FR2651633A1 - Electroacoustic transducer element and devices, with piezoelectric polymer bimorph, especially for producing a loudspeaker with a linear-type radiation diagram - Google Patents

Abstract

The invention supplies a novel electroacoustic transducer element, capable of exhibiting an impedance value of the same order as that of conventional loudspeakers (typically 4 to 8 Ohm), allowing direct matching to the elements of a sound reproduction system (amplifier, filters, etc.). The element consists of two symmetric quadrangular membranes (11, 12), arranged face to face, two opposite edges (13, 14) of which are embedded together, and spaced apart by spacers (15) in the mid-parts, the said membranes each being formed by a film (11, 12) made of piezoelectric polymer material, metallised on its two faces (111, 112, 121, 122), the two metallised films (11, 12) being excited in phase opposition by the same modulated electrical source. Preferential application to the production of linear loudspeakers, with one or more frequency-specialised elements.

Description

 Element, and devices for electroacoustic bimorph transducers of piezoelectric polymer, in particular for producing a speaker with a radiation diagram of the linear type.

 The field considered is essentially that of electroacoustic transducers, and more particularly but not exclusively, loudspeakers constituting linear electroacoustic sources.

 A linear source is for example rectangular, of width equal to or less than the wavelength, and of length (height) greater than the wavelength.

In near field, the acoustic intensity emitted by the source does not depend on the position along the large dimension; in far field acoustic acoustics is focused in the plane perpendicular to the large dimension, mid
distance from the ends of the source.

 The linear source being vertical, this focusing in a horizontal plane is advantageous for the listener, since, contrary to the case of the point acoustic source, the acoustic intensity is less in the volume outside the listening plane; it results in particular that there is less intensity reflected by the walls of the listening room (floor and ceiling), hence a gain in sound reproduction quality.

 These linear sources are difficult to achieve using conventional speaker technology.

 According to a common embodiment, the emission cone can thus be given an elliptical section, but it is rare that the major axis / minor axis ratio exceeds 2.

 The known techniques most suited to the constitution of linear sources are the ribbon loudspeaker and the electroacoustic loudspeaker.

 In the first case, the impedance is generally very low, since the conductor constitutes the membrane. In the second case, the impedance is very high, since it is that of an air capacitor, a few millimeters thick.

These loudspeakers must therefore be adapted to the impedance of the amplifier, by step-down and step-up transformers respectively. Because of the cost of these transformers added to that of the speakers, these technologies are only used in the very high end of electroacoustics.

 The invention aims to overcome in particular the drawbacks of these existing techniques.

 More specifically, a first objective of the invention is to provide an electroacoustic transducer element, capable of having an impedance value of the same order as that of conventional loudspeakers (typically 4 to 8 Ohm), allowing direct adaptation to the elements a sound reproduction system (amplifier, filters, ...).

 Another object of the invention is to provide transducer elements of this type, capable of having, in different embodiments of an element, or in different modes of assembling several elements, various characteristics useful for the constitution of high -speakers.

These objectives, as well as others which will appear subsequently, are achieved according to the invention using an electroacoustic transducer element, characterized in that it consists of two symmetrical quadrangular membranes, placed face to face, including two opposite edges, are embedded together, and spaced by spacer in their middle parts
and in that said membranes are each formed by a film of piezoelectric polymer material, metallized on its two faces, the two metallized films being excited in phase opposition by the same modulated electrical source.

 According to the embodiments of the transducer element, the means for embedding two opposite edges of the membranes can impart mechanical tension to said membranes.

 Advantageously, said jointing means of two opposite edges of said membranes consist of a single frame made of a polymer material with a coefficient of expansion substantially identical to that of said films constituting the membranes.

 In an advantageous embodiment of the invention, said element comprises a complementary damping film, extending in the plane of symmetry of said membranes. Said damping film is for example made of viscoelastic polymer material such as polyurethane foam.

 According to another preferred embodiment characteristic, each of said membranes consists of a laminate of n films of piezoelectric polymer material metallized on their two faces and electrically excited in parallel. This embodiment makes it possible in particular to modify the impedance of the element, for example to bring it to standardized values.

 Advantageously, said spacer spacer is formed by a bar extending substantially parallel to said embedding edges of the membranes, made of material of the type of an expanded polymer.

 The objectives of the invention are also achieved by means of an electroacoustic transducer device characterized in that it consists of two symmetrical tapes, arranged face to face, each consisting of a film of piezoelectric polymer material with metallized faces, said pair of tapes cooperating with a plurality of embedding means and spacing means defining a plurality of electroacoustic transducer elements as presented above.

This embodiment makes it possible to obtain sets of aligned transducers, according to an advantageous manufacturing process.

 According to the invention, an electroacoustic transducer device can also consist of the assembly of at least two electroacoustic transducer elements, mechanically independent, and having distinct resonant frequencies. Each of said elements has the same length, and a different width and / or voltage corresponding to its resonant frequency.

 A broadband loudspeaker has thus been produced, when each of said electroacoustic transducer elements is connected to the same electrical source, possibly through a specific filtering element.

Other characteristics and advantages of the invention will appear on reading the following description of some preferred embodiments of the invention, given by way of illustration and not limitation, and the attached drawings in which
- Figure 1 illustrates the structure of an electroacoustic bimorph transducer element of piezoelectric polymer, according to the invention;
- Figures 2A, 2B illustrate two modes of excitation of the metallized films constituting an electroacoustic transducer element according to claim 1;
- Figure 3 is a perspective diagram of an electroacoustic transducer element mounted in a frame;
- Figure 4 is a diagram of an electroacoustic transducer device according to the invention consisting of three complementary, mechanically independent elements, intended for example to form a broadband speaker;
- Figure 5 is a diagram illustrating an embodiment of an electroacoustic transducer element of the invention, provided with a central damping membrane;
- Figure 6 is a diagram illustrating the production of an electroacoustic transducer element membrane of the invention, according to a laminate of films of metallized piezoelectric polymer, in particular for reducing the impedance value of the element; ;
- Figures 7A, 7B are respectively a front diagram, and in section, of an electroacoustic transducer device of the invention, consisting of a pair of metallized piezoelectric polymer ribbons, defining a series of aligned transducer elements;
- Figure 8 is an enlarged view of one of the tranductive elements formed in the device of Figures 7A, 7B.

 The electroacoustic transducer element of the invention uses the piezoelectric properties of certain polymers as presented for example in the article entitled "Towards a new generation of sensors thanks to piezoelectric polymers" published by FranSois Micheron in the review "Measurement - regulation - Automatism "dated November 1981.

 The prototype of the piezoelectric polymers is polyvinylidene fluoride (PVF2) n, of formula (CH2-CF2) n.

 PVF2 is presented as a thin material, with a large surface, very flexible, with low density and great mechanical damping. Because this material is thermoformable, it is possible to give it any shape, for example a quadrangular cut as used in the electroacoustic transducer element of the invention.

 Other polymeric materials also have piezoelectric characteristics, such as polyvinyl chloride (PVC), polyvinyl fluoride (PVF), polyamides (nylon 66 (registered trademark)).

 Piezoelectric properties are also found in biopolymers, such as the constituent tissues, tendon, horn, or silk, among others. However, although synthetic constituents of these materials have been prepared with the same piezoelectric coefficients as natural biopolymers, the stability of these constituents seems precarious for the moment.

 The present invention therefore aims to use more particularly, for the time being, synthetic piezoelectric polymers, of the type presented in the preceding non-exhaustive list, without excluding the extension to embodiments using other piezoelectric materials.

 As shown in FIG. 1, an electroacoustic transducer element according to the invention has a bimorph structure, consisting of two quadrangular membranes 11, 12, of the same dimensions, formed from a film of piezoelectric polymer material. The two membranes 11, 12 are embedded at their end edges 13, 14, and slightly stretched. We will see later that the tension force is a bimorph adjustment characteristic.

 In an advantageous embodiment illustrated in FIG. 3, the bimorph 30 is embedded at its two end edges 33, 34 in a single frame 35. The frame is provided, in its upper part, with two contacts 31, 32 of excitation of the metallized faces of the films constituting the bimorph 30, for example according to one of the arrangements in FIGS. 2A and 2B. The bimorph 30 moves freely inside the frame 35. The latter can be fixed on a base, or in a box, at the fixing points 36, 37.

 A light wedge 15, forming a spacer, is introduced between the two membranes 11, 12, substantially halfway up the bimorph. In the diagram of FIG. 1, the length of the spacer 15 is considerably increased compared to the dimensions of the bimorph, for reasons of clarity. In reality, the length of the shim 15 is rather of the order of a few millimeters for a height of the bimorph of the order of 15 cm.

 The two films 11, 12 are metallized on all of their two internal faces 112, 12 and external, 11 12 and are electrically excited in opposition.

 Due to the properties of the piezoelectric polymer materials, one will thus obtain under excitation an elongation of one of the films each time the other film will undergo a shrinkage. There follows a displacement of the bimorph, having a maximum intensity at the level of the spacer 15, in a direction 16 perpendicular to the embedding direction 17.

 The displacement is considerably amplified in relation to the variations in dimensions of each of the two films; for a height of the bimorph h = 50 cm and a spacer of e = 5 mm, the coefficient of amplification of displacement is h / e = 100.

 For example, for a bimorph made up of two PVF2 films, 30 µm thick, the displacement at the center is 17 µm per applied volt. Assuming that in dynamic operation (1 k) the static displacement values are retained, a few tens of volts applied to the bimorph are sufficient to produce an acoustic level of 90 to 95 dB at 1 meter, sufficient for domestic listening.

 FIGS. 2A and 2B illustrate two modes of excitation of the metallized faces 121, 122, 111, 112, the constituent films of the bimorph of FIG. 1.

 In these FIGS. 2A, 2B, the arrows in solid lines 20 correspond to the remanent polarization of the piezoelectric polymer constituting the films 11 and 12.

This remanent polarization is characteristic of the polar nature of PVF2, and comes in particular from the ionic asymmetry created at the level of the monomer: one carbon in two carries two hydrogens, the other two fluorines. Appropriate mechanical and electrical treatments, during the synthesis of PVF2, make it possible to accentuate this phenomenon (see above-mentioned article).

 The dotted arrows 21 represent the polarization induced by the excitation of the metallized faces of the films.

 In the embodiment illustrated in FIG. 2A, the modulated source 22 induces the polarizations 20 in opposition, while the remanent polarizations of the films 11, 12 are parallel. In the case where the film 12 lengthens, the film 11 then shrinks.

 In the case of excitation illustrated in FIG. 2B, the polarizations induced 20 by the modulation source 22 are in parallel, while the remanent polarizations 21 are in opposition. In this case, a retraction of the film 11 is also observed at the time of an elongation of the film 12.

 The first resonance mode of the bimorph embedded at its two ends is at very low frequency, in the absence of mechanical tension applied along axis 17: with the previous illustrative dimensions, the fundamental resonance mode is located between 10 and 13 Hz .

 This frequency can be increased, as in the case of a vibrating cord, by applying a mechanical tension to the films 11, 12 along the axis 17, at the cost of a reduction in amplitude in the center.

 This property has two implications illustrated in Figure 3 and 4 respectively.

 The first implication relates to the compatibility of the constituent materials of the frame and the bimorph of the same transducer element (fig 3).

 If the loudspeaker frame does not have the same temperature coefficient as that of the polymer (approximately 10-4 K1), the voltage of the bimorph 30 is a function of the temperature. It is therefore preferable that the frame 35 is itself made of polymer (for example PVC) and held, for example, in its middle part 36, 37 so that it can expand freely in direction 17.

 The second implication leads to consider the association of several transducer elements set on separate resonance frequencies (fig 4).

 A broadband loudspeaker can thus be produced by association of several bimorphs 41, 42, 43 of the same length, subjected to mechanical voltages T1, T2, T3, the higher the higher the frequency range to be reproduced. .

 On the other hand, advantageously, each of these transducer elements 41, 42, 43 will have a different surface. Indeed, in far field, the acoustic pressure is proportional to Vb.f2, with f: emitted frequency and Vb: volume swept by the membrane (surface x displacement).

 I1 results from this that, in the represented figure, the surfaces of the bimorphs 41, 42, 43 are increasing, and the tensions T1, T2, T3 of the membranes are decreasing, so as to allow the element 41 to restore the frequencies most and at element 43 the lowest frequencies.

 On the other hand, in order to effect a good distribution of the frequencies to be emitted between the three elements 41, 42, 43, it is advantageous to provide a filtering of the electrical signals supplied by the modulated source 40. More particularly, the bimorphs of large surface should not be supplied with high frequency signals, in order to maintain, at least at these high frequencies, a linear acoustic source operation. The referral filters can be constituted by simple resistors R1, R2, R3, placed in series with the metallized membranes 41, 42, 43 which by themselves have a capacitive impedance.

 FIG. 5 illustrates a structure of a transducer element comprising a complementary membrane. The resonance modes of the bimorphs of the invention being related to those of strings held at their ends, they can be the source of distortions in "waves", sources of harmonic distortion. These parasitic modes must be absorbed, if not eliminated. The proposed damping technique consists in placing between the two films 11, 12 of the bimorph a film 51 of viscoelastic polymer, that is to say having high mechanical losses (polyurethane foam for example). This family of materials is widely used in the loudspeaker industry: they are used in particular at the membrane-chassis junction and avoid the reflection of acoustic waves propagating in the membrane, therefore standing waves in the membrane. the spacer spacer for the bimorph then advantageously consists of two half shims 52, 53 attached and fixed by gluing, for example on either side of the damping membrane 51.

 The damping membrane 51 thus makes it possible both to obtain an adjustment of the resonant frequency of the transducer element, as well as the damping of the natural modes, etc.

Impedance matching
The capacitive impedances of the bimorphs used in the invention will generally be greater than the usual output impedances of the amplifiers (4 to 8 Ohm). Rather than performing the impedance matching by an expensive step-up transformer, it is advantageous (fig 6) to reduce the impedance of each film of thickness z by using n layers 601, 602, 603, 604, d 'thickness z / n, placed electrically in parallel on the electrical source 63 and bonded together. The bimorph compliance remains essentially the same as for the thickness z, while the apparent dielectric constant is multiplied by n2 and the apparent piezoelectric coefficient by n (the electrical calculation of this assembly establishes its exact equivalence with a step-up transformer of ratio n It should be noted that this laminated structure can be obtained by superimposing already polarized polymer films, by inverting at each layer the remanent polarization direction 61i and induced 62i. this structure can also be obtained from non-polarized films, which will then be polarized together once assembled: the alternation of the remanent polarizations takes place by itself.

Using the example of Figure 4 of a 3-bimorph speaker 41, 42, 43, and setting as a rule that the impedance of a bimorph must not be less than 4 Ohm, at its frequency high cutoff, we obtain the different numbers of layers presented in table I
TABLE I
S (m2) fc (Hz) C1 (F) Z1 (Ohm) nn Zn (Ohm) 131 2.10-2 20k 6.7.10.8 119 4 7.4
B2 0.16 5k 5.3.10-7 60 3 6.7
B3 1,300 3.3.1 on6 159 5 6.4
S: surface, fc: high cut-off frequency, Cl: capacitance for a layer of 30, am, Zî: impedance at fc for a layer, n: number of layers, Zn impedance at fc for n layers.

 Given that the impedances Zn are all close, the resistors R1, R2, R3 constituting the routing filters in FIG. 4 can all be equal to 7 Ohm. However, if the three bimorphs 41, 42, 43 are put in parallel at the output of the same amplifier, the impedance at the highest frequencies tends towards approximately 2 Ohm which is too low; you have to use more sophisticated routing filters or supply these bimorphs with separate amplifiers.

Aligned sound sources (Fig 7At 7B. And 8!
Bimorphs according to the structure described can be assembled online and behave as a linear acoustic source for frequencies such that the corresponding wavelength is greater than the distance separating their centers.

 As shown in Fig 7A, 7B, the source 70 consists of a pair of ribbons 71, 72 mounted in a frame 73 having an opening of 2 m x 12 mm. The two laminated strips 71, 72 are embedded therein at their ends 74, 75, where the contacts are made. Every 2 cm, are placed in the opening of the intermediate embedding means 76 consisting for example of two cylindrical rods 77, 78 with a diameter of 2 mm, separated by about 50 μm and which periodically enclose the double ribbon 71, 72: an elementary bimorph 80 is therefore constituted by the surface of ribbon compnse between two pairs of successive embedding rods 77, 78. The shims 79, of thickness 1 mm, are advantageously cylinders of expanded polymer (polyethylene for example); they are inserted and glued between the two tapes 71, 72 midway between the pairs of mounting rods 77, 78.

Or for example to produce a linear source two meters high, 1 cm wide, in the band 3 at 16 kHz. At 16 kHz, the wavelength is 2 cm, the transducer elements therefore have a height h = 2 cm. A frequency below 3 kHz is chosen for the resonant frequency, ie fr = 1500 Hz. Under these conditions,
e (wedge width 79) = fr.h2 / 600 = lmm.

The capacity of the acoustic source is
C = 10-10.4.10-2 / 3.10-5 1.33.10-7 F
and its impedance at 16 kHz:
Z = 75 n
This impedance is reduced to 75/9 = 8.3 Ohm by stratification of 3 layers of 10 µm.

 The embodiments presented, as well as the numerical examples in no way limit the scope of the present invention, and those skilled in the art will imagine for themselves the many other variants covered by this patent.

Claims (11)

 1. Electroacoustic transducer element, characterized in that it consists of two symmetrical quadrangular membranes (11, 12), arranged face to face, of which two opposite edges (13, 14), are embedded together, and spaced by spacer (15 ) in their middle parts,  and in that said membranes are each formed by a film (11, 12) of piezoelectric polymer material, metallized on its two faces (11î, 112, 121, 122) the two metallized films (11, 12) being excited in opposition to phase by the same modulated electrical source (22).  2. electroacoustic transducer element according to claim 1 characterized in that the embedding means of two opposite edges of the membranes (11, 12) impart mechanical tension to said membranes.  3. electroacoustic transducer element according to any one of claims 1 and 2 characterized in that the said jointing means of two opposite edges of the said membranes consist of a single frame (35) made of a polymeric material with a coefficient of expansion substantially identical to that of said films constituting the membranes (11, 12).  4. electroacoustic transducer element according to any one of claims 1 to 3 characterized in that it comprises a complementary damping film (51), extending in the plane of symmetry of said membranes (11, 12).  5. electroacoustic transducer element according to claim 4 characterized in that said damping film is made of viscoelastic polymer material such as polyurethane foam.  6. electroacoustic transducer element according to any one of claims 1 to 5 characterized in that each of said membranes consists of a laminate of n films (601, 602, 603, 604) of piezoelectric polymer material metallized on their two faces and excited electrically in parallel.  7. electroacoustic transducer element according to any one of claims 1 to 6 characterized in that said spacing spacer (15, 52, 53, 79) is formed by a bar extending substantially parallel to said embedding edges of the membranes (13, 14, 74, 75), made of material of the type of an expanded polymer.  8. Electroacoustic transducer device characterized in that it consists of two symmetrical tapes (71, 72), arranged face to face, each constituted by a film of piezoelectric polymer material with metallized faces, said pair of tapes (71, 72) cooperating with a plurality of embedding means (74, 75, 77, 78) and spacing means (79) defining a plurality of electroacoustic transducer elements (80) according to any one of claims 1 to 7.  9. Electroacoustic transducer device characterized in that it consists of the assembly of at least two electroacoustic transducer elements (41, 42, 43), according to any one of claims 1 to 7, mechanical. ment independent, and having distinct resonant frequencies.  10. Device according to claim 9 characterized in that each of said elements has the same length, and a width and / or a tension (T1, T2, T3) distinct corresponding to its resonant frequency.  11. Device according to any one of claims 9 and 10 characterized in that each of said electroacoustic transducer elements is connected to the same electrical source (40), possibly through a specific filtering element (R1, R2, R3).