EP0032082A2 - Electro acoustic transducer with active dome - Google Patents

Abstract

The invention relates to an electro-acoustic transducer using a self-supporting active radiating membrane made from a polymer material. The invention provides a transducer in which a resilient shape restoring member fixed to the case capped by the radiating membrane takes on the shape of the concave parts of the membrane, so as to oppose the definitive staving-in of the membrane by an accidental thrust force acting on the dome shaped protuberance on its outer face and restore the member to its initial shape when the force is removed.

Description

The present invention relates to transmitters and receivers of acoustic waves in which a transducer element of non-developable form is used to convert an alternating electric voltage into vibrations or vice versa. It relates more particularly to loudspeakers and microphones in which the dome-shaped membrane is constituted by a self-supporting structure made of polymer material. The concave and convex faces of this structure are coated with electrodes forming a capacitor. The transducer effect implemented in these structures is manifested throughout the extent of the electro-sensitive zones located between the electrodes, which makes it possible to produce fully active domes. The polymeric materials used to manufacture the active domes are in the form of homogeneous or bimorphic films, the thicknesses of which are generally between a few tens and a few hundred microns. In this case, the final shape can be obtained by thermoforming or by electroforming. Self-supporting structures with very thin walls can also be obtained by molding or coating.

Whatever the manufacturing technique used. the dome obtained has good mechanical resistance due to the self-supporting properties which. set it apart from a flat film of comparable thickness. Nevertheless, by exerting a thrust in the center of the convex face of a dome, one can create a mechanically stable knockout which completely distorts the electroacoustic properties. This buckling phenomenon is reversible, but to regain the initial shape it is necessary to exert a thrust in the opposite direction to that which caused the knockout. In practice, the user does not have access to the convex face of a dome-shaped membrane, which implies a delicate disassembly of the transducer when its membrane has been accidentally smashed. To overcome this drawback, the convex radiating face of an active dome can be protected by a grid, but this means is ineffective when the knockout results from an overpressure. In addition, certain knock-outs can cause breakages such that the dome can no longer fully return to its original shape. In addition to the accidental knocks which can occur during the use of an electroacoustic transducer with active dome, it should be pointed out that parasitic vibratory modes can appear and give rise to irregular deformations by standing waves. On the other hand, the vibration of an active dome tends to amplify by resonance in a narrow range of the acoustic spectrum, which is detrimental to good sound reproduction. The control of the frequency response characteristic of an active polymer dome is based on a damping of its own resonance and on those which can be made to act by acoustic coupling. However, the modest efficiency of the piezoelectric polymer transducers does not make it possible to envisage a purely electric damping of the resonances which is both simple to implement and sufficiently effective.

In order to overcome the drawbacks listed above, the present invention proposes to associate with a ^- self-supporting active structure made of polymer material an elastic support that is acoustically permeable and conforms to the shape of its concave face. The pressure exerted by this support ensures resistance to the knockouts of the dome and contributes to its mechanical and acoustic damping.

The subject of the invention is an electroacoustic transducer comprising a rigid casing capped by a self-supporting active radiating membrane made of polymer material having at least one bulge, characterized in that the casing contains an elastic support that is acoustically permeable and conforms to the shape of the concave parts of the internal face of. radiant membrane; the shape taken by the support face of the support being determined by the proper shape of the radiating membrane.

• - la figure 1 est une vue en coupe d'un transducteur électroacoustique comportant une membrane en polymère piézoélectrique ;
• - la figure 2 est une vue en coupe d'une membrane bimorphe ;
• - la figure 3 illustre le défoncement d'une membrane en forme de dôme et ses modes vibratoires parasites ;
• - la figure 4 est une vue en coupe d'un transducteur électroacoustique selon l'invention ;
• - la figure 5 est une vue en plan d'une grille thermoformée ;
• - la figure 6 indique, les réponses en fréquence avec ou sans support de membrane ;
• - les figures 7 et 8 représentent des structures compressibles perméables acoustiquement.

The invention will be better understood by means of the description below and the appended figures among which:

• - Figure 1 is a sectional view of an electroacoustic transducer comprising a piezoelectric polymer membrane;
• - Figure 2 is a sectional view of a bimorph membrane;
• - Figure 3 illustrates the knockout of a dome-shaped membrane and its parasitic vibration modes;
• - Figure 4 is a sectional view of an electroacoustic transducer according to the invention;
• - Figure 5 is a plan view of a thermoformed grid;
• - Figure 6 indicates, the frequency responses with or without membrane support;
• - Figures 7 and 8 show compressible structures acoustically permeable.

In Figure 1, we can see an electroacoustic transducer capable of operating as a loudspeaker, earpiece or microphone. It comprises a self-supporting active membrane obtained by thermoforming, electroforming, molding or coating a film 3 of piezoelectric polymer material. The film 3 is coated on its two faces with conductive deposits 1 and 2 forming capacitor electrodes. The membrane 1, 2, 3 is in the form of a dome, for example a spherical cap with center 0 and radius of curvature R. The membrane assembly is electrically equivalent to a capacitor and when applied between the electrodes an alternating electric voltage, this active structure vibrates according to a thickness mode accompanied by a tangential alternating elongation mode. The membrane 1, 2, ₃ covers a rigid housing 8 and is fixed by its periphery to the edge of the housing 8 by means of a metal flange 4 A metal ring 7 placed in an annular housing of the edge of the housing 8 serves to establish electrical contact with the electrode 2 which forms the concave face of the membrane. The ring 7 is electrically connected to a terminal 6. The collar 4 which pinches the periphery of the membrane also serves as an elastic connection for the electrode 1 which forms the convex face of the membrane. A terminal 5 is fixed to the flange 4. The interior of the housing 8 communicates with the exterior through an orifice 9 which serves to balance the static pressures acting on either side of the membrane 1, 2, 3. The volume inside the case is partially filled with absorbent material 10 to prevent the establishment of standing waves. The volume 11 immediately adjacent to the electrode 2 is an air cushion at the static pressure of the air medium 12 in which the acoustic waves emitted or received propagate. The frequency response characteristic of the electroacoustic transducer depends on the diameter D of the vibrating piston constituted by the radiating membrane 1, 2, 3, the compliance and the inertia thereof, as well as the acoustic impedance constituted by the case. 8. The acoustic impedance of the box 8 is reduced to an acoustic capacity resulting from the volume of air enclosed and the active surface of the vibrating piston; the absorbent material 10 increases this capacity and introduces damping; the balancing hole 9 connects in parallel an acoustic inertia placed in series with an acoustic resistance.

The membrane shown in Figure 1 consists of a homogeneous film of piezoelectric polymer material. The piezoelectric effect is of dipolar origin. The materials which can be used to make the membrane are polymers such as polyvinylidene fluoride PVF ₂ , polyvinyl fluoride once substituted PVF and polyalkyl chloride, copolymers such as the copolymer of polyvinylidene fluoride and polyethylene tetrafluoride can also be used. The manifestation of the piezoelectric properties is linked to a preliminary treatment which includes an intense electric polarization phase preceded or not by a mechanical stretching phase.

Without departing from the scope of the invention, the membrane shown in FIG. 1 can be substituted for the one whose cross-section is given in FIG. 2.

The membrane of Figure 2 is of the bimorph type. It comprises two layers of polymer materials 13 and 14 which adhere perfectly to each other. The layers 13 and 14 can be made of dielectric materials devoid of piezoelectric properties. At least one of these layers has undergone an electrical charge implantation treatment producing an excess of permanent charge. When an alternating excitation voltage is applied to electrodes 1 and 2, the action of electrostatic forces produces elongations which can be made different by an appropriate choice of materials and excess charge. In the presence of a differential elongation proportional to the excitatory electric fields, bending torques M are obtained which cause an alternating curvature of the membrane. By way of nonlimiting example, a bimorph membrane can be produced using an electrically charged polyethylene tetrafluoride film which adheres perfectly to a polyvinyl chloride film. Of course, the bimorph structures can be made wholly or in part of piezoelectric polymer materials.

Figure 3 shows schematically most of the structures which have just been described. The housing 8 which contains a volume of air is capped by an active self-supporting membrane whose shape at rest is represented by the dotted line 15. This membrane vibrates in as a whole when it is subjected to an electrical or acoustic excitation. However, due to the peripheral fixation, phenomena of standing waves can give rise, at certain frequencies, to parasitic vibrations 17 (curve in phantom). In addition, the membrane may undergo permanent depression 16 under the effect of an accidental push acting on the convex face. As the membrane is fixed on the housing 8, it is not possible to erase this depression since; without delicate disassembly, there is no access to the concave face. Such sinking can result from awkward manipulation by the user, but it can also result from overpressure on the convex face of the membrane. Anyway, it must be considered that the self-supporting characteristic of non-developable surfaces such as spherical caps, conical trunk with straight or exponential profile, with concentric undulations goes hand in hand with a significant reduction in the thickness of the membranes (a few tens to a few hundred microns). As a result, these membranes are vulnerable to the sinking of their convex parts.

In Figure 4, we can see a sectional view of an electroacoustic transducer according to the invention. It comprises a housing 8 made of insulating material provided with a bottom 26 equipped with connection terminals 27 and 28. A membrane 18 similar to those of FIGS. 1 or 2 covers a circular opening situated at the top of the housing 8. The membrane 18 rests on the rim of the circular opening of the housing 8 via a recessed metal ring 21. It is pinched by its planar annular periphery by means of a metal collar 4. Thus, the electrodes which cover the faces of the membrane 18 are electrically connected to the collar 4 and to the ring 21 and these metal parts are in turn connected to the output terminals of a voltage step-up transformer 29. Input terminals of the transformer 29 are connected to the terminals 27.and 28 which pass through the bottom of the housing 26,

According to the invention, the case 8 immediately encloses an acoustically permeable elastic support under the membrane 18. This elastic support comprises at least two elements which are the cushion 19 and the grid 20, but these elements which are lightly pressed against the internal face of the membrane 18 are not lift elements. Indeed, the membrane 18 is self-supporting and it imposes its shape on the cushion 19 thanks to the convex shape of the grid 20. A plan view of the grid 20 is given in FIG. 5. The texture of the materials used to make the cushion 19 is illustrated by FIGS. 7 and 8. As shown in FIG. 7, it is possible to use a low density felt mattress whose packing has been stabilized with the aid of a binder, but which has retained a high porosity and good acoustic permeability.

By way of example, mention may be made of glass wools used in the field of thermal or acoustic insulation. FIG. 8 shows a mattress of cellular material with communicating cells; due to the low density, the open partitioning is reduced to its simplest expression, i.e. a three-dimensional network of meshes. Mention may be made of various polymer foams such as polyurethane and polyester. The cushion 20 being slightly compressed between the membrane 18 and the grid 20, it is the convex shape given to the latter which determines, with the concave shape of the membrane 18, the thickness of the cushion 20. This thickness can vary from center to the periphery of the membrane, or on the contrary be uniform if the center of curvature of the membrane 18 coincides with that of the grid 20. The grid 20 is fixed by the inside of the housing against the rim which delimits the circular opening capped by the membrane. A washer 22 held in place by the spacer 30 which bears on the bottom of the case 26 ensures the clamping of the periphery of the grid 20, because of the acnustic permeability of the membrane support 18, it is possible to envisage mounting inside the housing another self-supporting active membrane such as 24. This internal membrane 24 is clamped between two contact rings 23 and 25 which are interposed between the washer 22 and the spacer 30. The rings 23 and 25 are also connected to the transformer 29, so that the two membranes can cooperate with the sound radiation. The interior of the case 8 can be lined with absorbent material 40 to increase its acoustic capacity and to fight against standing waves. The mechanical compliance of the grid 20 and its mass can be chosen to form a mechanical resonator coupled to the membrane 18 by the cushion 19

By way of nonlimiting example, the grid 20 can be produced from a polyvinyl chloride lattice having a thickness of 2 mm and diamond-shaped meshes whose diagonals measure 6 mm and 4.5 mm. The cushion 19 is then constituted by two superimposed discs cut from a polyester wool mattress having a load-free thickness of 3mm. For a membrane 18 having a piston diameter D of 7 cm, one of the discs has a diameter of 7 cm and the other a diameter of 4 cm. The distance between the membrane 18 and the grid 20 is of the order of 3 mm, which ensures the compression of the superimposed discs.

In FIG. 6, two readings of the frequency response curve corresponding to the transducer of FIG. 4 can be seen with the dimensions which have just been indicated. The sound pressure level SPL was measured with a microphone placed in the axis of the transducer at a distance of 30 cm from the membrane 18. The electrical excitation power or white noise is adjusted to an effective watt. Curve 31 gives the response of the transducer of Figure 4 without the support 19, 20 and without the membrane 24, the curve 32 gives the response of the same transducer fitted this time with the support 19, 20, it can be seen that the natural resonance of the membrane 18 which extends between 10 and 18 kHz is flatter in presence of cushion 20 which improves the response in this region of the acoustic spectrum. The response is also improved between 0.63 and 5 kHz, because the resonance of the membrane support is used to accentuate its vibrational amplitude. The dip which occurs on the curve 32 between 2 kHz and 5 kHz can be filled by introducing the own radiation from the membrane 24 which can be designed to radiate in this region of the spectrum.

Thanks to the presence of the membrane support according to the invention, it has been possible to verify experimentally that the transducer has a high impact resistance, since the membrane 18 recovers its shape after a fall on its convex face. The membrane 18 also withstands finger pressure well. As regards the damping of parasitic vibrations of the membrane 18, the cushion 19 introduces a mechanical coupling which cooperates with the dissipative properties of the material constituting this cushion.

The cushion also acts as a coupling element between the membrane 18 and the resonant structure that constitutes the grid 20. It is therefore possible to mechanically increase the ability to radiate from the membrane in another region of the acoustic spectrum. than the one where its own resonance is located. The acoustic permeability of the cushion 19, grid 20 assembly also provides acoustic coupling with the other passive or active impedances which are contained in the casing 8.

Although the essential features of the present invention applied to preferred embodiments of the present invention, which have been described in the drawings and represented in the drawings, it is obvious that a person skilled in the art can bring about them. any modification form or detail that it deems useful, without departing from the scope of the invention.

In particular, the acoustic transparency can go hand in hand with the air permeability of the cushion and of the grid supporting this cushion, but it can also be eliminated when replacing the grid with a self-supporting shell of good mechanical compliance and of low mass and when a closed cell foam is used as a cushion.

The two elements of the elastic membrane support Can be melted into one, for example by treating with one suitable binder one of the faces of a fiber cushion so that it fulfills the function of a grid or a wall thin carrier.

The proposed device naturally extends to structures which provide a static pressure of non-uniform value along the membrane. This effect can result from the choice of an inhomogeneous thickness without load of the damping cushion and / or of a shape of the grid such that the interval separating it from the membrane varies in thickness.

It is also possible to sandwich the membrane 18 between two supports 19, 20 one of these supports extending outside the case 8 of the electroacoustic transducer.

Claims (12)

1, electroacoustic transducer comprising a rigid shoemaker (8) capped by a radiating self-supporting member (1, 2, 3) made of polymer material having at least one bulge, characterized in that the casing (8) contains an elastic support (19, 20) acoustically permeable and conforming to the shape of the concave parts of the internal face of the radiating membrane (1, 2, 3); the shape taken by the support face of the support (19, 20) being determined by the proper shape of the radiating membrane (1, 2, 3). 2. Transducer according to claim 1, characterized in that the elastic support (19, 20) comprises a grid (20) mechanically connected to the housing (8) and a compressible cushion (19) pinched between the internal face of the membrane and this wire rack. 3. Transducer according to claim 1, characterized in that the elastic support comprises a compressible cushion (19) having one of its faces stiffened by a binder; the stiffened face being fixed to the housing (8). 4. Transducer according to any one of claims 2 and 3, characterized in that the cushion (19) is made of a material consisting of mineral or synthetic entangled fibers. 5. Transducer according to any one of claims 2 and 3, characterized in that the cushion (19) is made of an organic material of cell type. 6. Transducer according to claim 5, characterized in that the cells making up the organic material are communicating. ¹ 7. A transducer according to claim 5, characterized in that the cells making up the organic material are closed. 8. Transducer according to any one of claims 1 to 7, characterized in that the housing (8) contains at least one active radiating element (24). acoustically coupled to the membrane (18) covering the housing (8), 9. A transducer according to any one of claims 1 to 8, characterized in that the radiating membrane (18) follows the shape of a dome having its convexity turning outwards from the housing (8). 10. Transducer according to any one of claims 1 to 9, characterized in that it comprises outside the housing a second elastic support which is acoustically permeable and cooperates with the first elastic support so as to wedge the radiating membrane (18). 11. Transducer according to any one of claims 1 to 10, characterized in that it comprises at least one radiating membrane (1, 2, 3) made of piezoelectric polymer material. 12. A transducer according to any one of claims 1 to 10, characterized in that it comprises at least one radiating membrane (1, 2, 13, 14) of the bimorph type.

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