EP0728535B1 - Process and device for reducing the resonant frequency of the cavities of submerged transducers - Google Patents

Description

The present invention relates to methods and devices to decrease the resonant frequency of the transducer cavities submersible.

The technical sector of the invention is that of production an electroacoustic submersible transducer.

The main application of the invention is to reduce this resonance frequency for so-called "Tonpilz" double transducers and in particular for those used to emit waves acoustics with high performance and around a master plan given and omnidirectionally according to this plan.

We know such electroacoustic transducers submersible, and in particular piezoelectric, comprising a rigid, hollow cylindrical housing open at both ends axial, and inside which are arranged coaxially with this one, two identical electro-acoustic motors, placed on the side and on the other side of a central counter-mass, and whose ends opposite are surrounded by a flag. Such transducers are said to be "Tonpilz" doubles. Said electro-acoustic motors can be made by two stacks of aligned piezoelectric plates. The external faces of the two pavilions are located in the plane of the axial ends of the housing, so that they are in contact with the liquid, in which the case is immersed, and the perimeter outer of these pavilions comes closer to the edge of the ends axial open of said housing.

Thus, these external faces emit waves in the liquid acoustic when electro-acoustic motors are energized electronically: these transducers are used in particular for emit low frequency acoustic waves into the water in a determined direction; for an application of this type of transducer mono or double "Tonpilz" for high power emissions, we can cite the request FR. 2,663,182 from Gilles GROSSO published on 13 December 1991, which describes additional devices for obtaining increased power.

To avoid the propagation of acoustic waves emitted by back sides of the pavilions, inside the case, especially when this one is just full of liquid, and which are then retransmitted in the ambient environment despite the rigidity of said housing, we place in the cavity filled with ambient liquid at the back of the flags of such non-waterproof housings, various means such as closed elastic tubes, sealed and filled with gas, and such as the Helmholtz resonance frequency of the cavity is close to the fundamental frequency of the axial vibrations of the vibrating assembly; such a device is described in the patent application FR. 2,665,998 of 05 May 1988 filed by the French State General Delegate for Armament. This brings back the problem of resistance to pressure of the outer casing, to the resistance of said tubes elastic, which, being smaller diameters, allow to have a lighter package: other means can be developed in the same objective and apply to the present invention, knowing that these devices require retaining a cavity, behind the pavilions, of sufficient dimensions.

Furthermore, when we want to decrease according to the objectives of the present invention the low frequency of use of such transducers below 500 Hz while increasing if possible or at least keeping the same electroacoustic efficiency and the power actually emitted, it is necessary to reduce the resonant frequencies of both the motor and the resonator. Yes other means can be developed and are the subject of the rest other patent applications, regarding the decrease in resonant frequency of the motor, the characteristics of these means can however be combined with the main essential ones of the present invention as will be described below, for the decrease in the resonant frequency of the resonator.

For that, it would certainly be possible and it is known to this day, to increase the mass of the pavilions as well as the dimensions of the back cavity of these and / or the number of compliant tubes like described and indicated above, but such solutions weigh down and increase the total volume of the transducer, thus requiring a higher sound power. Depending on the applications, especially for those where the volume of energy storage is critical and whose transducer must be autonomous for long periods, such solutions are not satisfactory.

The main problem posed is therefore to be able to reduce the resonant frequency of the resonator, i.e. of the resonant cavities of a type transducer, in particular, double "Tonpilz" without increasing its dimensions and weight from a classic transducer of the same type, and with at least the same acoustic power for at most the same power consumed.

• on obture ledit orifice par un radiateur passif dont la définition est explicitée ci-après, constitué d'un matériau plus dense que ledit fluide ;
• on suspend ce dit radiateur passif à la périphérie du dit orifice par un matériau élastique ;
• on émet des ondes acoustiques à des fréquences plus basses que celle de la plage de fréquence initiale donnée et au plus avec la même puissance consommée.

One solution to the problem posed is a method of transmitting acoustic waves in a fluid, at low frequency, from a transducer comprising at least one horn attached to the end of a motor pillar, a hollow rigid case surrounding said pavilion and delimiting therewith at least one cavity which communicates through at least one orifice with said external fluid, which can thus also fill it, and having dimensions and a determined external volume, which transducer transmitting waves in a range of frequency and at a given electroacoustic power, the process according to the invention is then such that:

• the said orifice is closed by a passive radiator, the definition of which is explained below, made of a material denser than the said fluid;
• said passive radiator is suspended from the periphery of said orifice by an elastic material;
• acoustic waves are emitted at frequencies lower than that of the given initial frequency range and at most with the same power consumption.

• on obture cette ouverture par un radiateur passif constitué en plusieurs secteurs indépendants et reliés entre eux par des liaisons élastiques.

In one embodiment for a dual "Tonpilz" transducer, a peripheral opening is made in the wall of said transducer housing between its two said pavilions, thus constituting one of said communication orifices between said external fluid and the interior cavity located between these pavilions;

• this opening is closed by a passive radiator made up of several independent sectors and connected to one another by elastic connections.

In addition, as indicated above, if we want to decrease also the resonant frequency of the transducer motor and so ability to increase sound efficiency and allow then increase the sound power for the same power given consumption, placed behind said roof inside of said housing a dynamic load integral therewith, closing partially its interior section and sharing said cavity interior in two parts back and front communicating and brings the outer peripheral edge of the pavilions closer to the wall internal of the case preferably at a distance of a few tenths of millimeters. Preferably, said transducer is used as well performed in a frequency range surrounding that specific to the rear cavity thus delimited by said dynamic load: the definition and a type of realization thereof is given in the description below.

So we were able to generate from a known type transducer double "Tonpilz", without cavity in front of the pavilions as described below, a sound signal of around 200 db at a frequency of 300 Hz without increase the volume of the cavity or the number of compliant tubes in this one as it was done until this day. In addition, by using simultaneously a dynamic load which decreases the frequency of the motor without increasing either the mass of the pavilions for the same level noise, the mechanical constraints applied to the ceramics of these motors are reduced and the power consumed also.

Indeed, the presence of said dynamic load actually allows indirectly increase the mass of the pavilion by combining a mass of liquid situated between the latter and said dynamic charge; as the latter only partially closes the interior section of the case, the liquid can however pass from the so-called front cavity towards the so-called rear cavity but being braked according to the ratio of surface between the free surface of the conduit left by said charge dynamic and the total internal surface of the case: we thus obtain a virtual mass of the pavilion all the more important as this surface ratio is high.

In a particular embodiment, when we want to use a transducer according to the invention and of the double "Tonpilz" type for also issue with a high yield and preferably around a given master plan and omnidirectionally in this plan, we extends said rigid case defined above, which is cylindrical of axis XX 'and encloses the two associated electroacoustic motors each to a pavilion, beyond these and in the axis XX '; we thus constitutes two cavities each comprising an axial orifice from end to front of the pavilions and the resonance of which is determined to match the desired transmission frequency range; we are closing then completely the part of the case located between the two said pavilions and enclosing a central cavity and we close both end orifices of said housing, each by a passive radiator.

One of the main applications of this latter mode of realization is the possibility of transmitting and / or receiving waves high-performance acoustics in horizontal planes to study the different properties, by layers or by slices, of the oceans, such as temperature, salinity, density, currents etc ..., both for understanding the phenomena and their fluctuation over time.

A publication of the journal "Pour la Science" No. 158 of December 1990, pages 66 and following and presented by Messieurs Robert SPINDEL and Peter WORCESTER describes such an application, equipment and measurements obtained to date in this area.

A technique called "tomography" has been developed for this. ocean acoustics "to generate a three-dimensional image of the area crossed by sound waves, as is done in medicine with X-ray beams, or in crust geology terrestrial with seismic waves: in the oceanic domain, we uses low frequency acoustic waves.

By adapting the resonance of said cavities thus created in this embodiment as described according to Figure 4 to that of transmission frequencies and this thanks to various embodiments such as described from the attached figures, one can obtain a yield 90 to 95% of the power consumed, especially in the bass frequencies from 100 to 500 Hz, but the performance is also improved in the high frequencies.

In addition, if we want to obtain a good directivity effect, we extends said rigid housing so that its total length is about half, to within 20%, or in fact 0.8 to 1.2 times half the wavelength of the acoustic waves emitted by the transducer. The more we deviate from the exact half-wavelength, the more we will emit large emission lobes in the axis of the transducer and not in the desired blueprint but this loss emission in an unnecessary axis does not represent a very powerful important if we stay within the above tolerances.

To obtain the desired resonance of the cavities, as described more precisely below, these can enclose either tubes closed elastic bands, waterproof and filled with gas, at least one flexible bladder occupying part of its volume and filled with a fluid more compressible than immersion liquid: the advantage of presence of such so-called compliant tubes and / or a bladder is also reduce the loss of efficiency and frequency attenuation between the two resonance peaks specific to the transducer, one of which is related to the mechanical resonance of the whole transducer, and the other to that of its cavity.

The result is new processes and transducers that can be equipped with devices according to the invention for emitting waves acoustic in a liquid with the main objective of reducing the transmission frequency without increasing the volume and weight of known transducers with the same flags and the same electroacoustic motors: beyond this objective and following the desired application, we can also then by combining various devices according to the invention as indicated above and described below, improve acoustic performance and get some directivity.

Other advantages of the present invention could be mentioned but those cited above already show enough to prove novelty and interest. The description and figures below represent an embodiment of the invention but have no limiting nature: other embodiments are possible, in particular by changing the shape of the dynamic load as shown in dotted as an example on the left in Figure 1.

Figure 1 is an axial sectional view of a transducer type indicated above with central cavity open to the outside and defined below and fitted with a passive radiator according to the invention.

Figure 2 is a perspective view of a passive radiator according to Figure 1.

Figure 3 shows comparative power curves acoustic, between a known classic transducer with double "Tonpilz" and a transducer of the same type equipped with a passive radiator, for relative to the frequency transmitted.

Figure 4 is an axial sectional view of a half-transducer of the type indicated above, with three cavities of which two say before are each equipped with a passive radiator according to the invention.

We note first of all that the present invention can apply to all types of submersible transducers composed of less of a flag and a driving pillar, even if in the examples cited above and below, it is described, for questions of simplification of description and the fact that these are applications main features of the invention, that pavilions coupled to motors electro-acoustic transducers type double "Tonpilz" of form cylindrical of revolution.

The transducer as shown in section in this figure 1 therefore comprises in a known manner, two electro-acoustic motors 1 aligned on an axis XX ', placed on either side of a counterweight central 2 and coaxially inside a cylindrical housing 5, which can be called external covering all of said motors 1 to the pavilions 3 at the end thereof, the cavity 7, as well bounded by said pavilions and said housing being filled with liquid 4 in which the entire transducer is immersed, such as Seawater.

Said electro-acoustic motors 1 and the intermediate mass 2 are on the one hand, held together by a preloading rod 9, also immobilizing the two pavilions by any means of assembly 3 on the ends of the pillar thus formed, and on the other hand, assembled using different connecting pieces 11, themselves associated to different fasteners 12 to the outer case 5. The various fixing means are such that they allow freedom of displacement of the ends of the electro-acoustic motors on the side of pavilions 3, which are integral with them and whose outer edge peripheral 16, not connected and independent of the internal wall of the case 5, can then vibrate freely so as to ensure full emission of acoustic waves in the ambient environment.

An inner sleeve 13 isolates the preload rod from said motors 1, and an external sealing envelope 8 ensures the insulation of these motors 1 and of the pavilions 3 with respect to the medium ambient 4.

The supply of said electro-acoustic motors 1 is supplied by any power cable 10 fixed on said connecting pieces 11 by an electrical connector 14. The making of such transducer and all of the different connecting pieces the constituent are of the known field and realizable by any man of profession: all the other elements making it possible in particular to obtain the Helmholtz resonant frequency of the cavity as shown in the introduction, as well as the various connecting elements improving the mechanical performance of the assembly are not figured here; some have been the subject of various other requests for patents like those in particular cited in the introduction for so-called compliant tubes as shown in Figure 4.

To allow the filling of the cavity 7 with said liquid 4, said external housing 5 comprises at least one opening 6 of communication with the outside, said opening being able to be consisting of holes distributed around the cylindrical part of the housing or even consisting of a circular peripheral opening complete.

According to the present invention, said opening or orifice 6, or also called a vent, is closed by a passive radiator 15 constituted one or more solid plates of thickness "L", produced in a denser material than said fluid 4, and suspended at the periphery said orifice or vent by an elastic material 23: this or these plates constituting said passive radiator 15 preferably match the shape of the opening 6, 20 which they close and that of the case 5 which they then ensure surface continuity. The material of plates of said passive radiator 15 can be metallic of bronze type aluminum or steel etc ...: if we thus increase in a ratio of the order of eight the acoustic mass of the neck of the vent 20 thus closed the resonant frequency of the cavity is reduced such that shown in Figure 3 and according to the equivalent acoustic mass thus obtained, one can also widen the range of this frequency of resonance.

In addition, as this acoustic mass is determined by the product of the density of the material multiplied by the height of the neck the vent, that is to say in the present invention, the thickness "L" plates constituting the passive radiator 15, and divided by the total area of the vent opening 20, if the density of the material without changing the dimensions of the vent, we get actually a higher acoustic mass; conversely for the same acoustic mass, if we increase the density, we increase the radiation surface of this vent and thus the impedance of the acoustic radiation.

In the case where this opening 6, according to the transducer shown in Figure 1, is peripheral and continuous, said passive radiator 15 consists of several plates or sectors 16 independent and interconnected by elastic links 17, as shown in perspective in Figure 2, in which the total surface area of the cylindrical vent or opening 6 of revolution is thus closed by eight sectors 16.

In addition, said transducer as shown in dotted lines on the left in FIG. 1 may comprise behind each pavilion 3 and inside the case 5 a dynamic load 22 associated with each of the pavilions, integral with said case 5 and partially closing its inner section by dividing the internal cavity 7 into two parts, rear 7 ₁ and front 7 ₂ , communicating: in the representation of Figure 1, this in fact corresponds to dividing the entire internal cavity of the housing into three cavities, of which a single rear central unit 7 ₁ is median and two so-called "front" cavities 7 ₂ are each located behind each of the two pavilions 3.

According to the representation of the dynamic load to the left of the figure, it consists of a solid wall 27 matching the shape of the internal surface of the wall of the housing 5 of which it is integral, surrounding the engine pillar 1 and pierced with at least one orifice 26 through which the latter passes, which orifice carries a conduit 28 extending behind the wall 27 relative to the roof 3 and leaving a free peripheral passage around said driving pillar 1.

Other embodiments with other conduits and orifices distributed over said partition 27 around the engine pillar 1 are possible. It is necessary, specifically in this embodiment, that the distance "e" between the peripheral external edge of the pavilions 3 and the internal wall of the housing is as small as possible, in any case less than 1 mm and preferably a few tenths of a millimeter, so that this space allows only little loss of leakage of the liquid from the cavity 7 ₂ towards the outside during vibration, so that said fluid stresses above all the dynamic load 22 to thereby virtually increase the mass of the flag 3, and thus reduce, at equal weight and volume thereof, its emission frequency for the same power consumption.

In Figure 3, a curve 24 of emission of sound power of a known transducer as shown in the Figure 1 with a passive radiator according to the invention, while the curve 25 represents the same transducer but not equipped with said passive radiator, said emission powers being noted by compared to frequencies in herz: we note that the frequency of resonance of approximately 580 Hz basic is decreased by more than 100 Hz for the same transmission power. This was of course measured at from a given type of transducer, but with others transducers, we would get equivalent results with always the same offset to decrease the resonance frequency and resignation.

In another embodiment, the transducer submersible electroacoustics as shown in section on the Figure 4, comprises in a manner known as that of Figure 1 two electroacoustic motors 1, aligned along an axis XX ', placed on either side of a central counterweight 2 and coaxially at the interior of a rigid cylindrical case 5 with the same axis XX ′, covering all of said motors 1 up to pavilions 3 end thereof and open itself at both ends; the cavity 7 thus delimited between and at the rear of said pavilions and by the housing itself is in communication with the immersion liquid outside 10, by the only annular spaces "e" between the shape interior of said rigid case 5 and the peripheral edges of ends of pavilions 3: this space "e" should be as small as possible possible, less than 0.5 mm to avoid pumping of the liquid between the front and rear of said pavilions, as in the example of Figure 1 with dynamic load 27, although in the present example this distance is less critical. No further communication or vent is not practiced in the housing outside the holes necessary for the passage of the supply hull 10 and the fasteners 21 external, but then closed and sealed to prevent loss acoustic.

According to this embodiment, said rigid case 5 extends along its axis XX 'beyond the two pavilions 3 and constitutes with them two cavities 19 whose resonance corresponds to the frequency desired program.

For this, said cavities 16 can contain tubes elastic 18 closed, waterproof and filled with gas, which is called compliant tubes, such as those described in the patent application FR. 2,665,998 of May 5, 1988.

Furthermore, to allow a better tuning of the resonance cavities with the desired emission frequency, aperture 20 said cavities 19 on the outside are of a smaller diameter d that the internal diameter D of the rigid case 5: a compromise dimensional must then be found between the total dimension of the cavity, compliant tubes or other devices as described below, and the diameter of this opening.

In one embodiment, said cavity 7 inside said case 5 and located between the two pavilions 3 and in which is locate said electroacoustic motors 1, can indeed also enclose elastic tubes 18, closed, sealed and filled with gas, therefore called compliants.

The realization of such a transducer and all of the different parts the constituent is of the known and achievable field by any person skilled in the art without the need to give more details of construction, such as in particular the fixings, the compliant tubes and the various connecting parts of the elements relative to each other.

According to the present invention, each orifice 20, or opening axial end or vent, said front cavities 19 is closed by a passive radiator 15 constituted by a plate of a material of higher density than medium 4: as indicated previously, it may be a metal part made of steel or aluminum bronze; this plate is suspended at the periphery of the opening or vent by an elastic suspension 23, and can be domed.

To allow greater immersion of this transducer, said compliant tubes 18 can be replaced in one any of the cavities 7, 19 by at least one flexible bladder occupying at least part if not all of the entire volume of the cavity concerned and filled with a more compressible fluid than the ambient liquid 4: this can be applied either for cavity 7 or for the end cavities 19, that is to say for all of said cavities.

• soit plusieurs vessies indépendantes, qui sont glissées par des ouvertures dans le boítier 5, après avoir été de préférence remplies, lesdites ouvertures devant être ensuite refermées pour assurer la continuité du boítier rigide extérieur 5 au niveau de cette cavité centrale 7 ;
• soit une seule membrane occupant au moins une partie si ce n'est l'ensemble de la surface interne de la cavité 7 du transducteur et réalisée par une peau en élastomère par exemple, et que l'on remplit ensuite dudit fluide, mais la difficulté est alors de pouvoir assurer ce remplissage sans qu'il reste de bulles d'air qui compromettraient l'efficacité d'un tel dispositif, avec la profondeur.

In fact, taking into account the presence of the acoustic motors 1 and of the various assembly parts 12 and of the supply cable 10, it may preferably be placed in the central cavity 7:

• either several independent bladders, which are slid by openings in the housing 5, after having preferably been filled, said openings having to be then closed to ensure the continuity of the rigid external housing 5 at this central cavity 7;
• or a single membrane occupying at least a part if not the entire internal surface of the cavity 7 of the transducer and produced by an elastomer skin for example, which is then filled with said fluid, but the difficulty is then to be able to ensure this filling without any air bubbles remaining which would compromise the effectiveness of such a device, with depth.

Indeed, the fluid occupying the volumes delimited by the skin said bladders should fill at best and preferably practically the entire cavity, because its volume must in fact be greater than that compliant tubes 18 represented and as described above, so as to have equivalent compressibility characteristics to that of said tubes as used to date in other types transducers.

For this, the compressibility of said fluid must in fact be less than 10 ⁹ N / m ² , defined by the product of its density P _f with the square of the speed of propagation of sound in this fluid C _f .

• volume de la cavité 6 ou 16 = volume du fluide + volume de l'eau 10 résiduelle pouvant exister dans la cavité correspondante.
• compliance globale du système = (volume du fluide / Pf x Cf 2 du fluide) + (volume de l'eau / 2,22 x 109).

To then have the value of the overall compliance of the cavity, we must have both:

• volume of cavity 6 or 16 = volume of fluid + volume of residual water 10 which may exist in the corresponding cavity .
• overall system compliance = (fluid volume / P f x C f 2 fluid) + (volume of water / 2.22 x 10 9 ) .

A fluid from the family of fully fluorinated organic compounds of the C8H18 type is thus preferably chosen; in addition, the viscosity should not be too high, or lower than that of water, preferably less than 6.5 x 10 ^-7 m ² per second which is the viscosity of the silicone oil.

Claims (10)

1. A process for emitting acoustic waves into a fluid 4 at low frequency from a transducer comprising at least one horn 3 solid with the end of a motor pillar 1, a hollow rigid box 5 surrounding the said horn and delimiting with the latter at least one cavity 7, 19 which communicates by at least one port 6, 20 with the said external fluid 4 which can thus fill it also and having determined dimensions and external volume, the said transducer transmitting waves in a given range of frequency and with the given electro-acoustic power, characterised in that :

   said port 6, 20 is obturated by a passive radiator 15, made of a material denser than the said fluid 4 ;

   this said passive radiator 15 is hung at the periphery of said port 6, 20 by an elastic material 23 ;

   acoustic waves are emitted at frequencies lower than that of the initial range of frequency and with the same maximum power consumption.
2. A process for emitting acoustic waves according to claim 1 from a transducer whose rigid and cylindrical box 5 on axis XX', surrounds two horns 3 and encloses two acoustic motors 1 each of them being associated with one of the latter, identical and located coaxially on both sides of a central counter-mass 2, characterised in that :

   said rigid box 5 is prolonged on axis XX' beyond both horns 3, and thus making two cavities 19 each of them comprising an axial end port 20 and the resonance of which is determined to correspond to the range of emission frequency wanted ;

   the part of the box located between both said horns and enclosing a central cavity 7 is completely closed ;

   Each of the two end ports 20 of said box 2, is obturated by a passive radiator 15.
3. A process for emitting acoustic waves according to claim 1 from a transducer 2 whose rigid and cylindrical box 5 on axis XX', surrounds two horns 3 and encloses two electro-acoustic motors 1, each of them being associated with one of the latter, identical and located coaxially on both sides of a central counter-mass 2, characterised in that:

   a peripheral opening 6 is made in the wall of box 5 between the two said horns making one of the said ports of communication between said external fluid 4 and internal cavity 7 located between both horns ;

   this opening is obturated by a passive radiator 15 made of several sectors 16 independent and connected the ones with the others by elastic connections 17.
4. A device to reduce the resonant frequency of the cavities of the submersible transducers comprising at least one horn 3 solid with the end of a motor pillar 1, a hollow rigid box surrounding the said horn and delimiting with the latter at least one cavity 7, 19 which communicates by at least one port 6, 20 with the said external fluid 4 which is filled by the said cavity, and having determined dimensions and external volume, the said transducer transmitting waves in a given range of frequency with the given power, characterised in that it comprises a passive radiator 15 made of a material denser than the said fluid 4, obturating said port 6, 20 and hung at the periphery of the edge of said port 6, 20 by an elastic material 23.
5. A device according to claim 4, characterised in that said box 5 is cylindrical around axis XX' and encloses two identical electro-acoustic motors 1 located coaxially in this box on both sides of a central counter-mass 2 and whose opposed ends are each surrounded by a horn 3, said box 5 comprising between both horns 3 a peripheral opening 6 constituting a port of communication between internal cavity 7 and external fluid 4, the said opening being obturated by a passive radiator 15 made of several independent sectors 16 connected to each other by elastic connections 17.
6. A device according to claim 5, characterised in that the said transducer comprises at the rear of said horn 3 and inside box 5 a dynamic load 22 solid with the latter partially closing its internal section and dividing the internal cavity 7 into two communicating parts, the rear one 7₁ and the front one 7₂.
7. A device according to claim 6, characterised in that the said dynamic load 22 is made of a solid wall 27 following the shape of the internal surface of box 5 which it is solid with, surrounding said motor pillar 1 and drilled with at least one port 26 through which the latter passes, the said port comprising a conduct 28 extending behind wall 27 in relation to horn 3 and leaving a free peripheral passage around said motor pillar 1,
8. A device according to claim 4, characterised in that said box 5 is cylindrical around axis XX' and encloses two identical electro-acoustic motors 1 located coaxially in this box on both sides of a central counter-mass 2 and whose opposite ends are each surrounded by a horn 3, said rigid box 5 extending along axis XX' beyond both horns 3 and constituting with the latter two cavities 19 whose resonance corresponds to the emission frequency wanted and each of them comprising an axial end port 20 each being obturated by a passive radiator 15.
9. A device according to claim 4, characterised in that cavities 7, 19 delimited by horns 3 and box 5 contain closed elastic tubes 18, sealed and filled with gas.
10. A device according to claim 4, characterised in that each of said cavities 7, 19 encloses at least one flexible bladder occupying at least part of the volume of the cavity which contains it and filled with a fluid more compressible than liquid 4 of immersion.

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