GB2111799A - Electro acoustic transducer - Google Patents

Description

SPECIFICATION A polarized solid dielectric capacitor electroacoustic transducer The invention relates to electroacoustic transducers. These may be used in microphones and hydrophones in which the acoustic pressure acts directly on the transducer which comprises a vibrating structure of the electrically polarized solid dielectric capacitor type. Capacitor microphones using a solid electrically polarized dielectric are generally formed by one or more dielectric films coated with electrodes. Depending on the mechanical tensile or compression stresses resulting from the elastic deformation induced by the incident acoustic pressure, electrical charges are created by piezoelectric effect or by electric charge excess.

To obtain a linear transducer effect, the induced electric charge must vary exactly as the incident acoustic pressure. The use of a flat membrane resisting the deformation due to the extensile stresses alone produced by the increase of its area within a rigid mounting contour does not allow a linear response to be obtained in amplitude, except when the membrane is a bimorph structure. In fact, the deformed shape of an homogeneous film outside the plane creates extensile stresses whichever the side on which the thrust is exerted. To get over this disadvantage, an homogeneous film may be given an enveloping shape which, by expanding and contracting under the effect of the acoustic pressure, will produce alternate stresses.

However, the shaping of a film also presents drawbacks, particularly in so far as the stability in time of the shape is concerned, which has repercussions on the electroacoustic characteristics.

The mechanical compliance of a vibrating microphone structure influences operation thereof, for it fixes the resonance frequency and thus the upper limit of the frequency band reproduced at a constant level. In the case of an omnidirectional microphone, it must be arranged so that the rear face of the membrane is not subjected to the acoustic pressure and, for this purpose, the membrane is mounted in a rigid case so as to cause it to compress a certain volume of air. The rigidity of the vibrating structure is thus reinforced by the presence of the volume of air, but since the deformation stress is in part stored up in a medium void of transductor properties, the sensitivity of the microphone is less than if its membrane were the only member resisting the acoustic pressure. The influence of an air cushion loading a membrane is preponderant when this latter is very flexible, when its area is large and when the volume of air compressed is reduced.

In addition, a flexible membrane of large area is generally thin, so that the capacitor formed by this membrane equipped with electrodes has a high electric capacity. According to electrostatic laws, the potential difference of a charged capacitor is proportional to the charges carried by the plates and inversely proportional to the capacity. The open circuit voltage delivered by a thin membrane microphone is then relatively low, requiring voltage amplification and efficient screening against the disturbing electric fields.

Piezoelectricity phenomena have given rise to numerous studies showing that concurrently with intrinsic piezoelectricity defined as is known by a tensor of rank three and implying that the material considered does not have a centrosymmetric structure, there exists a flexion piezoelectricity. The electric polarization induced by flexion piezoelectricity is determined by the piezoelectric coefficients of a tensor of rank four and it appears when there exists a stress gradient within the material subjected to deformation. Contrary to intrinsic piezoelectricity, flexion piezoelectricity does not involve any previous electric or structural anistropy of the mechanically stressed material for it is the inhomogeneous stress which creates the structural defect giving rise macroscopically to the induced electrical polarization. Nevertheless experience shows that occurrence of flexion piezoelectricity is substantially increased when the material considered has received an electric anisotropy of the polar type or by charge excess.

The simple recognition of flexion piezoelectricity and measurement thereof form an integral part of the state of the technique such as illustrated in the article by BREGER et al entitled "BENDING PIEZOELECTRIC ITY IN POLY VINYLIDENE FLUORIDE" and appearing on pages 2239 and 2240 of the revue: "Japan J. APL.

PHYS. Vol. 15(1976), No. 11".

With a view to overcoming the above-mentioned drawbacks, the present invention aims at applying flexion piezoelectricity to a solid polarized dielectric capacitor microphone structure.

More precisely, the invention provides a solid polarized dielectric capacitor electroacoustic transducer comprising at least two collector electrodes, a vibrating structure formed by said dielectric and subjected to the incident acoustic pressure and a support to which said vibrating structure is secured by its edges; said collector electrodes being carried by said vibrating structure and connected to two output terminals, the improvement being that said vibrating structure is provided as a flat structure in the form of a sufficiently thick plate so that the middle layer does not undergo any significant deformation during flexion of said plate.

The invention will be better understood from the following description and accompanying Figures in which: Figures ito 6are explanatory Figures; Figure 7 is a meridian section of a first variation of a microphone in accordance with the invention; Figure 8 is a partial isometric view of a second variation of a microphone in accordance with the invention; Figure 9 is an electrical diagram of the amplifier circuit; Figure 10 is a meridian section of a third variation of a microphone in accordance with the invention; Figure 11 shows a detail of construction of the vibrating plate microphone in accordance with the invention; Figure 12 shows a detail of construction of the vibrating plate microphone according to another variation of the invention; Figure 13 shows a constructional detail of the vibrating plate microphone according to yet another variation of the invention; and Figure 14 is a sectional view of a microphone with integrated impedance adapter circuit.

In Figure 1,two meridian sections (a) and (b) can be seen corresponding respectively to elastic structures embedded at their periphery 2. The structure shown at (a) is a flat plate 1 of thickness e1 whereas the structure shown at (b) is a flat membrane 3 of thickness e2 considerably less than e1.

If we assume that these structures have the same radius R and are subjected to the same acoustic pressure p, it can be seen that deflection AZ1 assumed by the deformed shape 4 of plate 1 is less than thickness e1 whereas the deflection AZ2 assumed by the deformed shape 5 of membrane 3 is considerably greater than e2 A remark should be made here as regards the mechanical resistance to pressure. In fact, if pressure p increases the plate will behave like a membrane and if pressure p decreases it is the membrane which will have the behaviour of a plate.

Within the scope of the present invention, this lack of determination is removed for a deformable plate is used which is subjected directly to the acoustic pressure. Now, the highest value of the acoustic pressure which the receiver element of a microphone has to withstand corresponds to a sound intensity level of 125dB above the audibility threshold fixed art2.10~5 Pascal, i.e. a maximum pressure of the order of 35 Pascal. Deflection w0 in the center of a circular embedded plate of height h and radius R subjected to a pressure p is given by the approximate theory of low flexion plates by the formula:

with as validity condition: w0 0.1 h We can then evaluate the ratiowhich is not to be exceeded for a given elastic material.

Taking for example a material with relatively low rigidity, such as vinylidene polyfluoride (PVF2) which has an elasticity modulus E = 3.5 1 09N.m-2 and a Poisson coefficient v = 0.3, we find that the pressure p = 35Pa fixes a ratio Rh S 100.

Thus, a circular embedded structure of radius R = 1 Omm satisfies the criterion of low flexion if its thickness h is at least equal to 100 lim. A piezoelectric ceramic material of the PZT type twenty times more rigid than PVF2 leads to a minimum thickness smaller by half according to the same criterion. However, it should be pointed out that the permittivity of ceramic PZT is one hundred times higher than that of PVF2, which means that the open circuit voltage of a ceramic microphone capsule is, all other things being equal, a hundred times smaller.

Piezoelectric ceramics and intrinsically piezoelectric crystals are rather reserved for the microphonic detection of very high or ultrasonic frequency acoustic pressures.

Tha analysis of the piezoelectric effect which follows amounts to considering the elastic deformation stress within a polarized material, in deriving from the stresses generated by the acoustic pressure the electric charge densities induced on the surface and in seeing how these charge densities are collected by the electrodes.

In so far as the elastic deformation stress is concerned, the notions of plate and membrane are clearly separate, even if use is made of a classification criterion based on low flexion. In fact, the deformation stress of an elastic structure breaks down into several terms which bring in expansion, flexion and shearing tensions. If W is the total deformation stress and if WT represents the fraction of this stress corresponding to expansion tension alone, the structure shown at (a) in Figure 1 may be characterised by a ratio W - WT/(WT) very much greater than unity, for the expansion tensions play only a very small part in the resistance of the plate to the acoustic pressure. On the contrary, the structure shown at (b) in Figure 1 has a ratio W - WT/(WT) close to unity if deflection AZ2 is close to thickness e2 and which tends towards zero when membrane 5 becomes infinitely thin, for then it is bereft of rigidity to flexion.

Another means of making a distinction between flat plate and flat membrane consists in noting that the expansion tensions do not change sign when the thrust on a membrane is reversed in the presence of an alternating acoustic pressure. On the other hand, the other tensions which correspond to the stress W - WT change sign in harmony with the alternating stress. The result is a greater or lesser linearity of the electro-acoustic transducer effect which determines the part assumed by the different mechanical tensions in the elastic resistance to the incident acoustic pressure.

Piezoelectricity may appear in two distinct forms which are intrinsic piezoelectricity and flexion piezoelectricity. Intrinsic piezoelectricity implies that the material subjected to deformation has properties comparable to those of a crystalline body of a non centro-symmetric class. This is the case for polar polymer materials such as polarized vinylidene polyfluoride. On the other hand, flexion piezoelectricity may exist in any dielectric body, for it can be attributed to the formation of dipolar moments in the presence of a mechanical tension gradient. The intensity with which flexion piezoelectricity may appear is considerably increased when the material is electrically biassed by charge excess (electret) or by creation of a polar phase on a macroscopic scale.

In Figure 2, an experimental device can be seen for deducing the two forms of piezoelectricity one from the other. This device comprises a prismatic section beam 6 made for example from vinylidene polyfluoride polarized parallel to axis Z. This beam is carried at one end by an embedment 2 and it may be caused to bend by applying thereto at the other end a force F in a direction parallel to axis Z. The longitudinal axis of the beam OX forms with an axis OY and axis OZ a trirectangle trihedron; the figures in brackets designate these axes according to the practice in crystallography. The faces of beam 6 normal to axis Z carry two electrodes 7 and 8 forming a capacitor. These electrodes are connected to terminals 9 and 10 between which an electric voltage V appears. The state of stress in the right-hand section 13 of the beam is shown by a triangular distribution with zero center 12 which shows the flexion stresses. The length of the embedded beam 6 is L and the system of electrodes 7 and 8 is situated on the variable abscissa x = -1. The faces of beam 6 carrying electrodes 7 and 8 have as equations: Z = h and Z = -h and their width is equal to b.

At a current point C of coordinates X, Y, Z of beam 6, the non zero components of the stress tensor are: 3 F 3 F (1 Z2 xl = -4 bh 2 = X.Zandx5 = ~~~~ X1 describes the distribution 12 with zero mean over the height 2h of the beam and X5 describes the distribution 11 whose mean X5 over height 2h of the beam is necessarily equal to - F/(2bh). No expansion of tension along OX exists since force F is perpendicular to the axis of the beam. The electric charge Q induced in electrodes 7 and 8 of area S, at abscissa x = -e corresponds to the charge density: Q #X1 S = d3sX+f3113(8Z )x= -t The term d35.X5 represents the charge density relative to the intrinsic piezoelectricity for which it is known that the induced polarization Pi is given by the tensorial expression: P = dijk Xjk where dijk are the piezoelectric modules belonging to a tensor of rank three.

The term f3113 (6 X/6)x = -e varies as a function of the abscissa of the electrodes and represents the charge density relative to the flexion piezoelectricity for which it is known that the induced polarization Pj' is given bythetensorial expression: bXjk Pi = fijkl #xl where fibre are piezoelectric modules belonging to a tensor of rank four and xl the fth coordinate. Knowing that the coordinates are three in number, the tensor expressing flexion piezoelectricity contains 81 piezoelectric modules which are not all zero whatever the dielectric material considered. With the measuring device of Figure 1, the two forms of piezoelectricity can be easily deduced one from the other.

By way of example, there are given below three measured values of the flexion piezoelectric modules.

Material f313 f3223 Polarized PVF2 210-16C.m/N 0.510-15C.m/N Non Polarized PVF2 0.510-18C.m/N It appears then that any structure formed from an insulating material and in a state of inhomogeneous stresses is capable of delivering an electric signal between electrodes which is a measure of the stress undergone by the structure.

The flexed beam of Figure 2 may be considered as an illustration of the phenomena which exist preponderantly in plate of Figure 1.

Figure 3 shows an elementary plate volume dx, dy, dz at rest (cubic shape) and under pure flexion stresses.

The volume 15 of thickness e comprises at mid-height a neutral layer 16 whose surface does not vary between the rest state and the deformed state. If we accept this assumption, we can calculate for a layer 14 of height dZ at distance Z from the neutral layer what are the elastic stresses induced by curvatures Px and py undergone by the flexed plate. By expressing the equilibrium of the volume of Figure 3, by following a known mathematical reasoning, the differential equation may be obtained governing the low flexion of transversely loaded plates.

The theory of low flexion may be better understood from Figure 4 which shows the spherical skull-cap deformation of radius p of a flat circular plate subjected at its periphery to a uniform flexion couple M. Arc Ab has as length the diameter 2a of the plate and its highest point corresponds to the deflection 6. This is true if we assume that the mean does not undergo any meridian deformation. There may then be calculated the radius an of the circular deflection undergoing the highest circumferential shortening. This radius has as value: a a1 = psin4)with4)-- p the circumferential expansion is then equal to 1 that is, approximately 2=4)2 a ##2 # By introducing the value of 6 which is approximately 2 , we find: #=3# This represents an upper limit for the expansion of the middle layer 16 of Figure 3 which should be compared to expansions at the level of the faces equal to e/2 i/p The notion of low flexion amounts then to assuming that the expansion of the middle layer is negligeable at mid-height with respect to the flexion expansions of the faces, which amounts to writing that 6 < < e.

In practice, the limit is set at 6 s 0.1 e, as already pointed out previously.

The theoretical account of how to calculate the sensitivity of a solid polarized dielectric capacitor microphone is much too complex to be given at length in this description, but the main lines thereof will be recalled for the case shown in Figure 5. It is a flat circular plate 1 with peripheral embement 2 also flat.

Electrodes 7 and 8 cover completely the free faces of radius R of plate 1. In the meridian section of Figure 5 where axis Z is an axis of revolution, the middle layer of the plate is shown by a broken rectilinear line in the undeformed state. When the acoustic pressure exerts a transverse uniformly distributed thrust, the plate is deformed along a curve with bending point shown with a dashed line in Figure 5. The current point P0 of the middle layer moves to P1 and the center of the plate presents a deflection w0 given by the formula: w0 = 3 p 1 - v2 R4 16 ' E h3 The movement w at distance r from the center is given by the expression: w = wo[l(r)2]2 R In the normalized diagram of Figure 6, the parameter p =lS shown in abscissa and a normalized scale as ordinates. Curve 17 represents as a function of p the law of movements w/wO defined by the preceding formula.

To calculate the induced electric charge contribution, it is required to know the mechanical stresses Xr and Xe present in the deformed plate. Curve 18 of the diagram of Figure 6 gives the values of the expression Xo / kEZ/1 -V2 where k is given the expression: 3 1-v2 R2 K = qP E T and v=0.3 Xe is the circumferential stress in a system of cylindrical coordinates (0 r, z).

The curve 19 of the diagram of Figure 6 gives the values of the expression kE Xr/ .Z 1-# Xr is the meridian stress.

It is interesting to see that, because of the assumption of low flexion, the stresses increase linearly with z on each side of the middle laver.

Furthermore. it mav be noted that dw dr is zero at abscissa R, that stress Xr is cancelled out at abscissa

and that stress Xe is cancelled out at abscissa

From the knowledge of stresses Xr and Xe which are a function of the cylindrical coordinates (8, r, z), it is necessary to calculate layer by layer the polarization induced by intrinsic piezoelectricity and flexion piezoelectricity. After integration along z, a charge surface density is disposed on the faces of the plate which must be further integrated over the whole extent of electrodes 7 and 8 so as to obtain the total charge induced by the acoustic pressure.

After relatively tedious calculation which it would be pointless to reproduce here, it may be shown that the contributions to the induced charge by intrinsic piezoelectricity and by flexion piezoelectricity are in the aggregate zero.

Experience confirms this surprising result that a flat circular plate with flat embedment in transversely polarised homogeneous material completely covered on both its faces with electrodes has a substantially zero sensitivity as a piezoelectric transducer subjected to the acoustic pressure.

As the calculations progress it can be seen however that the electric charges induced on the faces of the plate change sign with the radial position and that the overall cancelling out is the result of an exact compensation related to the precise location of the + and - charges over the extent of the faces.

The preceding considerations result in practice in the existence of a sensitivity which may be used when the weakly flexed plate configuration is used. In fact, there exist in practice structural abnormalities which prevent the charges collected by the electrodes from being exactly cancelled out. Slight warping of a plate, imperfect embedment or structural inhomogeneity are so many factors which contribute to obtaining an effective sensitivity.

This natural sensitivity may be compared with that of membrane microphone systems but the superiority of the plate lies in the possibility of being associated with a sealing case of reduced volume and in a better operating linear. A substantial gain in sensitivity has been observed with respect to embedded transducer plates with electrodes completely covering the faces. This gain is obtained by creating a systematic incurvation of the plate through a non flat embedment. The curved or wavy shape given to the plate by the embedment modifies the stress condition by giving to the two forms of piezoelectricity the possibility of appearing in a precise way.

However, this means is not the only one capable of increasing the acoustic sensitivity.

In Figure 7, the meridian section of a microphone capsule in accordance with the invention can be seen in which the vibrating plate is not embedded. According to this variation, plate 1 covered completely over both its faces with electrodes 7 and 8 is simply supported instead of being embedded. Unlike the embedment shown in Figure 5, the simple bearing support of Figure 7 does not generate a flexion couple at the securing point of the plate flexed by the acoustic pressure p. The deformed shape shown with a broken line in Figure 7 has a shape which is simply curved without any inflexion point. The result is a state of mechanical tension very different from that of Figure 6, but which remains governed by the resistance to the flexion. The charges collected as a whole by electrodes 7 and 8 do not offset each other exactly and the sensitivity of the microphone capsule is substantially increased.

To achieve the conditions of a simple support, the section of Figure 7 shows that the bottom of case 21 comprises an annular collar in which there has been formed a bearing surface 22 with a pointed top on which the plate assembly 1,7,8 rests. The application of this assembly against the bearing surface 22 is provided by a seal 23 made from a compressible insulating material provided inside crown 20 which fits on to the bottom of case 21. When plate 1 vibrates, its periphery pivots about the knife-edge crest of bearing surface 22 which does not generate any flexion couple at the securing point. This pivoting causes the bearing face of seal 23 to rock, but so as to avoid creating a resisting couple, this seal is made from a polymer or elastomer foam. This seal may be made conducting, so as to provide contact with electrode 7. Electrode 7 plays the role of ground electrode connected to the metal parts 20 and 21 of the case and electrode 8 is stopped at a short distance from bearing surface 22. Of course, nothing prevents parts 21 and 22 of the case from being made of an insulating material and from forming the electrostatic screening by crimping in an outer metal case, as illustrated in Figure 14.

The intensification of sensitivity with which the device of Figure 7 is concerned rests essentially on the mode of securing the vibrating plate to the case. By comparing the deformed shape of Figure 7 with that of Figure 5, it can be seen that they have a similar trend if the crown is neglected close to the embedment which presents a curvature reversal.

This remark ieads quite naturally to defining what is the charge QF induced by flexion piezoelectricity on electrodes completely covering a circular embedded plate.

The mathematical expression for this charge is:

Stresses Xr and Xe may be expressed as a function of the ratio p =R by the expressions (a) or (b) which follow.

the value of K is given above in relation with the description of Figure 6.

These expressions allow the induced charge QF to be expressed.

With the relationships (b), we have

which may be written: #E 6w QF = - 1 - # (f3113+f22)R(6r)=R (c) With relationships (a) we find:

the change of sign of the integrant (1 - 2 2) takes place for

The relationship (c) shows that the induced charge is zero if the plate is fixed by a flat embedment, for 6w XSr)r=R The relationship (d) shows that if R' = , the crown included between radii R and R' collects a charge equal and of opposite sign to the charge collected by the inner circle of radius R'.

From what has just been said, a considerable gain in sensitivity of the device of Figure 5 may be obtained by subdivision of one of the electrodes 7 or 8 along a circular cut of radius R'.

This variation is illustrated by the partial isometric view of Figure 8. The bottom of case 21 and collar 20 form here a flat embedment nipping a flat plate 1. Electrode 27 completely covers the outwardly turned face of plate 1. The inner face of plate 1 carries two concentric electrodes 26 and 25. The central electrode is a disk 25 whose radius is close to the above defined value R'. The peripheral electrode 26 is a ring with inner radius close to R'. The circular cut which separates the two electrodes 25 and 26 is situated at radius R', i.e. at 70% from the center with respect to the non embedded zone of plate 1. If we take for reference electrode 27, the microphonic voltages delivered by electrodes 25 and 26 are of opposite signs and greater than the voltage which would be delivered by electrodes 25 and 26 joined together. Several methods for using the voltages supplied by the electrodes may be envisaged. The simplest solution consists in providing on the internal face of plate 1 only one of the electrodes 25 and 26. In this case a single input impedance matching circuit is appropriate.

The microphonic sensitivitys =where v is the off-load voltage delivered and p the acoustic pressure may be derived from the expression Q'F v = -- c where c is the interelectrode capacity given by c = e-seR'2/h and Q'F the collected charge calculated by integrating the expression (d) of charge 0F between the integrating limits 0 and R'/R.

We obtain 3 1+v R 2 S = 8 (f3113 + f3223) By way of non limiting example, this voltage sensitivity can be estimated in the case of a polarized vinylidene polyfluoride plate.

The values Rand h are fixed so as to obtain a first resonance frequency of 3 kHz, i.e. : R = 7.5 mm and h = 240 microns. A sensitivity s = 1 mV/Pa is obtained.

Since the interelectrode capacity is the same for the central electrode 25 and for the annular electrode 26, the same sensitivity is obtained in both cases. It goes without saying that electrode 27 does not need to extend beyond the zone facing that of electrodes 25 and 26 which serves for collecting the induced charge.

The sensitivity of the microphone capsule may however be doubled by using electrodes 25 and 26 as output terminals. In this case, electrode 27 covers the whole of plate 1 and it must be efficiently screened against external electrostatic influences, for it is floating. The electric circuit of Figure 9 illustrates this mode of connection with respect to a differential amplifier comprising two unipolartransistors with isolated gate T1 and T2. The sources of transistors T1 and T2 are connected to the negative pole 30 of a symmetrical power supply having pole 29 as common pole. The drain of transistor T1 is connected directly to the positive pole 28 of the power supply whereas the drain of transistor T2 is connecte thereto via a load resistor R1 at the terminals of which appears the amplified voltage. Electrodes 25 and 26 are resp existed, which solution also comes within the scope of the present invention, but it is not the double of what the capsule of Figure 9 supplies between electrode 27 and one of the electrodes 25 or 26. All other things being equal, the capsule of Figure 10 offers an electric impedance four times less than that of Figure 8, which may be useful if energization of the amplifier circuit is provided by the short-circuit current and not by the developed off-load voltage. It is useful to note that the short-circuit current with constant induced charge is a function proportional to the acoustic frequency and that if a current amplifier with low input impedance and high output impedance is used, a capacitive load must be provided at the output to rectify the response curve.

In the applications described which use a radius R' for separating the electrode zones from the polarized zones, it may be considered that the value V 2/2R is optimal. With a slight deviation from this value there exists a gain in sensitivity. In general, when polarization of the dielectric only exists in a part of the plate, it is advantageous to use electrodes which do not exceed the polarized area. The localized polarization may be easily obtained by conventional polarizing processes between electrodes or by Corona effect with keeper ring.

The provision of radial inhomogeneities is only one aspect of what may be done to obtain better microphone sensitivity.

Figure 11 is a partial sectional view of a stratified vibrating plate comprising a layer 35 which is inert from the piezoelectric point of view to which adheres a piezoelectrically active layer 34 having identical elastic properties. The diagram of the flexion tensions retains the triangular shape with apex on the middle fibre 36, but only the tensions existing in layer 34 contribute to developing an induced charge in the surface. Intrinsic piezoelectricity may then supply a non zero contribution to which is added the flexion piezoelectricity contribution.

Figure 12 is a partial sectional view of a stratified vibrating plate comprising a layer 38, inert from the piezoelectric point of view, which adheres to a piezoelectrically active layer 34 having greater compliance.

The flexing of layer 38 causes fairly uniform stretching of layer 34 which is represented by the stress diagram 39.

To form the stratified plates of Figures 11 and 12 which may moreover comprise more than two superimposed layers, assembly may be achieved by bonding. The plate of Figure 11 may be formed for example by two layers of PVF2, only one being electrically polarized. The stratified layer of Figure 12 may be formed for example by a polarized layer 34 of PVF2 bonded or grafted to a metal plate 38 of greater rigidity. In this case, the metal plate serves as electrode.

Instead of using a vibrating plate partially polarized in thickness, as illustrated in Figures 11 and 12, a monolithic plate may be formed such as plate 40 of Figure 13. By creating along axis z a profile 42 of conductivity having a low value above a fibre 41 and a much higher value below this fibre, an offset of the electric field may be provided in the region underlying fibre 41. As well as being piezoelectrically active in thickness, this plate is partly passivated by conductivity reinforcement. Such a plate may be obtained by doping the material to a penetration depth limited to a fraction of the total thickness. To this end, electronic bombardment technology may be used allowing, with an energy of a few tens of keV, penetration to a depth of a few tens of microns in a polymer material. A conductivity profile may also be obtained by diffusion of alkaline ions carried by a solvent.

In Figure 14, there is shown a meridian section of a microphone capsule for telephonic applications.

The flat vibrating plate 1 is clamped in a peripheral embedment formed by the flat edges of a metal cover 44 and the bottom of a case 45 also made from metal. The face of plate 1 turned towards cover 44 is completely covered over with an electrode 7 grounded to the case, which takes place at the end of assembly by crimping a metal casing 43. The bottom of cover 44 is pierced with apertures 48 forming a grid permeable to sound; the inside of the cover is lined with a textile lining 47 also permeable to sound. Cover 44 and plate 1 define a first acoustic cavity 46. A second acoustic cavity is formed by an upper recess in the bottom of case 45 which has an inner wall pierced with an aperture 50. A lower recess at the bottom of case 45 forms a third acoustic cavity 53 with a printed circuit plate 54. The aperture 50 for communication between the second acoustic cavity and the third acoustic cavity is closed off by means of a textile damping pad 49. The iower face of plate 1 carries an annular electrode 8 connected electrically to an amplifier circuit 51 carried by the center of plate 1. Power supply circuits 55 carried by the printed circuit 54 are connected by broken line connections to circuit 51. Output terminals 56 carried by the printed circuit 54 are also connected to the impedance matching circuit 51. A leak resistor 53 is provided between electrode 7 and electrode 8 by plugging a hole formed through plate 1 with a conducting paste. This resistor serves to limit the electroacoustic response towards the low frequencies. Damping means 47 and 49 contribute to damping the resonance frequency of plate 1.

It goes without saying that the different means by which the sensitivity of a flat vibrating plate may be improved may be combined together. The description of Figure 4 shows that the use of a flat plate is relatively easy to put into effect and that it allows a high degree of integration of the electronic components while retaining simple forms for the assembly parts. The stability in time of the electroacoustic characteristics is remarkable and the compactness of the microphone capsule in no way impairs its electroacoustic performances. The device of Figure 14 illustrates more particularly a pressure microphone receiving the acoustic pressure on one of the faces of the plate. However, the invention also applies to pressure gradient microphones which prove to be particularly efficient in noisy environments to privilege nearby sound sources.

When a microphone such as described above is plunged in water, it can be seen that it forms a good hydrophone. The resonance frequency is lowered because of the water load. It should also be pointed out that the plates may be formed not only from vinylidene polyfluoride, but also from one of its co-polymers.

Claims (25)

1. A polarized solid dielectric capacitor electro-acoustic transducer comprising at least two collector electrodes, a vibrating structure formed from said dielectric and subjected to the incident acoustic pressure and a support to which said vibrating structure is secured by its edges; said electrodes being carried by said vibrating structure and connected respectively to two output terminals, said vibrating structure is a flat structure in the form of a plate sufficiently thick for the middle layer to undergo no significant deformation during flexion of said plate.

2. The transducer as claimed in claim 1, wherein the securing means connecting the edge of said plate to said support allows same to pivot freely when it bends alternately under the effect of said incident acoustic pressure.

3. The transducer as claimed in claim 2, wherein the bearing surface for said plate comprises a pointed projection at the top of which the edge of the plate may freely pivot.

4. The transducer as claimed in claim 1, wherein the edge of said plate is clamped in an embedment with flat faces provided in said support.

5. The transducer as claimed in claim 1, wherein said plate presents an inhomogeneous electric polarization over the whole of its extent.

6. The transducer as claimed in claim 5, wherein a central region of said plate has an electric polarization whose sign is reversed with respect to the electric polarization existing in a region extending between the edge of said plate and said central region.

7. The transducer as claimed in claim 1, wherein the electric polarization of said plate is inhomogeneous in a direction perpendicularto its faces.

8. The transducer as claimed in claim 8 wherein said plate is a stratified structure formed from at least two superimposed layers adhering to each other; one of said layers being made from a polarized dielectric.

9. The transducer as claimed in claim 8, wherein the other one of said layers is made from metal.

10. The transducer as claimed in claim 8, wherein said other one of said layers is made from the same dielectric but not polarized.

11. The transducer as claimed in claim 1, wherein the electric conductivity of said plate is in homogeneous.

12. The transducer as claimed in any one of claims 1 to 11, wherein said electrodes cover respectively the faces of said plate.

13. The transducer as claimed in claim 1, wherein one at least of said electrodes covers partially one of the faces of said plate.

14. The transducer as claimed in claim 13, wherein said electrodes are situated on one of the faces of said plate; the other face of said plate being covered with a counter electrode.

15. The transducer as claimed in claim 13, wherein said plate is an embedded circular plate and said electrode covers a part of said face limited by a circle having a radius close to 0.7 times the radius of the non embedded part of said plate.

16. The transducer as claimed in claim 14, wherein said electrodes are connected to the differential input of an electronic amplifier circuit.

17. The transducer as claimed in claim 1, wherein said plate carries an electronic amplifier circuit connected to said electrodes.

18. The transducer as claimed in claim 1, wherein a single face of said plate is accessible to the acoustic pressure.

19. The transducer as claimed in claim 1, wherein the two faces of said plate is accessible to the acoustic pressure.

20. The transducer as claimed in claim 1, wherein the polarization of said plate-is dipolar.

21. The transducer as claimed in claim 1, wherein the polarization of said plate is formed by a charge excess.

22. The transducer as claimed in claim 20, wherein said plate is made from vinylidene polyfluoride or one of its co-polymers.

23. The transducer as claimed in claim 1, wherein the response is limited towards the low frequencies by a resistor connecting together said electrodes.

24. The transducer as claimed in claim 23, wherein said resistor is integrated in said plate.

25. A polarized solid dielectric capacitor electroacoustic transducer substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.