EP0657868B1 - Process and transducer for emitting very low frequency acoustic waves with high power - Google Patents

Description

The present invention as defined in the claims relates to a method of transmitting waves high power very low frequency acoustics, and transducers correspondents.

The technical sector of the invention is that of production electro-acoustic transducers for the emission of waves acoustic immersion in a liquid. Document EP-A-0 363 032 discloses such a device.

The main application of the invention is the possibility to emit very low frequency and high acoustic waves power, regardless of the depth of immersion.

We know in particular for such low emissions frequencies and in immersion, electro-acoustic transducers so-called flextensional submersibles, which include one or more electro-acoustic motors, generally piezo-electric motors, which are placed in the same plane inside or in peripheral crown of a flexible shell or shell. It's this shell placed in contact with water, which then forms the surface emission of acoustic waves.

These transducers convert the oscillations of the hull generated in the plane where the electro-acoustic motors are located, and due to the effects of their expansion-compression, in oscillations perpendicular to this plane and therefore of flexion of the shell, hence their name of flextensional transducers. They amplify as well the amplitude of the motor oscillations by a factor of 3 and even 4 and thus obtaining very low frequencies.

• la classe I regroupe les transducteurs ayant une coque en ellipsoïde de révolution autour d'un axe et un seul moteur électroacoustique, qui peut être piézoélectrique ou magnétostrictif, qui est coaxial avec l'axe de révolution de la coque, et qui est couplé mécaniquement et acoustiquement par ses deux extrémités avec les deux portions de surfaces de la coque situées sur l'axe de révolution.
• la classe II regroupe les transducteurs ayant une coque en forme de disque ou de tore de révolution autour de l'axe du disque ou du tore. Ces transducteurs comportent plusieurs moteurs piézoélectriques, disposés radialement autour de l'axe de révolution de la coque et couplés mécaniquement et acoustiquement avec celle-ci, soit par une de leurs extrémités dans le cas de forme en disque, et l'autre extrémité s'appuyant sur un pilier central, soit par leurs deux extrémités dans le cas de forme en tore;
• la classe III regroupe les transducteurs ayant une coque qui comporte deux renflements à ses deux extrémités et qui a une forme générale de type os ou diabolo;
• la classe IV regroupe les transducteurs dont la coque est un cylindre allongé, dont la section en coupe droite a la forme d'une ellipse ou d'une courbe fermée présentant un étranglement dans sa partie centrale. Les transducteurs de cette classe comportent plusieurs moteurs électro-acoustiques qui sont parallèles entre eux et dont les axes sont placés dans des plans transversaux perpendiculaires aux génératrices du cylindre et suivant la direction du plus grand axe de la section : les deux extrémités de chaque moteur sont couplées mécaniquement et acoustiquement avec la coque cylindrique;
• la classe V regroupe les transducteurs dont la coque est une combinaison de la forme de ceux de la classe I avec ceux de la classe IV, ou ceux de la classe II, et que l'on peut qualifier de forme en "huítre" : il s'agit en fait d'une forme de type ellipsoïde, éventuellement de révolution, mais aplatie et non allongée comme dans la classe I, ou sphéroïde très aplatie, ou encore torique engendrée par une circonférence comportant un côté rectiligne autour duquel elle tourne et qui constitue son axe.

It will be recalled that the known flextensional transducers can be grouped into five classes according to their general shell shape:

• Class I includes transducers having an ellipsoid shell of revolution around an axis and a single electroacoustic motor, which can be piezoelectric or magnetostrictive, which is coaxial with the axis of revolution of the shell, and which is mechanically coupled and acoustically by its two ends with the two portions of shell surfaces located on the axis of revolution.
• Class II groups together transducers with a shell in the form of a disc or torus of revolution around the axis of the disc or torus. These transducers comprise several piezoelectric motors, arranged radially around the axis of revolution of the shell and mechanically and acoustically coupled with it, either by one of their ends in the case of disc form, and the other end s' pressing on a central pillar, either by their two ends in the case of a torus shape;
• class III groups together transducers having a shell which has two bulges at its two ends and which has a general shape of the bone or diabolo type;
• Class IV includes transducers whose hull is an elongated cylinder, the cross-section of which is in the form of an ellipse or a closed curve presenting a constriction in its central part. Transducers of this class include several electro-acoustic motors which are parallel to each other and whose axes are placed in transverse planes perpendicular to the generatrices of the cylinder and in the direction of the largest axis of the section: the two ends of each motor are mechanically and acoustically coupled with the cylindrical shell;
• class V groups together transducers whose hull is a combination of the shape of those of class I with those of class IV, or those of class II, and which one can qualify as shape in "oyster": it it is in fact an ellipsoid type shape, possibly of revolution, but flattened and not elongated as in class I, or very flattened spheroid, or toric generated by a circumference having a rectilinear side around which it turns and which constitutes its axis.

The present invention relates more particularly, but not exclusively, flextensional transducers of classes IV and V above.

All flextensional transducers have many advantages, for the emission of low and very low frequency waves, but they also have certain drawbacks, including that of require that the hulls or envelopes are either closed and waterproof, either open and not waterproof, but then with devices individuals who to date limit the effects of this non sealing.

Indeed, the acoustic waves that we want to emit in bass frequencies, should only be caused by shell vibrations external waves, and no other waves caused by internal vibrations shell and electro-acoustic motors must not escape from inside it so as not to disturb said low emission frequency. For this, a first solution is that this shell can be closed and full of gas: it must then withstand the pressure immersion; this however limits the use of such transducers to shallow depths, because if we want to increase the resistance of the shell to withstand the pressure from great depths, we lose elasticity and therefore possible emission; for not having to increase this resistance and keep this elasticity, devices specific ones were then developed and some were even the subject various patents.

• la demande de brevet No. FR. 2.634.292 de Monsieur Gilles GROSSO et intitulée "procédé et dispositif pour maintenir le gaz contenu dans une enceinte immergée en équilibre de pression avec l'extérieur", publiée le 19 Janvier 1990, qui consiste à associer à ladite enceinte immergée telle que le boítier d'un transducteur piézo-électrique, plusieurs bouteilles contenant chacune une poche déformable, prégonflées à des pressions différentes, et permettant ainsi de compenser la pression hydrostatique à différentes profondeurs d'immersion;
• la demande de brevet FR. 2.665.814 déposée le 10 Août 1990 par la société THOMSON sur des "transducteurs électro-acoustiques destinés à être immergés" et comportant un système de compensation automatique de la pression d'immersion grâce à des chambres remplies de gaz et de volumes réduits, de manière à ne compenser que les efforts axiaux s'exerçant sur le pilier central de céramiques du transducteur.

All these devices consist in fact in using the possibility of compensating the external pressure by an increase in the internal pressure, and this in different ways according to the inventions, so as not to make a sealed housing withstand the forces of resistance to the external pressure . We can quote for example:

• patent application No. FR. 2.634.292 by Mr. Gilles GROSSO and entitled "process and device for maintaining the gas contained in an immersed enclosure in pressure balance with the outside", published on January 19, 1990, which consists in associating with said submerged enclosure such as the casing a piezoelectric transducer, several bottles each containing a deformable pocket, pre-inflated to different pressures, and thus making it possible to compensate for the hydrostatic pressure at different depths of immersion;
• the FR patent application. 2,665,814 deposited on August 10, 1990 by the company THOMSON on "electro-acoustic transducers intended to be immersed" and comprising an automatic compensation system for the immersion pressure thanks to chambers filled with gas and reduced volumes, so as to compensate only for the axial forces exerted on the central ceramic pillar of the transducer.

One could cite other patent applications using pneumatic external pressure compensation systems immersion, but all including mechanical means and / or fairly large gas supply or storage and / or complicated.

• soit en utilisant un fluide différent du milieu ambiant et en disposant une membrane de séparation faisant office également de diaphragme flexible compensant les différences de volume interne dues à la compressibilité du fluide intérieur, en général de l'huile, ceci est utilisé dans la demande de brevet FR. 2.671.927 publiée le 24 Juillet 1992 et déposée par l'Etat Français, sur des "transducteurs électro-acoustiques directifs et procédés et dispositifs de fabrication"; mais dans la présente application aux transducteurs très basses fréquences de type flextensionnel, on ne peut pas supprimer alors les ondes dues aux vibrations internes, ce qui est en dehors du but recherché;
• soit en utilisant le liquide du milieu ambiant : la présente invention se situe dans cette dernière catégorie, dans laquelle à ce jour pour répondre à la fois au problème de tenue en immersion, et à celui des ondes internes, on réalise dans la coque du transducteur des ouvertures de communication, ayant des dimensions données pour que la fréquence fondamentale des vibrations de l'ensemble soit voisine de la fréquence de Helmholtz de la cavité et, de plus, on rajoute suivant le cas, divers dispositifs dans ces dites ouvertures et/ou dans la cavité formée par la coque pour ajuster les deux dites fréquences et obtenir l'effet recherché.

Another known solution is then to have a non-waterproof shell, by filling it with liquid to avoid the above compensation devices:

• either by using a fluid different from the ambient environment and by having a separation membrane also acting as a flexible diaphragm compensating for the differences in internal volume due to the compressibility of the internal fluid, in general oil, this is used in the request for FR patent. 2,671,927 published on July 24, 1992 and filed by the French State, on "directive electro-acoustic transducers and manufacturing processes and devices"; but in the present application to very low frequency transducers of the flextensional type, it is therefore not possible to suppress the waves due to internal vibrations, which is outside the desired goal;
• either by using the liquid from the ambient medium: the present invention falls into this latter category, in which to date to respond both to the problem of resistance to immersion, and to that of internal waves, it is produced in the shell of the transducer communication openings, having given dimensions so that the fundamental frequency of the vibrations of the assembly is close to the Helmholtz frequency of the cavity and, in addition, various devices are added in these said openings and / or in the cavity formed by the shell to adjust the two said frequencies and obtain the desired effect.

However, even in this case, the broadcast is disturbed, especially in the range of very low frequencies and more, unlike objectives of this application, we lower the emission level, which limits the possibility of transmitting at high power.

The objective of the present invention is to be able indeed emit very low frequency acoustic waves in a liquid, without being bothered by the depth, without complicating the realization of transducers which for these frequency ranges are flextensional type and while allowing high emissions powers.

• on laisse pénétrer ledit liquide dans toute la cavité intérieure délimitée par cette enveloppe grâce à toute ouverture communicante sur l'extérieur;
• on dispose dans la cavité un baffle en matériau alvéolaire résistant à la pression d'immersion, qui occupe la majeure partie du volume de la cavité et dont la surface extérieure est à une distance constante de celle de l'enveloppe;
• on solidarise ledit baffle au dispositif d'assemblage des moteurs acoustiques;
• on sollicite lesdits moteurs pour émettre lesdites ondes acoustiques de très basses fréquences et à forte puissance.

This objective is achieved by a method for emitting such very low frequency acoustic waves into a liquid, by means of a transducer of known type comprising one or more electro-acoustic motors placed in the same plane inside or in a ring. peripheral of a flexible envelope, of which they are secured at least at one of their ends by any assembly device, to which they transmit their vibrations so that it is this which, in contact with said liquid, constitutes the surface for emitting acoustic waves in it, and which is of a form such that the amplitude of these vibrations is amplified, and that these are of low frequencies; according to the process of the invention:

• said liquid is allowed to penetrate the entire interior cavity delimited by this envelope thanks to any communicating opening on the outside;
• there is in the cavity a baffle made of cellular material resistant to the immersion pressure, which occupies most of the volume of the cavity and whose outer surface is at a constant distance from that of the envelope;
• said cabinet is secured to the assembly device of the acoustic motors;
• said motors are used to transmit said acoustic waves of very low frequencies and at high power.

The objective of the present invention is also achieved with a transducer of the same type known as that above, and in which its enclosure formed by part of said envelope comprises at minus any opening allowing said liquid to penetrate the entire interior cavity delimited by said enclosure and a baffle in Immersion pressure resistant foam material occupying the most of the volume of the cavity, the external surface of which is a constant distance from that of the vibrating envelope, and which is integral with the assembly device of electro-acoustic motors.

In a preferred embodiment, said material alveolar has an acoustic impedance of less than 0.7 times that of water, for example a cellular plastic type material to base of P.V.C. (polyvinyl chloride) and with closed porosities.

In an application to the class V transducers described previously, in which the envelope is of shape and type ellipsoid, it consists of two discs placed opposite from each other, convex, and connected at their periphery by at least a torus-shaped piece, such that these two discs are just supported and immobilized at their periphery, each in a groove of said toric piece.

The result is a new wave emission process very low frequency and high power acoustics, as well as new corresponding transducers, allowing such emissions.

Indeed, these processes and transducers whose characteristics indicated above overcome the various disadvantages previously mentioned in the flextensional transducers existing at today respond to the problem and the objectives set; the opening of the enclosure allowing the communication of the internal cavity with the external liquid, makes it possible to have a permanent system equipression, not requiring a compensation system: it is important to note that the maximum immersion depth of transducers according to the invention is moreover only limited to the pressure resistance of this material.

The presence of this cellular material occupying the interior of said cavity, absorbs the effect on the one hand, of a rear radiation of the hull, and on the other hand, vibrations of electro-acoustic motors themselves which could radiate inside the cavity, as is used in other types of transducers; however, in the present invention, the presence additional film of water between the body of the cabinet and the hull vibrating, increases its inertia, which allows to be able transmit even lower frequencies, of the order of at least 30% compared to an air-filled system.

In addition, the combination of different characteristics indicated above, allows to maintain a power level about constant emission compared to known systems filled air, and the drawbacks of which are mentioned above, which is not not the case of the other known transducers in equipression.

In addition, the total immersion of all the parts of the transducer and in particular of the cavity, allows a better cooling of electro-acoustic motors by diffusion calorific facilitated by the presence of the external fluid which is water, whereas in air-filled systems, this cooling is good on more difficult.

These advantages are of course found in submerged systems as also exists elsewhere, using devices allowing to play on the natural frequencies, but which as indicated above, decrease the transmission power since we then obtain bandwidth expansion, which is not the goal is achieved in the present invention.

For class V flextensional transducers such as indicated above, and for which, in a particular mode of realization, the two half-shells are immobilized in a support peripheral toric, in which these are neither welded nor rigidly fixed, a greater bending effect is obtained at the supports of the peripheral edges, which allows a more large displacement of said shells for the same displacement of ceramics, enabling the emission level to be increased and get an even lower frequency.

We could cite other advantages of the present invention, but those cited above already show enough to demonstrate novelty and interest.

The description and one of the figures below represents a exemplary embodiment of the invention, but have no character limiting: other embodiments are possible within the framework of the scope and scope of the invention, in particular, by changing the basic shape of flexible hull and arrangement of engines electro-acoustic, as in the different classes indicated previously.

Figure 1 is a perspective view of a transducer flextensional class IV as it can exist to date.

Figure 2 is a sectional view of a transducer flextensional class V and produced according to the present invention.

In these figures, the transducers shown are therefore as an example for FIG. 1 a type IV transducer, and for Figure 2 a type V transducer: they include, in a way known. one or more electro-acoustic motors 1, placed according to the same plane defined by the axes xx '/ yy', inside which relates to FIG. 1 and in the peripheral crown as regards the Figure 2 of a flexible envelope 5, which they are integral with less at one of their ends by any assembly device either in FIG. 1 for the connecting parts or masses 2, and in the Figure 2 for peripheral parts 10 for fixing and connection with counterweights 9 and electro-acoustic motors 1 connected between them by assembly screws 8; said electro-acoustic motors 1 and their counter-masses 9 transmit their vibrations to said flexible envelope 5 which is in contact with said liquid exterior 4, constituting the acoustic wave emission surface; said flattened ellipsoid shape of said surface allows to amplify the amplitude of the vibrations and to obtain a range of waves very low frequencies.

In Figure 1, the type IV transducer shown has a flexible casing 5 in the form of an elongated cylinder, the cross section of which straight cut has the shape of an ellipse, and electro-acoustic motors 1 represented in number of three in this example, are parallel to each other, along an axis yy 'and their axes are placed in transverse planes perpendicular to the generatrices of the cylinder along the axis xx 'of the envelope 5.

In this figure 1, the two ends of each motor are mechanically and acoustically coupled by the connecting pieces 2 with the cylindrical shell 5, but in other cases, in particular when it comes to class II transducers in the form of the hull thereof, said motors 1 may be integral only at one of their ends, the other being able to bear on a central pillar acting as a counterweight.

The ends of the cylindrical casing 5 are closed by flanges 3 which therefore do not participate directly in the amplification vibrations of the shell 5 caused by those of the motors electro-acoustic 1: to allow equipression and penetration of the liquid inside the cavity 7, one of said end flanges 3 has at least one opening 6, the dimensions according to existing systems to date, associated with the characteristics of the envelope 5, are determined in such a way that by coupling the elasticity of this envelope with the mass of the liquid 4 located in said opening 6, the Helmholtz frequency of the cavity 7 is close to the fundamental frequency of the vibrations of the assembly constituted by said electro-acoustic motors 1 and of any element associated with them, such as the connecting parts 2.

Given the drawbacks mentioned above in this mode of embodiment, such a type IV transducer according to FIG. 1 could be equipped according to the present invention with an internal cabinet 15, as shown in the example in FIG. 2, and connected to the central part, for example electro-acoustic motors 1, since it is not necessary that such a cabinet can be driven by the outer envelope 5 and can vibrate with it.

In the sectional representation of Figure 2 along the plane xx '/ zz' radial to the type V transducer selected, this includes a motor ring 13 formed of a crown of several pillars of ceramic or electro-acoustic motors 1 in number for example of eight, and associated with counterweights 9, connected by various screws assembly 8 allowing the continuity of the peripheral crown 13. On the other hand 18 central rods ensure the prestressing of the ceramic pillars 1.

The description of this crown motor element is not further clarified in the context of this description, since is not an object of the invention, and it already exists to this day different transducer systems using such an arrangement.

According to this figure 2, the envelope 5 forming said shell vibrator is composed of two convex discs connected to their periphery by at least one part 10 in the shape of a torus, so that these two convex discs 5 are just supported and immobilized at their periphery each in a groove 17 of the part 10. The two convex discs can both be turned outwards, but also and possibly both inwards or one outward and the other inward, but preferably, we will choose the orientation of the convexity of the two discs towards the exterior as shown in the figure.

Each of the grooves 17 is in fact carried by elements arranged in a crown, and constituting the pieces 10, independent of from each other for each of the two opposite and convex discs, and linked two by two, on either side of the plane in which locate the acoustic motors 1, by any means 11 of adjustment in relative distance; which crowns of pieces 10 have each a conical surface 12, cooperating with a surface of the same shape, carried by another continuous outer crown 13 peripheral, so that the action of bringing together elements of the part 10 by the means 11, decreases the length of the circumference of grooves 17 by pinching effect and ensures disc preload 5.

This technical choice of prestressing solution each part of the hull removes the fixing points which would be complex and expensive, as would any equipment that would bring manufacturing difficulties. In addition, it allows a easy disassembly of the domed hulls and clearly maintenance improved transducer.

The enclosure comprising the envelope 5, thus constituted comprises any opening, not shown in this figure cut, but being able to let said liquid 4 penetrate into the entire cavity 7 interior delimited by the envelope, such as for example a opening between two motors 1 in the outer ring 13.

A cabinet 15 made of pressure-resistant foam material immersion, occupies most of the volume of cavity 7, of which the external surface is at a constant distance 14 from that of the envelope 5, and which is integral with the assembly device 8 of the electro-acoustic motors 1 in ring 13. This assembly can be produced by elastic studs 16 spaced on the rear structure transducers 1, such as that constituted by the supports and the counter-masses 9 over the entire periphery of this ring 13 outside.

Preferably, the elastic material constituting said pads 16 is of elastic characteristics such as its radial speed of displacement in compression is less than ten times that of the motors electro-acoustic.

To obtain an additional inertia effect, while allowing the absorption of rear wave emissions, said distance 14 separating the surfaces of the cabinet 15 from the casing 5 and filled with liquid 4 is between 3 to 6 mm.

So that said cabinet allows to obtain the maximum effect sought, it is necessary that the acoustic and specific impedance, again called characteristic impedance of the material which constitutes it, and defined by the product of its density rm and the speed Cm of wave propagation in the material, such that: (rm x Cm) of material <0.7 of (ro x Co) of water.

This is obtained for example and preferably with a plastic cell based on P.V.C (polyvinyl chloride) and porosity closed, which also allows, for certain qualities of these foams, withstand depths of up to 1,000 meters.

Claims (10)

1. A method for emitting very low frequency acoustic waves in a liquid by means of a transducer of a known type comprising one or more electro-acoustic motors located according to the same intemal plane or as a peripheral ring of a flexible envelope to which they are interdependent at least at one of their extremities by any assembling device to which they transmit their vibrations so that the envelope which must be in contact to said liquid constitutes the emitting surface of the acoustic waves in the latter, and whose shape is such that the amplitude of the vibrations is amplified, making them low frequencies, wherein :

   the said liquid is allowed to enter the entire internal cavity delimited by this envelope by an orifice communicating with the outside ;

   A baffle is located in cavity made up of alveolar material withstanding the immersion pressure, which fills the greater part of the cavity volume and whose external surface is at a constant distance from that of envelope

   said baffle is made interdependent to the assembling device of acoustic motors

   said motors are requested to emit the said very low frequency and high power acoustic waves.
2. A transducer comprising one or more electro-acoustic motors located on the same plane, inside or as a peripheral ring of a flexible envelope to which they are interdependent at least at one of their extremities by any assembling device and to which they transmit their vibrations, the said flexible envelope being in contact with said liquid making up the acoustic wave emitting surface, and with a shape allowing the amplitude of these vibrations to be amplified in a range of very low frequencies, characterized in that the chamber partly constituted by said envelope includes at least one orifice allowing said liquid to enter the entire internal cavity delimited by this chamber and a baffle made up of alveolar material resisting the immersion pressure and filling the main part of cavity volume, whose external surface is at a constant distance from that of the envelope and which is interdependent to the assembling device of the acoustic motors .
3. A transducer according to claim characterized in that said distance separating baffle surfaces and envelope filled with liquid is between 3 - 6 mm.
4. A transducer according to claim 2, wherein said alveolar material has an acoustic impedance 0.7 times lower than the water impedance.
5. A transducer according to claim 4, wherein said alveolar material is a P.V.C.-based (Vinyl polychloride) cellular plastic with closed porosities.
6. A transducer according to claim 5, wherein the said baffle is connected to assembling device of motors by elastic studs .
7. A transducer according to claim 6, wherein the elastic characteristics of the material making up saids studs are such that its displacement radial speed in compression is ten times lower than that of acoustic motors.
8. A transducer according to claim 7, and such that the shape and type of external envelope are ellipsoid, comprising two convex disks located opposite one another, and connected at their periphery by at least one ring-shaped piece characterized in that both disks are supported and immobilized on their periphery, each one in a groove of piece .
9. A transducer according to claim 8, wherein each one of grooves is composed of elements arranged in two rings and constituting piece connected two by two, on both sides of the plane in which the acoustic motors are located by any means of adjustment in relative distance, each one of the said rings of piece comprising a conical surface cooperating with a surface having the same shape, supported by another continuous peripheral external ring in such a way that the bringing together of the elements of piece elements by means reduces the groove length and ensure the prestressing of disk.
10. A transducer according to claim 9, wherein the said electro-acoustic motors and their associated countermasses are arranged inside the said external peripheral ring.

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