insulating jacket. Its lateral surface has a profile which, by its recesses, makes it possible to reduce by approximately 50% the parasitic capacity between the body 31 and the spacer 40. We note on FIG. 4 external recesses 45 which alternate with internal recesses 46 Other forms are possible, with the same advantages. In general, we will retain for this part the advantage of a crenellated shape which, while ensuring the centering of the diaphragm and the spacer to the assembly, gives rise to a low parasitic capacity thanks to the layers of air that 'it introduces between body and spacer.

Another design detail of this microphone relates to the realization of the static pressure equalizing leak at the rear of the diaphragm. Instead of drilling an orifice passing right through the printed circuit, it is possible to produce radial capillary leaks breaking the tightness of the clamps of the spacer and of the body crimped on the printed circuit. The etching on both sides of the circuit is such that air passages are created in the thickness of the copper layer of the printed circuit. The rear cavity of the microphone is thus connected to atmospheric pressure. These capillary leaks have a sufficiently high acoustic impedance so as not to disturb the response of the microphone even at low frequency. In the same way, it is possible, by rupturing seals between the different parts of the assembly, to ensure that the cavity 38 is brought to atmospheric pressure under high acoustic impedance.

The assembly of the microphone capsule described in FIG. 3 has two aspects: the shaping of the non-planar diaphragm and the assembly by crimping.

A first way to dome the diaphragm of PVF 2 is to stack a flat diaphragm, for example from a die cut, with the other parts of the assembly and to crimp the body, as described more far. The type of diaphragm used in this microphone is sufficiently thick, therefore has a sufficiently high flexural rigidity, _for that the tightening of the frustoconical recess generates a dome shape without angular point. This deformation is extremely difficult to analyze and model because it is a problem of hyperstatic mechanics. In order to obtain the profile associated with suitable values of the microphone sensitivity and the frequency of the first resonance mode of the diaphragm, it is therefore necessary to proceed experimentally by studying several arrangements differing for example by the thickness of the diaphragm and the installation angle. As an indication, it can be noted that a PVF ₂ diaphragm 200 micrometers thick formed according to this process in the capsule shown in FIG. 3, presents its first mode of resonance at a frequency between 3600 and 4400 Hz , which turns out to be well suited to the telephone application.

When tightening by crimping, the elastic limits of the material are only exceeded at the periphery of the fitting or the polymer must conform to the angular profile of the anchoring zone. As a result, if the diaphragm is then removed, only this region retains its shape, the central part of the dome losing its arrow to approach a plane geometry. This shows that it is necessary to stabilize the shape of the recessed dome by relaxing the constraints of which it is the seat. From this point of view, an appropriate heat treatment consists in placing the completely assembled microphone in an enclosure at 90 ° C. for one hour.

This diaphragm shaping process and assembly of the microphone capsule had already been described in broad outline in the patent application cited above. This process is particularly advantageous by its simplicity. However, it has limitations. Indeed, if the embedding angle is increased beyond a certain value, about 7 °, this does not result in an increase in the arrow of the dome but a very aspherical shape in attitude. This process therefore does not make it possible to obtain very convex spherical or aspherical shapes, that is to say those whose arrow to diameter ratio exceeds 0.03.

A significant bending of the diaphragm is however advantageous if it is desired to have a high stability of the microphone sensitivity for operation at temperatures below ambient temperature. In fact, due to the large differential expansion between the polymer diaphragm and metal mounting parts, a decrease in temperature results in a radial contraction of the diaphragm, first resulting in a decrease in its deflection, then in the appearance of 'a radial tension when the geometry of the diaphragm becomes close to the plane. In this second phase, the microphone sensitivity drops suddenly. It is understood that this situation is encountered at a temperature all the lower that the diaphragm is initially more domed, because the initial arrow determines the provision of arc length that the diaphragm can absorb by approaching the chord plane, before its contraction doesn't start to stiffen it.

à 20° (H représentant la flèche et D le diamètre de la partie du diaphragme non encastrée). La courbe 50 a été tracée pour

= 0,020, la courbe 51 pour

= 0,024, la courbe 52 pour

= 0,027 et la courbe 53 pour

= 0,039. D'après ce diagramme, on constate que le rapport

doit être d'au moins 0,04 pour que l'écart entre les sensibilités à - 20° C et + 20° C soit inférieur à 3 dB. Les conditions de mise en forme du diaphragme et d'encastrement doivent être telles que le rapport

soit au moins égal à 0,04 à la température ambiante. Il existe plusieurs façons d'y parvenir.FIG. 5 is an explanatory diagram which shows the influence of the arrow on diameter ratio on the microphone sensitivity as a function of the temperature. The diagram has the temperature T in degrees Celsius on the abscissa and the sensitivity S in decibels on the ordinate, depending on the parameter.

at 20 ° (H representing the arrow and D the diameter of the part of the diaphragm not embedded). Curve 50 has been plotted for

= 0.020, the curve 51 for

= 0.024, the curve 52 for

= 0.027 and the curve 53 for

= 0.039. According to this diagram, we finds that the report

must be at least 0.04 so that the difference between the sensitivities at - 20 ° C and + 20 ° C is less than 3 dB. The diaphragm forming and embedding conditions must be such that the ratio

or at least 0.04 at room temperature. There are many ways to do this.

The diaphragm can be forced into the shirt. It is first cut to a diameter greater than a fraction of a percent of the inner diameter of the jacket. The insertion of the diaphragm into the jacket therefore causes its bending, its concavity being directed by pressure from the body of the capsule on the spacer. After tightening, the arrow of the diaphragm is greater than the arrow that one would obtain with a diaphragm returning just free in the shirt.

Another method is cold crimping, which amounts to treating the problem by the cause that caused it, namely the differential expansion between the embedding and the diaphragm. It simply involves tightening the diaphragm by crimping at a temperature below room temperature. When returning to room temperature, the opposite effect to that described above occurs, the diaphragm expanding to acquire a deflection greater than that resulting from shaping by simple embedding. With regard to the capsule shown in FIG. 3, a crimping temperature slightly higher than 0 ° C., in order to avoid icing, proves to be suitable for subsequently obtaining a stable microphone sensitivity at + 0.5 dB between - 5 and + 35 ° C and reduced by approximately 3 dB at - 20 ° C. The implementation of this method supposes that the automatic crimping bench of the capsule is enclosed in a cabin maintained at the crimping temperature and that the capsules, pre-assembled and ready to be crimped, remain there for a time sufficient to heat them up.

A final way of obtaining the desired shapes is to insert a non-planar preformed diaphragm into the assembly. To do this, we can first cut the diaphragm from a non-flat disc and then thermoform it in a mold of suitable geometry before crimping it into its recess. This process has the disadvantage of introducing an additional operation between cutting the diaphragm and assembling the capsule. This preforming operation can be integrated into the cut using a tool such as that shown in Figure 6. This figure shows a cookie cutter whose forming punch 60 heating and spherical in the case considered is likely to 'Impose the desired shape on a piezoelectric polymer film through the matrix 61. The forming punch 60 is integral with the cutting punch 62 thanks to the screw 63. The sidewall 64 prevents the film from sliding during the operation. Cutting takes place after the forming operation. The sidewall press, for example, has the shape of a paraboloid in its part in contact with the film. It can be made of a material known under the registered trademark ELADIP. Unlike the thermoforming of a previously cut disc, this process implies that the film undergoes an over-stretching which it is important to make irreversible so as to subsequently ensure the shape stability of the embedded diaphragm. This requires that the thermoforming before cutting is carried out at a high temperature (90 to 100 °), moreover compatible with the heat treatment for stabilizing the material in the form of a strip or of plates previously carried out at 110-120 ° C.

A technique suitable for crimping the body of the capsule after pre-assembly of the parts by simple stacking consists of a rotary rolling using the tool shown in FIG. 7, the main axis of which coincides with that of the capsule. It comprises a mandrel 70 which rotates the body 71 of the tool which supports the chuck 72. The axis of the chuck does not coincide with the main axis of the tool. The capsule whose body 74 alone is visible is placed in a support 75 on a stop 76. The rotation speed of the mandrel is of the order of a few hundred revolutions per minute. The dowel is also rotated and gradually lowered. It addresses tangentially vertical lip _J body of the capsule according to one of its generating lines to make it conform ur. rounded profile. The axis of rotation 73 of the dowel describes a cone around the main axis of the tool. In about ten turns, the lip is finally completely folded down and tightens the stack of parts by pressing 1: printed circuit. The complete operation is carried out in a few seconds, including the insertion of the preassembled capsule into the support and ejection after crimping.

In the general description of the capsule as in the embodiment, the body and the spacer are made of metal. This choice of material, although very well suited to the functions that these parts provide and to their manufacturing processes, is not limiting. Indeed, the same general transducer structure can be adopted while making these parts or at least one of these parts, made of plastic. The choice of the appropriate material is first of all guided by the requirement of very good mechanical and thermal resistance. In particular, excellent creep resistance is necessary to ensure good stability of the stack of parts after tightening. This leads to choosing a material having a glass transition temperature much higher than the highest temperature to which the transducer can be exposed.

In the case where the body and the spacer of the capsule are made of plastic, the problem of their electrical conductivity can be considered in two ways. A volume conductivity can be obtained thanks to a. charge of these parts with carbon black. The conductivity of materials thus charged is very low compared to that of metals but is sufficient in the application to microphones. Indeed, the input impedance of the preamplifier being of the order of a few 10 ⁶ n, resistances of the order of 10 to 100 k2 placed in series with the diaphragm are perfectly tolerable. If higher conductivity is required, as in the case of an emitting transducer, surface conduction is also possible by coating the parts in question. The choice of the material constituting them is then guided by its ability to receive a conductive coating. The coating can be produced by varnishing, by vacuum metallization or even chemically. By a masking process it is possible to avoid metallizing the external lateral surface of the spacer. It is then possible to remove the insulating jacket and thus further reduce the parasitic capacity between the body and the spacer. The outer surface of the capsule body can be coated with a conductive layer serving as a shield.

The method of assembling the capsule by bolting can be maintained if a forgeable plastic is used. However, this tightening and assembly process is generally better suited to metals than plastics. FIG. 8 is a view in meridian section of a capsule with a molded plastic body and spacer, the crimping being carried out by knurling. Note in this figure a body 80 comprising a wall 81 pierced with holes 82. The diaphragm 83 is clamped at its periphery between the body 80 and the spacer 84. The printed circuit 85 is pressed against the spacer by the metal part 86 playing the role of external electrode for the printed circuit. The piece 86, obtained for example by spinning, is crimped on the body as indicated by the arrow in FIG. 8. The body and the spacer receive surface conductive coatings 87 and 88 which provide the electrical connections between the faces of the diaphragm and the printed circuit, either directly or by means of the metal part 86. The crimping has the effect on the one hand of generating a diametrical tightening ensuring the contact of the conductive coating 88 on the shoulder 89 of the part 86, on the other hand to tighten the stack axially and to ensure effective embedding of the diaphragm. Other means of assembling the capsule are possible by taking advantage of the ability of plastics to bond or weld, in particular by ultrasound.

By way of example of plastic materials which satisfy the various criteria of mechanical and thermal resistance, of ability to receive a conductive filler or a conductive coating, mention may be made of phenylene polycarbonates (reinforced or not) and polyphenylene oxide modified using polystyrene or polyacrylonitrile. To make the body and the spacer from these materials, molding or injection techniques can be used. The use of plastic materials is particularly well suited to cases where it is imperative to minimize the differential expansion between the diaphragm and its jaws.

Piezoelectric diaphragm emitting transducers such as: headphones, speakers or vibrators can also be designed according to the structure which is the subject of the present invention. The printed circuit can play a simple role of connection support there, but can also carry electronic components as in the case of microphones, so if necessary to integrate into the capsule an amplifier or a signal generator. The acoustic components which are monolithically part of the body or spacer can be adapted to the application considered in particular in the form of resonators or pavilions.

The capsule which is the subject of the invention lends itself very well to the use of visco-elastic materials in contact with the diaphragm in order to produce acoustic filtering and damping means. These materials can fill the cavities defined by the body and the spacer. A foam or elastomer cushion can for example be assembled with the other parts. One can also consider injecting into the cavities after assembly a dose of a resin undergoing a large expansion of volume upon polymerization, for example a foam of the registered trademark RHODORSIL.

Such methods can be used as damping means in an overhead transducer, but also make it possible to extend the invention to the encapsulation of underwater piezoelectric transducers. In such devices, filling the cavities adjacent to the diaphragm with an appropriate material (polyurethane resin for example) can ensure one or more of the following functions: sealing, adaptation of acoustic impedance, resistance of the diaphragm to the action of high hydrostatic pressures.

Figure 9 is a meridian sectional view of a microphone capsule according to a variant of the invention. It differs from the capsule shown in Figure 3 by the presence of a ring located towards the front of the capsule. It comprises a body 90, a front embedding ring 91, a spacer 92, an insulating jacket 93, a printed circuit 94 supporting electronic components and on which are fixed output terminals such as 95. The diaphragm 96 is embedded between the ring 91 and the spacer. A protective film 97 is held between the body 90 and the ring 91. The body is pierced on the front face of the capsule of holes 98. It is pre-formed by knurling along a circumference 99 on the embedding ring. This pre-tightening contributes to the tightening of the protective film and to ensuring the electrical connection between the electrode situated on the front face of the diaphragm and the rear face of the printed circuit. The spacer provides the electrical connection between the electrode located on the rear face of the diaphragm and the internal face of the printed circuit. The body, the ring and the spacer must therefore be electrically conductive. The frequency response is shaped by the cavities and orifices made in the ring 91. This device allows an easier mechanical production of the body compared to FIG. 3, the embedding ring has its sharp angles better made and the protective film is better fixed.

Figure 10 is a possible diagram of the microphone preamplifier. Circuit 100 corresponds to the microphone capsule. The voltage Ve delivered by the diaphragm is symbolized by the voltage generator 101, Ca represents its active capacity (approximately 83 pF), Cp the parasitic capacity of the capsule (approximately 64 pF). Circuit 102 is a DARLINGTON assembly in the form of a micro-housing. R ₁ is a resistance of ten megohms, R is an adjustable resistance. The circuit 100 is connected by a connection cable to a signal processing station comprising a supply circuit 103 and a transmission circuit 104. The circuit 103 makes it possible to supply direct voltage (+ V, - V) to the circuit 100 via Ra resistors (approximately 3 k Ω). It also makes it possible to transmit the signal coming from the capsule via the connection capacitors C to the transmission circuit 104 of which only the input impedance Ze has been shown. Ze is generally in the range of 100 to 200 Q.

The resistance R can be adjusted from the outside of the capsule by a hole made in the printed circuit. This adjustment can be made by laser beam.

(Vs étant sa tension de sortie) en fonction de la fréquence f. Cette figure a été tracée en prenant Ze = 200 Ω et pour quatre valeurs de R : R = 100 Q (courbe 110), R = 200 Ω (courbe 111), R = 300 Ω (courbe 112), R = 400 Q (courbe 113). L'étalonnage de l'axe des ordonnées a été réalisé en prenant pour origine des décibels Gy = 0,43. Au vu de la figure 11, on constate que le gain Gv diminue lorsque R augmente.The frequency response of the preamplifier is a function of the values given to the resistance R and to the impedance Ze. FIG. 11 is a diagram showing the influence of the resistance R on the gain of the preamplifier Gv =

(Vs being its output voltage) as a function of frequency f. This figure was drawn by taking Ze = 200 Ω and for four values of R: R = 100 Q (curve 110), R = 200 Ω (curve 111), R = 300 Ω (curve 112), R = 400 Q ( curve 113). The calibration of the ordinate axis was carried out taking decibels Gy = 0.43 as an origin. In view of Figure 11, we see that the gain Gv decreases when R increases.

The temperature coefficient of resistance R can advantageously be chosen to provide compensation for the variation in sensitivity of the diaphragm with temperature.

FIG. 12 is a diagram giving the shape of the gain Gv as a function of the frequency f with the input impedance Ze as a parameter. This figure has been plotted for R = 200 Ω and for three values of Ze: Ze = 100 n (curve 120), Ze = 200 ^Q (curve 121) and Ze = 430 Ω (curve 122). The calibration of the ordinate axis was carried out taking Gv = 0.43 decibels as an origin. In view of Figure 12, we see that the gain Gv increases when Ze increases. The curves in FIGS. 11 and 12 indicate the possibility of a compromise between the gain of the capsule with its preamplifier and the low cut-off frequency of the microphone system.

It is within the scope of the invention to apply the structure of the microphone capsule to the most general case: microphones with planar or non-planar piezoelectric polymer or mineral diaphragm, embedded or held by any other means of attachment between jaws. The invention is also applicable to cases of transducers operating as transmitters.

Claims (31)

1. Electroacoustic transducer, the vibrating element of which consists of a piezoelectric diaphragm subjected to acoustic pressure on at least one of its faces, the faces of said structure being provided with electrodes forming a capacitor connected to an electrical circuit arranged on a printed circuit, said diaphragm and said electrical circuit being enclosed in a housing, said transducer comprising means for embedding said diaphragm, means for electrical connection of said electrodes to said electrical circuit and at least one low-pass acoustic filter, characterized in that said housing is constituted by a tubular body whose bottom is a perforated wall corresponding to the front face of the transducer, said body cooperating with a spacer to ensure the embedding of the diaphragm, said body cooperating with said printed circuit to ensure closure of the transducer and the positioning of the spacer, the electrical connection means s being provided by the body and the spacer, said wall and the diaphragm defining a space constituting said filter. 2. Transducer according to claim 1, characterized in that said diaphragm is made of polymer or of a polymer alloy. 3. Transducer according to claim 2, characterized in that said polymer is made of polyvinylidene fluoride or of polyvinylidene fluoride copolymer. 4. Transducer according to any one of claims 1 to 3, characterized in that the body and the spacer are metallic. 5. Transducer according to claim 4, characterized in that the body and the spacer are made of aluminum. 6. Transducer according to any one of claims 1 to 5, characterized in that the body (3I) and the spacer (40) have undergone an anti-corrosion chemical treatment. 7. A transducer according to any one of claims 1 to 6, characterized in that it further comprises an insulating jacket (39) made of a material having a low dielectric constant, said jacket ensuring the electrical insulation of the body (31 ) and the spacer (40). 8. A transducer according to claim 7, characterized in that said insulating jacket has peripheral recesses (45, 46). 9. A transducer according to any one of claims 1 to 8, characterized in that it comprises on its front face an impermeable film (34). 10. Transducer according to any one of claims 1 to 9, characterized in that it comprises a pierced cup (35) placed on the front face of said transducer and defining with said wall (32) a cavity (38) constituting a pass filter -low. 11. Transducer according to any one of claims 1 to 10, characterized in that said printed circuit (41) comprises at least one orifice ensuring a static pressure equalizing leak between the two faces of the printed circuit. 12. A transducer according to claim 11, characterized in that said equalizing leak is effected by ruptures in the tightness of the clamps of the spacer (40) and of the body (32) crimped on the printed circuit, said ruptures in sealing being produced by etching faults in said printed circuit. 13. Transducer according to any one of claims 9 to 12, characterized in that sealing ruptures between said housing (90) and one of said embedding means (91) are provided in order to ensure a setting the atmospheric pressure of the front face of said diaphragm. 14. Transducer according to any one of claims 1 to 13, characterized in that the embedding of said diaphragm contributes to giving it a domed shape. 15. A transducer according to claim 14, characterized in that, H being the arrow of the diaphragm and D the diameter of its non-embedded part, the relation

≥ 0.04 is satisfied. 16. A transducer according to any one of claims 1 to 15, characterized in that at least one of the mounting parts (80, 84) of the diaphragm is made of plastic. 17. Transducer according to claim 16, characterized in that said plastic material has a glass transition temperature higher than the maximum temperature of use of the transducer. 18. Transducer according to one of claims 16 or 17, characterized in that said embedding parts are made conductive by incorporation of conductive elements. 19. Transducer according to one of claims 16 or 17, characterized in that said mounting parts have elements of their surface made conductive by a conductive coating (87, 88). 20. A transducer according to any one of claims 16 to 19, characterized in that said plastic material is based on polycarbonate. 21. A transducer according to any one of claims 16 to 19, characterized in that said plastic material is based on polyphenylene oxide. 22. Transducer according to any one of claims 1 to 21, characterized in that said diaphragm is in contact with a viscoelastic element serving as a shock absorber. 23. Transducer according to any one of claims 1 to 22, characterized in that said housing further comprises a conductive ring (91) placed between the diaphragm (96) and the bottom of said body (90), this ring ensuring functions diaphragm fitting and acoustic filtering. 24. A method of manufacturing a transducer according to any one of claims 1 to 23, characterized in that the assembly of the transducer is maintained by the mechanical connection of the body (31) and the printed circuit (41), this connection ensuring the pressing of the spacer (40) on the diaphragm (30). 25. The manufacturing method according to claim 24, characterized in that said mechanical connection results from the crimping of said body on said printed circuit. 26. The manufacturing method according to claim 25 characterized in that said crimping is carried out by a rotary die-cutting. 27. The manufacturing method according to any one of claims 24 to 26, characterized in that said transducer undergoes, after assembly, a heat treatment for dimensional stabilization of the diaphragm. 28. The manufacturing method according to claim 27, characterized in that said heat treatment is carried out at 90 ° C for 1 hour. 29. Manufacturing method according to one of claims 24 to 26, characterized in that said crimping is carried out at a temperature close to the lowest operating temperature of said transducer. 30. The manufacturing method according to any one of claims 24 to 29, characterized in that said diaphragm is preformed before assembly of the transducer. 31. The manufacturing method according to claim 30, characterized in that said preforming is carried out by cutting out said diaphragm.

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