EP2367640A1 - Acoustic wave transducer and sonar antenna with improved directivity - Google Patents

Acoustic wave transducer and sonar antenna with improved directivity

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Publication number
EP2367640A1
EP2367640A1 EP09805787A EP09805787A EP2367640A1 EP 2367640 A1 EP2367640 A1 EP 2367640A1 EP 09805787 A EP09805787 A EP 09805787A EP 09805787 A EP09805787 A EP 09805787A EP 2367640 A1 EP2367640 A1 EP 2367640A1
Authority
EP
European Patent Office
Prior art keywords
transducer
acoustic
housing
wall
axis
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
EP09805787A
Other languages
German (de)
French (fr)
Other versions
EP2367640B1 (en
Inventor
Frédéric MOSCA
Pascal Girardi
Robert Girault
Yann Cottreau
Guillaume Matte
Samuel Thomas
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
iXBlue SAS
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iXBlue SAS
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Publication date
Application filed by iXBlue SAS filed Critical iXBlue SAS
Publication of EP2367640A1 publication Critical patent/EP2367640A1/en
Application granted granted Critical
Publication of EP2367640B1 publication Critical patent/EP2367640B1/en
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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B06—GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS IN GENERAL
    • B06B—METHODS OR APPARATUS FOR GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS OF INFRASONIC, SONIC, OR ULTRASONIC FREQUENCY, e.g. FOR PERFORMING MECHANICAL WORK IN GENERAL
    • B06B1/00—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency
    • B06B1/02—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy
    • B06B1/06—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy operating with piezoelectric effect or with electrostriction
    • B06B1/0607—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy operating with piezoelectric effect or with electrostriction using multiple elements
    • B06B1/0611—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy operating with piezoelectric effect or with electrostriction using multiple elements in a pile
    • B06B1/0618—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy operating with piezoelectric effect or with electrostriction using multiple elements in a pile of piezo- and non-piezoelectric elements, e.g. 'Tonpilz'
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
    • H04R1/00—Details of transducers, loudspeakers or microphones
    • H04R1/44—Special adaptations for subaqueous use, e.g. for hydrophone
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
    • H04R17/00—Piezoelectric transducers; Electrostrictive transducers

Definitions

  • the present invention relates to an electro-acoustic transducer for a sonar antenna.
  • An electro-acoustic transducer is used for transmitting and / or receiving acoustic pressure waves.
  • an acoustic transducer transforms an electric potential difference into an acoustic pressure wave (and vice versa into a reception mode).
  • piezo-acoustic transducers there are different types of electro-acoustic transducers. In the remainder of this document we are particularly interested in the piezo-acoustic transducers of Tonpilz and Janus-Helmholtz type. These transducers comprise a piezoelectric motor, generally consisting of a stack of piezoelectric ceramics and electrodes, this piezoelectric motor being connected on the one hand to a counterweight and on the other hand to a flag.
  • the piezoelectric motor assembly, counterweight and horn is connected by a prestressing rod and constitutes a resonator whose resonance frequency depends in particular on the dimensions of the horn, the motor and the counterweight.
  • the piezo-acoustic resonator is usually placed in a waterproof protective case.
  • the outer face of the flag is in direct contact with the immersion medium or placed behind an acoustically transparent membrane.
  • the inner cavity of the housing is filled with either air or a fluid selected to have a good acoustic impedance without loss, without impedance breaking with water.
  • the fluid used is usually an oil.
  • the cavity is filled with air, the acoustic coupling between the transducer and the immersion medium is through the outer face of the flag.
  • the acoustic coupling between the transducer and the immersion medium is through the horn through the oil and the housing.
  • the immersed transducer transforms the vibration wave of the resonator into an acoustic pressure wave propagating in the immersion medium.
  • An electro-acoustic transducer makes it possible to sound an acoustic echo.
  • the specific response of a transducer depends on the frequency, the bandwidth and the direction of the echo with respect to the transmitting / receiving axis of the transducer.
  • the transducer In bathymetry applications, the transducer is placed vertically to probe echo from the sea floor. It is therefore essential to sound acoustic waves in a specific direction. Indeed, the secondary echo sources generate noise and reduce the sensitivity of the device.
  • a directivity diagram represents the acoustic intensity as a function of the measurement direction (angularly marked).
  • FIG. 2 The directivity diagram indicative of the response of a Tonpilz transducer as a function of the direction relative to the acoustic axis of the transducer is shown schematically in FIG. 2.
  • This diagram 12 being symmetrical with respect to the acoustic axis 7 transducer (axis 0-180 °) only a half-diagram is shown.
  • the curve of this diagram is a sound intensity level curve.
  • FIG. 2 shows a main lobe 13 centered on the acoustic axis 7 of the transducer and oriented in the X direction towards the front of the horn.
  • the diagram of FIG. 2 also has a rear lobe 14, on the acoustic axis and in the X 'direction opposite to the main lobe 13.
  • FIG. 2 also shows side lobes 15, 15', 15 "parasitic in directions between 40 ° and 140 ° with respect to the acoustic axis
  • the presence of secondary lobes impairs the directivity of the transducer, which receives and / or emits acoustic energy in different directions the X direction of the transducer axis towards the front of the pavilion.
  • Tonpilz transducers operate at frequencies between 1 kHz and 800 kHz.
  • the problem of side lobes appears when the characteristic dimension of the emitting face is of the order or greater than the working wavelength.
  • the problem of secondary lobes therefore appears more easily at high frequencies> 5OkHz (because the wavelengths become of the order of a centimeter).
  • a Janus-Helmholtz transducer comprises indeed two piezo-acoustic motors aligned along the same axis and fixed on a central contermasse, each piezo-acoustic motor being connected to a horn by a prestressing rod. The two flags are thus located at opposite ends on the axis of the device and symmetrical with respect to a plane transverse to the axis.
  • a Janus-Helmholtz transducer makes it possible to work at lower frequencies (from 150 Hz to 20 kHz) than a Tonpilz transducer.
  • the directivity diagram of a Janus-Helmholtz type transducer operating at very low frequency is generally very non-directive. This diagram is symmetrical with respect to the transverse plane of symmetry. However, it has two power maxima on the axis of the transducer in the forward direction of each flag. But the power emitted or received in the direction transverse to the acoustic axis can also induce disturbances. In addition, when a Janus-Helmholtz transducer is used at a relatively higher frequency, side lobes also appear. There are known solutions to improve the directivity of an electro-acoustic transducer.
  • the counterweight of the transducer serves as a vibration node and is therefore an important fixed point for the directivity of the transducer.
  • the directivity of the transducer is improved by connecting the counterweight to the housing by a metal plate (aluminum, stainless steel, steel, etc.).
  • the secondary lobes in site around the normal to the acoustic axis are one of the major restrictions of a sonar antenna, and whatever the type of transducer used (see Figure 2). Indeed these secondary lobes cause the presence of surface echoes and significantly degrade the contrast on the system's shadow.
  • One of the aims of the invention is to improve the directivity of an electro-acoustic transducer of the Tonpilz or Janus-Hemholtz type.
  • Another object of the invention is the reduction of the housing lobes in an electro-acoustic type transducer.
  • the invention relates to an acoustic wave transducer comprising at least one electro-acoustic motor, a horn having an inner wall and an outer wall, a counterweight, and a hollow housing having an inner wall and an outer wall and at least one opening. acoustic. Said electro-acoustic motor is connected on the one hand to the horn and on the other hand to the counterweight along an axis and said electroacoustic motor is able to excite the horn around at least one acoustic resonance frequency f.
  • said housing is connected to the counterweight and surrounds the motor and the roof, the outer wall of the roof being placed opposite an acoustic opening of the housing, and the space between the inner wall of the housing and the inner wall of the roof forming a cavity comprising a fluid.
  • said transducer comprises attenuation acoustic means integral with an outer wall of the housing for attenuating acoustic waves in transmission and / or reception at frequency f in at least one direction transverse to the transmission axis /reception.
  • the housing has a wall extending longitudinally along the axis of the transducer and having a thickness E, said thickness E being greater than the acoustic wavelength ⁇ corresponding to the frequency f in the housing of to absorb a portion of the acoustic waves at the frequency f in at least one direction transverse to the axis.
  • Said attenuation means may further comprise an absorbent sheath attached to an outer wall of the housing and adapted to absorb acoustic waves at the frequency f in at least one direction transverse to the axis.
  • Said attenuation means may further comprise a diffractive grating surrounding the absorbent sheath, said array being able to diffract waves acoustically in the passband of the transducer and suspension means capable of damping the coupling of acoustic waves between the diffractive grating and the absorbent sheath.
  • Said attenuation means may further comprise a reflective sheath around the diffractive grating and suspension means capable of damping the coupling of acoustic waves between the reflecting sheath and the absorbent sheath.
  • the reflective sheath is made of aluminum
  • the absorbent sheath is made of polymer resin or syntactic foam
  • the viscoelastic polymer suspension means is made of aluminum
  • the reflective sheath is of convex outer shape so as to attenuate a part of the acoustic waves originating from the immersion medium in directions transverse to the axis.
  • the transducer is a Tonpilz type transducer, comprising an elongated piezoelectric motor, said motor comprising a stack of piezoelectric components and electrodes, the stack being connected along an axis of symmetry. at one end to the pavilion and at the other end to the counterpart.
  • the transducer is a Janus-Helmholtz type transducer, comprising two elongated piezoelectric motors whose axes are aligned, each motor comprising a stack of piezoelectric components and electrodes, the stacking being connected along an axis of symmetry at one end to a horn and at the other end to a central counterweight common to both engines, said transducer comprising two housings surrounding each motor-horn subassembly.
  • the invention also relates to a sonar antenna comprising a plurality of transducers, said transducers being placed in a common housing according to one of the preceding embodiments.
  • the present invention also relates to the features which will emerge in the course of the description which follows and which will have to be considered individually or in all their technically possible combinations.
  • FIG. 2 represents an exemplary directivity diagram of a Tonpilz type acoustic antenna
  • - Figure 3 schematically shows a Tonpilz type acoustic transducer with its housing
  • FIG. 4 shows schematically a sectional view of attenuation means of the housing lobes;
  • FIG. 5 illustrates the representative directivity diagram of a Tonpilz acoustic antenna according to the invention
  • FIG. 6 schematically shows a sectional view of a Janus-Helmholtz-type acoustic transducer
  • FIG. 7 shows a sonar antenna comprising several transducers in the same housing.
  • FIG. 1 represents a partial view of a Tonpilz transducer (the housing is not shown), the transducer being symmetrical about the acoustic axis 7.
  • the transducer comprises an electro-acoustic motor 1 connected to a horn 4 and a counterweight 5 by a prestressing rod 6.
  • this motor comprises piezoelectric ceramics connected to electrodes 3 which are subjected to a sinusoidal voltage. Piezoelectric ceramics thus undergo a sinusoidal mechanical deformation in the polarization direction of the ceramics.
  • the flag 4 provides a dual function of broadening the bandwidth of the transducer due to its own mode of swirling and acoustic impedance matching between the ceramic and the fluid medium.
  • the counterweight 5 stabilizes the assembly and shifts the nodal plane of vibration towards the rear of the transducer ensuring a maximum transmission of energy in the desired direction of the acoustic axis forward of the flag 4.
  • the prestressing rod 6 maintains the acoustic motor-horn-counterweight unit under prestressing so as to ensure its compression-only operation.
  • the Tonpilz transducer is integrated in a housing 8 (not shown in FIG. 1) filled with oil 10 in order to ensure the pressure balance with the immersion medium into which the transducer is immersed.
  • the counterweight 5 is mounted in force in the housing 8.
  • the side lobes or housing lobes are a disadvantage known for many years in the transducers and in particular Tonpilz type transducers.
  • the inventors have analyzed the behavior of such a transducer. According to this analysis, the generation of these so-called “housing" side lobes is due to a coupling between the elements of the transducer (horn and counter), the fluid in which the resonator and the casing are immersed. This coupling results in the generation of four shear waves from two sources 16 and 16 'within the housing 8, each of the sources 16, 16' generating two shear waves in opposite directions. The origin of the secondary lobes is a coupling linked to a mode conversion of a shear wave propagating in the case. A first coupling acoustic coupling takes place between the fluid 9 and the housing 8.
  • This coupling generates a first source 16 of shear waves, schematically represented at the flag in the housing.
  • Unexpectedly coupling does not occur only at the interface between the fluid medium and the housing but a second mechanical coupling is at the counterweight.
  • the countermass is not necessarily a perfectly immobile vibration node, but undergoes displacements transverse to the axis. These displacements induce shear waves from a secondary focus 16 'shown schematically in Figure 3 in the housing opposite the counterweight.
  • the combination of coupling waves from the two foci 16 and 16 'further produces interfering waves.
  • FIG. 4 schematically represents a portion of a box seen in section, comprising various means for attenuating the acoustic waves. These means are advantageously arranged on the sides of the housing which extend longitudinally with respect to the acoustic axis 7 of transmission / reception of the transducer, so as to attenuate the acoustic waves propagating in substantially transverse directions (90 ⁇ 40 degrees ) the acoustic axis 7.
  • the attenuation means may be placed on one or more sidewalls around the axis, or form a continuous sheath surrounding the periphery of the housing around the acoustic axis.
  • a first means consists in increasing the thickness of the housing so that it is greater than the acoustic wavelength ⁇ corresponding to the frequency f in the housing.
  • the thickness of the housing is equal to about 2 ⁇ or 3 ⁇ .
  • Such a housing thickness makes it possible to convert the shear wave into a compression wave.
  • a Tonpilz transducer whose frequency is 100 kHz
  • a 2.5-3 cm thickness envelope is well suited.
  • the adapted thickness will be proportional to the frequency.
  • the thickness of the housing is uniform on all the faces of the housing extending longitudinally with respect to the axis.
  • the rear face of the housing also has a thickness greater than ⁇ , so as to attenuate the rear lobe 13 in the X 'direction opposite to the acoustic transmission / reception X direction.
  • a case thickness greater than ⁇ , or even equal to 2 ⁇ or 3 ⁇ can be obtained by directly manufacturing a case having such a thickness.
  • a second housing whose inner shape is adapted to the outer shape of the initial housing so that the total thickness of the housing thus obtained has a total thickness greater than ⁇ .
  • a second means consists in arranging around the casing 8 an absorbent sheath 17 so as to absorb the energy of the shear waves converted into compression waves.
  • the absorbent sheath is made of a softer material than the housing, for example a polymer resin.
  • a third means consists in placing a diffractive grating 19 on the surface of the absorbent sheath.
  • the grating 19 may be a one-dimensional grating with a pitch and a depth of the order of one half-wavelength.
  • the network 19 can also be two-dimensional.
  • a fourth means consists in placing a reflective sheath 18 around the absorbent sheath and the diffractive grating so as to increase the shear waves that are converted into compression waves in the absorbing medium.
  • the reflective sheath 18 may comprise for example a reflective envelope made of a material having a high impedance contrast with the absorbent sheath.
  • a strong impedance break is necessary for the reflective material which may for example be a metal.
  • This structure finally requires suspension means of the reflective material, so as to isolate this material and avoid transmission by vibratory coupling in the undesired direction.
  • the suspension means advantageously comprise a viscoelastic polymer.
  • the surface of the reflective layer 18 is of concave shape seen from the sources 16 and 16 '.
  • the order in which the side lobe attenuation means are assembled from the axis of the transducer to the outside of the housing is important and is preferably the order indicated above.
  • Attenuation means may be placed on the rear face of the housing.
  • FIG. 5 represents a simulation of the directivity diagram of the same Tonpilz transducer as that of FIG. 2, but provided with the means described above, and more precisely of all the accumulated means with the exception of the reflecting sheath.
  • FIG. 5 a very strong reduction of side lobes, which have almost disappeared.
  • the rear lobe 14 is also reduced.
  • the directivity of the transducer is thus considerably improved.
  • the device of the invention thus makes it possible to improve the directivity and the sensitivity of an electro-acoustic transducer.
  • the invention can adapt to any type of sonars with a slight modification of the outer envelope of the transducer.
  • the invention applies in particular to Janus-Helmholtz type transducers, as shown diagrammatically in section FIG. 6.
  • the Janus-Helmholtz transducer comprises two piezo-acoustic motors respectively 1 and 21 aligned along the same axis 7 and fixed on a contermass central 5.
  • Each piezoelectric motor 1, 21 is connected to a roof 4, 24 by a prestressing rod.
  • the two flags 4, 24 are thus located at opposite ends on the axis 7 of the device.
  • a housing 8, respectively 28 surrounds each motor-horn subassembly 1 and 4, respectively 21 and 24.
  • the counterweight is fixed by a metal plate on the one hand to the housing 8 and on the other hand to the housing 28.
  • the inner cavity each housing 8, 28 is filled with a fluid.
  • the housings 8 and 28 can be modified so that they comprise means of attenuation of acoustic waves emitted and / or received in directions transverse to the acoustic axis 7.
  • One or more wave attenuation means may be applied in a direction transverse to the housing of each of the two coaxial resonators.
  • the first means consists in using casings 8 and 28 with a thickness greater than ⁇ , and preferably equal to 2 ⁇ or 3 ⁇ .
  • a second means consists in fixing an absorbent sheath on a wall of the housing extending longitudinally along the axis 7.
  • a third means consists in placing on the surface of the absorbent sheath a diffractive grating.
  • a fourth means consists in placing a reflective sheath around the absorbent sheath and the diffracting grating so as to increase the shear waves that are converted into compression waves in the absorbing medium.
  • FIG. 7 schematically shows a sonar antenna seen from the front.
  • the antenna comprises a plurality of transducers.
  • four flags of Tonpilz transducers are aligned in the same casing 8.
  • FIG. 7 represents an absorbent sheath disposed on one of the sides of the sonar. Absorbent sheath portions may be disposed on the other sides of the housing which extend longitudinally along the axis 7 of the flags 4 of the transducers.
  • the absorbent sheath is placed on a wall of the housing whose thickness is greater than ⁇ in a direction of emission of the side lobes.
  • the absorbent sheath 17 advantageously cooperates with a reflecting medium 18, and a diffractive grating 18.
  • the absorption means may comprise separate elements on external sides of the housing, or a continuous sheath on the periphery of the housing in a plane perpendicular to the acoustic axis.
  • the invention thus makes it possible to eliminate the secondary lobes of a sonar antenna formed of a set of transducers having substantially the same acoustic axis.
  • the invention makes it possible to considerably improve the directivity of such a sonar antenna as well as its rear rejection.
  • the invention also applies to piezo-electric transducers of "sawed” or glue-type technology used in medical ultrasound or quarter-wave plate probes ("Diagnostic Ultrasound Imaging", Elsevier, Thomas L. Szabo). .

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Transducers For Ultrasonic Waves (AREA)
  • Measurement Of Velocity Or Position Using Acoustic Or Ultrasonic Waves (AREA)

Abstract

The invention relates to an acoustic wave transducer that includes at least one electroacoustic motor (1, 21), a flare (4, 24) having an inner wall and an outer wall, a counterweight (5), and a hollow housing (8, 28) having an inner wall and an outer wall and at least one acoustic opening. The electroacoustic motor is connected to the flare (4, 24) as well as to the counterweight (5) along an axis (7), and said electroacoustic motor (1, 21) is capable of exciting the flare at about at least one resonance frequency f. The housing (8, 28) is connected to the counterweight (5) and surrounds the motor (1, 21) and the flare (4, 24), the outer wall of the flare being arranged opposite an acoustic opening of the housing, and the space between the inner wall of the housing and the inner wall of the flare defines a cavity that contains a fluid. According to the invention, said transducer includes acoustic attenuation means connected to the outer wall of the housing in order to attenuate the transmission and/or reception acoustic waves at the frequency f at least in a direction transverse to the transmission/reception axis. The invention also relates to a sonar antenna that comprises at least one transducer according to the invention.

Description

La présente invention concerne un transducteur électro-acoustique pour antenne sonar. Un transducteur électro-acoustique est utilisé pour l'émission et/ou la réception d'ondes de pression acoustiques. En mode émission, un transducteur acoustique transforme une différence de potentiel électrique en onde de pression acoustique (et inversement en mode de réception). The present invention relates to an electro-acoustic transducer for a sonar antenna. An electro-acoustic transducer is used for transmitting and / or receiving acoustic pressure waves. In transmission mode, an acoustic transducer transforms an electric potential difference into an acoustic pressure wave (and vice versa into a reception mode).
Il existe différents types de transducteurs électro-acoustiques. Dans la suite de ce document nous nous intéressons en particulier aux transducteurs piezo-acoustiques de type Tonpilz et Janus-Helmholtz. Ces transducteurs comprennent un moteur piézoélectrique, constitué généralement d'un empilage de céramiques piezo-électriques et d'électrodes, ce moteur piezo-électrique étant relié d'une part à une contremasse et d'autre part à un pavillon. L'ensemble moteur piezo-électrique, contremasse et pavillon est relié par une tige de précontrainte et constitue un résonateur dont la fréquence de résonance dépend en particulier des dimensions du pavillon, du moteur et de la contremasse.There are different types of electro-acoustic transducers. In the remainder of this document we are particularly interested in the piezo-acoustic transducers of Tonpilz and Janus-Helmholtz type. These transducers comprise a piezoelectric motor, generally consisting of a stack of piezoelectric ceramics and electrodes, this piezoelectric motor being connected on the one hand to a counterweight and on the other hand to a flag. The piezoelectric motor assembly, counterweight and horn is connected by a prestressing rod and constitutes a resonator whose resonance frequency depends in particular on the dimensions of the horn, the motor and the counterweight.
Le résonateur piezo-acoustique est généralement placé dans un boîtier de protection étanche. La face externe du pavillon est en contact direct avec le milieu d'immersion ou placée derrière une membrane acoustiquement transparente. La cavité intérieure du boîtier est remplie soit d'air soit d'un fluide choisi pour avoir une bonne impédance acoustique sans perte, sans rupture d'impédance avec l'eau. Le fluide utilisé est généralement une huile. Quand la cavité est remplie d'air, le couplage acoustique entre le transducteur et le milieu d'immersion se fait par la face externe du pavillon. Quand la cavité est remplie d'huile, le couplage acoustique entre le transducteur et le milieu d'immersion se fait par le pavillon à travers l'huile et le boîtier. Le transducteur immergé transforme l'onde de vibration du résonateur en onde de pression acoustique qui se propage dans le milieu d'immersion.The piezo-acoustic resonator is usually placed in a waterproof protective case. The outer face of the flag is in direct contact with the immersion medium or placed behind an acoustically transparent membrane. The inner cavity of the housing is filled with either air or a fluid selected to have a good acoustic impedance without loss, without impedance breaking with water. The fluid used is usually an oil. When the cavity is filled with air, the acoustic coupling between the transducer and the immersion medium is through the outer face of the flag. When the cavity is filled with oil, the acoustic coupling between the transducer and the immersion medium is through the horn through the oil and the housing. The immersed transducer transforms the vibration wave of the resonator into an acoustic pressure wave propagating in the immersion medium.
Un transducteur électro-acoustique permet de sonder un écho acoustique. La réponse spécifique d'un transducteur dépend de la fréquence, de la bande passante et de la direction de l'écho par rapport à l'axe d'émission/réception du transducteur. Dans des applications de bathymétrie, le transducteur est placé verticalement de manière à sonder l'écho provenant du fonds sous-marin. Il est alors essentiel de sonder les ondes acoustiques dans une direction précise. En effet, les sources d'écho secondaires génèrent du bruit et réduisent la sensibilité du dispositif. Un diagramme de directivité représente l'intensité acoustique en fonction de la direction de mesure (repérée angulairement). Le diagramme de directivité indicatif de la réponse d'un transducteur de type Tonpilz en fonction de la direction par rapport à l'axe acoustique du transducteur est représenté schématiquement sur la figure 2. Ce diagramme 12 étant symétrique par rapport à l'axe acoustique 7 du transducteur (axe 0-180°) seule un demi-diagramme est représenté. La courbe de ce diagramme est une courbe de niveau d'intensité acoustique. On observe sur le diagramme de la figure 2 un lobe principal 13 centré sur l'axe acoustique 7 du transducteur et orienté dans la direction X vers l'avant du pavillon. Le diagramme de la figure 2 présente également un lobe arrière 14, sur l'axe acoustique et dans la direction X' opposée au lobe principal 13. On observe également sur la figure 2 des lobes secondaires 15, 15', 15" parasites dans des directions comprises entre 40° et 140° par rapport à l'axe acoustique. La présence de lobes secondaires nuit à la directivité du transducteur, qui reçoit et/ou émet une énergie acoustique dans des directions différentes la direction X de l'axe du transducteur vers l'avant du pavillon.An electro-acoustic transducer makes it possible to sound an acoustic echo. The specific response of a transducer depends on the frequency, the bandwidth and the direction of the echo with respect to the transmitting / receiving axis of the transducer. In bathymetry applications, the transducer is placed vertically to probe echo from the sea floor. It is therefore essential to sound acoustic waves in a specific direction. Indeed, the secondary echo sources generate noise and reduce the sensitivity of the device. A directivity diagram represents the acoustic intensity as a function of the measurement direction (angularly marked). The directivity diagram indicative of the response of a Tonpilz transducer as a function of the direction relative to the acoustic axis of the transducer is shown schematically in FIG. 2. This diagram 12 being symmetrical with respect to the acoustic axis 7 transducer (axis 0-180 °) only a half-diagram is shown. The curve of this diagram is a sound intensity level curve. FIG. 2 shows a main lobe 13 centered on the acoustic axis 7 of the transducer and oriented in the X direction towards the front of the horn. The diagram of FIG. 2 also has a rear lobe 14, on the acoustic axis and in the X 'direction opposite to the main lobe 13. FIG. 2 also shows side lobes 15, 15', 15 "parasitic in directions between 40 ° and 140 ° with respect to the acoustic axis The presence of secondary lobes impairs the directivity of the transducer, which receives and / or emits acoustic energy in different directions the X direction of the transducer axis towards the front of the pavilion.
Les transducteurs de type Tonpilz fonctionnent à des fréquences comprises entre 1 kHz et 800 kHz. Le problème des lobes secondaires apparaît lorsque la dimension caractéristique de la face émettrice est de l'ordre ou supérieure à la longueur d'onde de travail. La longueur d'onde λ étant définie reliée à la fréquence f par la relation λ= c / f, où c est la vitesse de l'onde acoustique dans le milieu d'immersion (la vitesse du son dans l'eau de mer est d'environ 1500m/s). Le problème des lobes secondaires apparaît donc plus facilement aux hautes fréquences > 5OkHz (car les longueurs d'onde deviennent de l'ordre du centimètre).Tonpilz transducers operate at frequencies between 1 kHz and 800 kHz. The problem of side lobes appears when the characteristic dimension of the emitting face is of the order or greater than the working wavelength. The wavelength λ being defined connected to the frequency f by the relation λ = c / f, where c is the speed of the acoustic wave in the immersion medium (the speed of sound in the seawater is about 1500m / s). The problem of secondary lobes therefore appears more easily at high frequencies> 5OkHz (because the wavelengths become of the order of a centimeter).
Ces lobes secondaires sont généralement attribués à un découplage imparfait entre le moteur piezo-électrique et le boîtier, d'où leur désignation « lobes de boîtier ». De plus, il est connu que les forces de pression en immersion profonde produisent des déformations et ne permettent pas un découplage du moteur et du boîtier.These secondary lobes are generally attributed to an imperfect decoupling between the piezoelectric motor and the housing, hence their designation "housing lobes". In addition, it is known that the deep immersion pressure forces produce deformations and do not allow decoupling of the motor and the housing.
Un autre type de transducteur est dérivé de la structure Tonpilz : il s'agit des transducteurs de type Janus-Helmholtz. Un transducteur Janus-Helmholtz comprend en effet deux moteurs piezo-acoustiques alignés selon un même axe et fixés sur une contermasse centrale, chaque moteur piezo-acoustique étant relié à un pavillon par une tige de précontrainte. Les deux pavillons sont ainsi situés aux extrémités opposées sur l'axe du dispositif et symétriques par rapport à un plan transverse à l'axe. Un transducteur Janus-Helmholtz permet de travailler à des fréquences plus basses (de 150 Hz à 20 kHz) qu'un transducteur de type Tonpilz.Another type of transducer is derived from the Tonpilz structure: these are Janus-Helmholtz type transducers. A Janus-Helmholtz transducer comprises indeed two piezo-acoustic motors aligned along the same axis and fixed on a central contermasse, each piezo-acoustic motor being connected to a horn by a prestressing rod. The two flags are thus located at opposite ends on the axis of the device and symmetrical with respect to a plane transverse to the axis. A Janus-Helmholtz transducer makes it possible to work at lower frequencies (from 150 Hz to 20 kHz) than a Tonpilz transducer.
Le diagramme de directivité d'un transducteur de type Janus-Helmholtz fontionnant à très basse fréquence (de 150 Hz à 20 kHz) est générallement très peu directif. Ce diagramme est symétrique par rapport au plan transverse de symétrie. Il présente toutefois deux maxima de puissance sur l'axe du transducteur dans la direction avant de chaque pavillon. Mais la puissance émise ou reçue dans la direction transverse à l'axe acoustique peut également induire des perturbations. De plus, lorsqu'on utilise un transducteur Janus-Helmholtz à une fréquence relativement plus élevée, des lobes secondaires apparaissent également. II existe des solutions connues pour améliorer la directivité d'un transducteur électro-acoustique. La contremasse du transducteur sert de nœud de vibration et est donc un point fixe important pour la directivité du transducteur. Ainsi, on améliore la directivité du transducteur en reliant la contremasse au boîtier par une plaque métallique (en aluminium, inox, acier...).The directivity diagram of a Janus-Helmholtz type transducer operating at very low frequency (from 150 Hz to 20 kHz) is generally very non-directive. This diagram is symmetrical with respect to the transverse plane of symmetry. However, it has two power maxima on the axis of the transducer in the forward direction of each flag. But the power emitted or received in the direction transverse to the acoustic axis can also induce disturbances. In addition, when a Janus-Helmholtz transducer is used at a relatively higher frequency, side lobes also appear. There are known solutions to improve the directivity of an electro-acoustic transducer. The counterweight of the transducer serves as a vibration node and is therefore an important fixed point for the directivity of the transducer. Thus, the directivity of the transducer is improved by connecting the counterweight to the housing by a metal plate (aluminum, stainless steel, steel, etc.).
Cependant, les lobes secondaires en site autour de la normale à l'axe acoustique sont une des restrictions majeures d'une antenne sonar, et ce quelque soit le type de transducteur utilisé (cf figure 2). En effet ces lobes secondaires occasionnent la présence d'échos de surface et dégradent considérablement le contraste sur ombre du système.However, the secondary lobes in site around the normal to the acoustic axis are one of the major restrictions of a sonar antenna, and whatever the type of transducer used (see Figure 2). Indeed these secondary lobes cause the presence of surface echoes and significantly degrade the contrast on the system's shadow.
Des outils de modélisation de la réponse en fréquence d'un transducteur de type Janus-Helmholtz existent, mais ces outils ne parviennent pas à simuler parfaitement le comportement d'un transducteur.Tools for modeling the frequency response of a Janus-Helmholtz type transducer exist, but these tools fail to perfectly simulate the behavior of a transducer.
Un des buts de l'invention est d'améliorer la directivité d'un transducteur électro- acoustique de type Tonpilz ou Janus-Hemholtz. Un autre but de l'invention est la réduction des lobes de boîtier dans un transducteur de type électro-acoustique.One of the aims of the invention is to improve the directivity of an electro-acoustic transducer of the Tonpilz or Janus-Hemholtz type. Another object of the invention is the reduction of the housing lobes in an electro-acoustic type transducer.
L'invention concerne un transducteur d'ondes acoustiques comprenant au moins un moteur électro-acoustique, un pavillon ayant une paroi interne et une paroi externe, une contremasse, et un boîtier creux ayant une paroi interne et une paroi externe et au moins une ouverture acoustique. Ledit moteur électro-acoustique est relié d'une part au pavillon et d'autre part à la contremasse suivant un axe et ledit moteur électroacoustique est apte à exciter le pavillon autour d'au moins une fréquence de résonance acoustique f. Ledit boîtier est relié à la contremasse et entoure le moteur et le pavillon, la paroi externe du pavillon étant placée en face d'une ouverture acoustique du boîtier, et l'espace entre la paroi interne du boîtier et la paroi interne du pavillon formant une cavité comprenant un fluide. Selon l'invention, ledit transducteur comprend des moyens acoustiques d'atténuation solidaires d'une paroi externe du boîtier pour atténuer les ondes acoustiques en émission et/ou en réception à la fréquence fdans au moins une direction transverse à l'axe d'émission/réception. Selon un premier mode de réalisation, le boîtier a une paroi s'étendant longitudinalement selon l'axe du transducteur et d'épaisseur E, ladite épaisseur E étant supérieure à la longueur d'onde acoustique λ correspondant à la fréquence f dans le boîtier de manière à absorber une partie des ondes acoustiques à la fréquence f dans au moins une direction transverse à l'axe. Lesdits moyens d'atténuation peuvent en outre comprendre une gaine absorbante fixée sur une paroi externe du boîtier et apte à absorber des ondes acoustiques à la fréquence fdans au moins une direction transverse à l'axe.The invention relates to an acoustic wave transducer comprising at least one electro-acoustic motor, a horn having an inner wall and an outer wall, a counterweight, and a hollow housing having an inner wall and an outer wall and at least one opening. acoustic. Said electro-acoustic motor is connected on the one hand to the horn and on the other hand to the counterweight along an axis and said electroacoustic motor is able to excite the horn around at least one acoustic resonance frequency f. Said housing is connected to the counterweight and surrounds the motor and the roof, the outer wall of the roof being placed opposite an acoustic opening of the housing, and the space between the inner wall of the housing and the inner wall of the roof forming a cavity comprising a fluid. According to the invention, said transducer comprises attenuation acoustic means integral with an outer wall of the housing for attenuating acoustic waves in transmission and / or reception at frequency f in at least one direction transverse to the transmission axis /reception. According to a first embodiment, the housing has a wall extending longitudinally along the axis of the transducer and having a thickness E, said thickness E being greater than the acoustic wavelength λ corresponding to the frequency f in the housing of to absorb a portion of the acoustic waves at the frequency f in at least one direction transverse to the axis. Said attenuation means may further comprise an absorbent sheath attached to an outer wall of the housing and adapted to absorb acoustic waves at the frequency f in at least one direction transverse to the axis.
Lesdits moyens d'atténuation peuvent comprendre en outre un réseau diffractant entourant la gaine absorbante, ledit réseau étant apte à diffracter des ondes acoustiques dans la bande passante du transducteur et des moyens de suspension aptes à amortir le couplage d'ondes acoustiques entre le réseau diffractant et la gaine absorbante.Said attenuation means may further comprise a diffractive grating surrounding the absorbent sheath, said array being able to diffract waves acoustically in the passband of the transducer and suspension means capable of damping the coupling of acoustic waves between the diffractive grating and the absorbent sheath.
Lesdits moyens d'atténuation peuvent comprendre en outre une gaine réfléchissante autour du réseau diffractant et des moyens de suspension aptes à amortir le couplage d'ondes acoustiques entre la gaine réfléchissante et la gaine absorbante.Said attenuation means may further comprise a reflective sheath around the diffractive grating and suspension means capable of damping the coupling of acoustic waves between the reflecting sheath and the absorbent sheath.
Selon un mode de réalisation particulier, la gaine réfléchissante est en aluminium, la gaine absorbante est en résine polymère ou en mousse syntactique, et les moyens de suspension en polymère viscoélastique.According to a particular embodiment, the reflective sheath is made of aluminum, the absorbent sheath is made of polymer resin or syntactic foam, and the viscoelastic polymer suspension means.
Selon encore un mode de réalisation particulier, la gaine réfléchissante est de forme extérieure bombée de manière à atténuer une partie des ondes acoustiques provenant du milieu d'immersion dans des directions transverses à l'axe.According to another particular embodiment, the reflective sheath is of convex outer shape so as to attenuate a part of the acoustic waves originating from the immersion medium in directions transverse to the axis.
Selon un mode de réalisation préféré, le transducteur est un transducteur de type Tonpilz, comprenant un moteur piezo-électrique de forme allongée, ledit moteur comprenant un empilage de composants piezo-électriques et des électrodes, l'empilage étant relié selon un axe de symétrie par une extrémité au pavillon et par l'autre extrémité à la contremasse.According to a preferred embodiment, the transducer is a Tonpilz type transducer, comprising an elongated piezoelectric motor, said motor comprising a stack of piezoelectric components and electrodes, the stack being connected along an axis of symmetry. at one end to the pavilion and at the other end to the counterpart.
Selon un autre mode de réalisation, le transducteur est un transducteur de type Janus-Helmholtz, comprenant deux moteurs piezo-électriques de forme allongées dont les axes sont alignés, chaque moteur comprenant un empilage de composants piezo- électriques et des électrodes, l'empilage étant relié selon un axe de symétrie par une extrémité à un pavillon et par l'autre extrémité à une contremasse centrale commune aux deux moteurs, ledit transducteur comprenant deux boîtiers entourant chaque sous- ensemble moteur-pavillon.According to another embodiment, the transducer is a Janus-Helmholtz type transducer, comprising two elongated piezoelectric motors whose axes are aligned, each motor comprising a stack of piezoelectric components and electrodes, the stacking being connected along an axis of symmetry at one end to a horn and at the other end to a central counterweight common to both engines, said transducer comprising two housings surrounding each motor-horn subassembly.
L'invention concerne également une antenne sonar comprenant une pluralité de transducteurs, lesdits transducteurs étant placés dans un boîtier commun selon l'un des modes de réalisation précédents.The invention also relates to a sonar antenna comprising a plurality of transducers, said transducers being placed in a common housing according to one of the preceding embodiments.
La présente invention concerne également les caractéristiques qui ressortiront au cours de la description qui va suivre et qui devront être considérées isolément ou selon toutes leurs combinaisons techniquement possibles.The present invention also relates to the features which will emerge in the course of the description which follows and which will have to be considered individually or in all their technically possible combinations.
Cette description est donnée à titre d'exemple non limitatif et fera mieux comprendre comment l'invention peut être réalisée en référence aux dessins annexés sur lesquels : - la figure 1 représente schématiquement les composants internes d'un transducteur acoustique de type Tonpilz à symétrie de révolution autour de son axe (vue en demi-section sans le boîtier) ;This description is given by way of nonlimiting example and will better understand how the invention can be made with reference to the accompanying drawings in which: - Figure 1 shows schematically the internal components of a tonpilz type acoustic transducer symmetry of revolution around its axis (view in half-section without the case);
- la figure 2 représente un exemple de diagramme de directivité d'une antenne acoustique type Tonpilz ; - la figure 3 représente schématiquement un transducteur acoustique de type Tonpilz avec son boîtier ;FIG. 2 represents an exemplary directivity diagram of a Tonpilz type acoustic antenna; - Figure 3 schematically shows a Tonpilz type acoustic transducer with its housing;
- la figure 4 représente schématiquement une vue en coupe de moyens d'atténuation des lobes de boîtier ; - la figure 5 illustre le diagramme de directivité représentatif d'une antenne acoustique Tonpilz selon l'invention ;- Figure 4 shows schematically a sectional view of attenuation means of the housing lobes; FIG. 5 illustrates the representative directivity diagram of a Tonpilz acoustic antenna according to the invention;
- la figure 6 représente schématiquement une vue en coupe d'un transducteur acoustique de type Janus-Helmholtz ;- Figure 6 schematically shows a sectional view of a Janus-Helmholtz-type acoustic transducer;
- la figure 7 représente une antenne sonar comprenant plusieurs transducteurs dans un même boîtier.- Figure 7 shows a sonar antenna comprising several transducers in the same housing.
La figure 1 représente une vue partielle d'un transducteur Tonpilz (le boîtier n'est pas représenté), le transducteur étant symétrique de révolution autour de l'axe acoustique 7. Le transducteur comprend un moteur électro-acoustique 1 relié à un pavillon 4 et une contremasse 5 par une tige de précontrainte 6. Dans l'exemple représenté ce moteur comprend des céramiques piézoélectriques reliées à des électrodes 3 qui sont soumises à une tension sinusoïdale. Les céramiques piézoélectriques subissent ainsi une déformation mécanique sinusoïdale dans la direction de polarisation des céramiques. Le pavillon 4 assure une double fonction d'élargissement de la bande passante du transducteur due à son mode propre de papillonnage et d'adaptation d'impédance acoustique entre la céramique et le milieu fluide. La contremasse 5 stabilise l'ensemble et décale le plan nodal de vibration vers l'arrière du transducteur assurant une transmission maximale de l'énergie dans la direction souhaitée de l'axe acoustique vers l'avant du pavillon 4. La tige de précontrainte 6 maintient l'ensemble moteur acoustique-pavillon-contremasse sous précontrainte de façon à assurer son fonctionnement en compression seule.FIG. 1 represents a partial view of a Tonpilz transducer (the housing is not shown), the transducer being symmetrical about the acoustic axis 7. The transducer comprises an electro-acoustic motor 1 connected to a horn 4 and a counterweight 5 by a prestressing rod 6. In the example shown, this motor comprises piezoelectric ceramics connected to electrodes 3 which are subjected to a sinusoidal voltage. Piezoelectric ceramics thus undergo a sinusoidal mechanical deformation in the polarization direction of the ceramics. The flag 4 provides a dual function of broadening the bandwidth of the transducer due to its own mode of swirling and acoustic impedance matching between the ceramic and the fluid medium. The counterweight 5 stabilizes the assembly and shifts the nodal plane of vibration towards the rear of the transducer ensuring a maximum transmission of energy in the desired direction of the acoustic axis forward of the flag 4. The prestressing rod 6 maintains the acoustic motor-horn-counterweight unit under prestressing so as to ensure its compression-only operation.
Le transducteur Tonpilz est intégré dans un boîtier 8 (non représenté sur la figure 1 ) rempli d'huile 10 afin d'assurer l'équilibre en pression avec le milieu d'immersion dans lequel est plongé le transducteur. Générallement, la contremasse 5 est montée en force dans le boîtier 8. Les lobes secondaires ou lobes de boîtier (cf Figure 2) sont un inconvénient connu depuis de nombreuses années dans les transducteurs et en particulier les transducteurs de type Tonpilz.The Tonpilz transducer is integrated in a housing 8 (not shown in FIG. 1) filled with oil 10 in order to ensure the pressure balance with the immersion medium into which the transducer is immersed. Generally, the counterweight 5 is mounted in force in the housing 8. The side lobes or housing lobes (see Figure 2) are a disadvantage known for many years in the transducers and in particular Tonpilz type transducers.
Les inventeurs ont analysé le comportement d'un tel transducteur. Selon cette analyse, la génération de ces lobes secondaires dits « de boîtier » est due à un couplage entre les éléments du transducteur (pavillon et contremasse), le fluide dans lequel baigne le résonateur et le boîtier. Ce couplage se traduit par la génération de quatre ondes de cisaillement à partir de deux sources 16 et 16' au sein du boîtier 8, chacune des sources 16, 16' générant deux ondes de cisaillement dans des directions opposées. L'origine des lobes secondaires est un couplage lié à une conversion mode d'une onde de cisaillement se propageant dans le boîtier. Un premier couplage acoustique a lieu entre le fluide 9 et le boîtier 8. Ce couplage génère une première source 16 d'ondes de cisaillement, schématiquement représentée au niveau du pavillon dans le boîtier. De manière inattendue le couplage ne se produit pas uniquement à l'interface entre le milieu fluide et le boîtier mais un second couplage mécanique se situe au niveau de la contremasse. Selon les applications et les type de montage la contremasse n'est pas nécessairement un nœud de vibrations parfaitement immobile, mais subit des déplacements transverses à l'axe. Ces déplacements induisent des ondes de cisaillement à partir d'un foyer secondaire 16' représenté schématiquement sur la figure 3 dans le boîtier en regard de la contremasse. La combinaison d'ondes de couplage provenant des deux foyers 16 et 16' produit en outre des ondes interférentes.The inventors have analyzed the behavior of such a transducer. According to this analysis, the generation of these so-called "housing" side lobes is due to a coupling between the elements of the transducer (horn and counter), the fluid in which the resonator and the casing are immersed. This coupling results in the generation of four shear waves from two sources 16 and 16 'within the housing 8, each of the sources 16, 16' generating two shear waves in opposite directions. The origin of the secondary lobes is a coupling linked to a mode conversion of a shear wave propagating in the case. A first coupling acoustic coupling takes place between the fluid 9 and the housing 8. This coupling generates a first source 16 of shear waves, schematically represented at the flag in the housing. Unexpectedly coupling does not occur only at the interface between the fluid medium and the housing but a second mechanical coupling is at the counterweight. Depending on the applications and type of mounting the countermass is not necessarily a perfectly immobile vibration node, but undergoes displacements transverse to the axis. These displacements induce shear waves from a secondary focus 16 'shown schematically in Figure 3 in the housing opposite the counterweight. The combination of coupling waves from the two foci 16 and 16 'further produces interfering waves.
Ces couplages se traduisent par la génération de quatre ondes de cisaillement au sein du boîtier représentées schématiquement sur la figure 3. Par conversion de mode, transformation de l'onde S en onde P, et après avoir interféré ces ondes se propagent sous forme d'onde de compression dans le milieu fluide et forment des lobes secondaires dits de boîtier.These couplings result in the generation of four shear waves within the housing shown diagrammatically in FIG. 3. By mode conversion, transformation of the S wave into a P wave, and after having interfered with these waves propagate in the form of compression wave in the fluid medium and form so-called secondary lobes of housing.
L'invention propose différents moyens complémentaires pour piéger l'énergie des lobes secondaires. La figure 4 représente schématiquement une portion de boîtier vue en coupe comprenant différents moyens d'atténuation des ondes acoustiques. Ces moyens sont avantageusement disposés sur les flancs du boîtier qui s'étendent longitudinalement par rapport à l'axe acoustique 7 d'émission/réception du transducteur, de manière à atténuer les ondes acoustiques se propageant dans des directions sensiblement transverses (90 ± 40 degrés) à l'axe acoustique 7. Les moyens d'atténuation peuvent être placés sur un ou plusieurs flancs autour de l'axe, ou bien former une gaine continue qui entoure la périphérie du boîtier autour de l'axe acoustique.The invention proposes various complementary means for trapping the energy of the side lobes. FIG. 4 schematically represents a portion of a box seen in section, comprising various means for attenuating the acoustic waves. These means are advantageously arranged on the sides of the housing which extend longitudinally with respect to the acoustic axis 7 of transmission / reception of the transducer, so as to attenuate the acoustic waves propagating in substantially transverse directions (90 ± 40 degrees ) the acoustic axis 7. The attenuation means may be placed on one or more sidewalls around the axis, or form a continuous sheath surrounding the periphery of the housing around the acoustic axis.
Plus précisément, un premier moyen consiste à augmenter l'épaisseur du boîtier pour que celle-ci soit supérieure à la longueur d'onde acoustique λ correspondant à la fréquence f dans le boîtier. De préférence, l'épaisseur du boîtier est égale à environ 2λ ou 3λ. Une telle épaisseur de boîtier permet de convertir l'onde de cisaillement en onde de compression. Par exemple pour un transducteur Tonpilz dont la fréquence est de 100 kHz, une enveloppe d'épaisseur 2,5-3 cm est bien adaptée. Pour un Tonpilz de fréquence inférieure, l'épaisseur adaptée sera proportionnelle à la fréquence.More specifically, a first means consists in increasing the thickness of the housing so that it is greater than the acoustic wavelength λ corresponding to the frequency f in the housing. Preferably, the thickness of the housing is equal to about 2λ or 3λ. Such a housing thickness makes it possible to convert the shear wave into a compression wave. For example for a Tonpilz transducer whose frequency is 100 kHz, a 2.5-3 cm thickness envelope is well suited. For a Tonpilz of lower frequency, the adapted thickness will be proportional to the frequency.
De préférence, l'épaisseur du boîtier est uniforme sur toutes les faces du boîtier s'étendant longitudinalement par rapport à l'axe. Avantageusement, la face arrière du boîtier a également une épaisseur supérieure à λ, de manière à atténuer le lobe arrière 13 dans la direction X' opposée à la direction X d'émission/réception acoustique.Preferably, the thickness of the housing is uniform on all the faces of the housing extending longitudinally with respect to the axis. Advantageously, the rear face of the housing also has a thickness greater than λ, so as to attenuate the rear lobe 13 in the X 'direction opposite to the acoustic transmission / reception X direction.
Une épaisseur de boîtier supérieure à λ, voire égale à 2λ ou 3λ peut être obtenue en fabriquant directement un boîtier ayant une telle épaisseur. Pour les dispositifs comprenant déjà un boîtier d'épaisseur initiale insuffisante, on peut disposer un second boîtier dont la forme intérieure est adaptée à la forme extrieure du boîtier initial de manière à ce que l'épaisseur totale du boîtier ainsi obtenu ait une épaisseur totale supérieure à λ.A case thickness greater than λ, or even equal to 2λ or 3λ can be obtained by directly manufacturing a case having such a thickness. For devices already comprising a housing of insufficient initial thickness, it is possible to arrange a second housing whose inner shape is adapted to the outer shape of the initial housing so that the total thickness of the housing thus obtained has a total thickness greater than λ.
Un second moyen consiste à disposer autour du boîtier 8 une gaine absorbante 17 de manière à absorber l'énergie des ondes de cisaillement converties en ondes de compression. Pour une conversion de mode il faut que la gaine absorbante soit réalisée dans un matériau plus souple que le boîtier, par exemple une résine polymère.A second means consists in arranging around the casing 8 an absorbent sheath 17 so as to absorb the energy of the shear waves converted into compression waves. For a mode conversion it is necessary that the absorbent sheath is made of a softer material than the housing, for example a polymer resin.
On peut aussi placer au-dessus de la structure absorbante une couche de mousse afin d'imposer un second trajet dans la structure et ainsi doubler l'atténuation. Un troisième moyen consiste à placer en surface de la gaine absorbante un réseau diffractant 19. Le réseau 19 peut être un réseau à une dimension avec un pas et une profondeur de l'ordre de la demi-longueur d'onde. Le réseau 19 peut aussi être à deux dimensions.Above the absorbent structure can also be placed a layer of foam to impose a second path in the structure and thus double the attenuation. A third means consists in placing a diffractive grating 19 on the surface of the absorbent sheath. The grating 19 may be a one-dimensional grating with a pitch and a depth of the order of one half-wavelength. The network 19 can also be two-dimensional.
Un quatrième moyen consiste à placer une gaine réfléchissante 18 autour de la gaine absorbante et du réseau diffractant de façon à augmenter la marche des ondes de cisaillement converties en ondes de compression dans le milieu absorbant. La gaine réfléchissante 18 peut comprendre par exemple une enveloppe réfléchissante constituée d'un matériau ayant un fort contraste d'impédance avec la gaine absorbante.A fourth means consists in placing a reflective sheath 18 around the absorbent sheath and the diffractive grating so as to increase the shear waves that are converted into compression waves in the absorbing medium. The reflective sheath 18 may comprise for example a reflective envelope made of a material having a high impedance contrast with the absorbent sheath.
Une forte rupture d'impédance est nécessaire pour le matériau réfléchissant qui peut par exemple être un métal. Cette structure nécessite enfin des moyens de suspension du matériau réfléchissant, de manière à isoler ce matériau et éviter la transmission par couplage vibratoire dans la direction non désirée. Les moyens de suspension comprennent avantageusement un polymère viscoélastique.A strong impedance break is necessary for the reflective material which may for example be a metal. This structure finally requires suspension means of the reflective material, so as to isolate this material and avoid transmission by vibratory coupling in the undesired direction. The suspension means advantageously comprise a viscoelastic polymer.
De préférence, la surface de la couche réfléchissante 18 est de forme concave vue des sources 16 et 16'.Preferably, the surface of the reflective layer 18 is of concave shape seen from the sources 16 and 16 '.
L'ordre dans lequel les moyens d'atténuation des lobes secondaires sont assemblés en partant de l'axe du transducteur vers l'extérieur du boîtier est important et est de préférence l'ordre indiqué ci-dessus.The order in which the side lobe attenuation means are assembled from the axis of the transducer to the outside of the housing is important and is preferably the order indicated above.
De manière analogue, pour réduire le lobe arrière, on peut placer des moyens d'atténuation sur la face arrière du boîtier.Similarly, to reduce the back lobe, attenuation means may be placed on the rear face of the housing.
Les différents moyens techniques mis en œuvre ont un effet additif pour améliorer la directivité du transducteur et réduire les lobes secondaires. La figure 5 représente une simulation du diagramme de directivité du même transducteur Tonpilz que celui de la figure 2, mais muni des moyens décrits ci-dessus, et plus précisément de tous les moyens cumulés à l'exception de la gaine réfléchissante. On observe sur la figure 5 une très forte réduction des lobes secondaires, qui ont quasiment disparu. Le lobe arrière 14 est également réduit. La directivité du transducteur est ainsi considérablement améliorée. Le dispositif de l'invention permet ainsi d'améliorer la directivité et la sensibilité d'un transducteur électro-acoustique.The various technical means used have an additive effect to improve the directivity of the transducer and reduce the sidelobes. FIG. 5 represents a simulation of the directivity diagram of the same Tonpilz transducer as that of FIG. 2, but provided with the means described above, and more precisely of all the accumulated means with the exception of the reflecting sheath. We see in Figure 5 a very strong reduction of side lobes, which have almost disappeared. The rear lobe 14 is also reduced. The directivity of the transducer is thus considerably improved. The device of the invention thus makes it possible to improve the directivity and the sensitivity of an electro-acoustic transducer.
L'invention peut s'adapter à tout type de sonars moyennant une légère modification de l'enveloppe externe du transducteur. L'invention s'applique notamment aux transducteurs de type Janus-Helmholtz, tel que représenté schématiquement en coupe figure 6. Le transducteur Janus- Helmholtz comprend deux moteurs piezo-acoustiques respectivement 1 et 21 alignés selon un même axe 7 et fixés sur une contermasse centrale 5. Chaque moteur piezo- acoustique 1 , 21 est relié à un pavillon 4, 24 par une tige de précontrainte. Les deux pavillons 4, 24 sont ainsi situés aux extrémités opposées sur l'axe 7 du dispositif. Un boîtier 8, respectivement 28 entoure chaque sous-ensemble moteur-pavillon 1 et 4, respectivement 21 et 24. La contremasse est fixée par une plaque métallique d'une part au boîtier 8 et d'autre part au boîtier 28. La cavité intérieure de chaque boîtier 8, 28 est remplie d'une fluide. De manière analogue à l'invention décrite plus haut en lien avec un transducteur Tonpilz, on peut modifier les boîtiers 8 et 28 pour qu'ils comprennent des moyens d'atténuation des ondes acoustiques émises et/ou reçues dans des directions transverses à l'axe acoustique 7. On peut appliquer un ou plusieurs moyens d'atténuation des ondes dans une direction transverse au boîtier de chacun des deux résonateurs coaxiaux. Le premier moyen consiste à utiliser des boîtiers 8 et 28 d'épaisseur supérieure à λ, et préférentiellement égale à 2λ ou 3λ. Un second moyen consiste à fixer une gaine absorbante sur une paroi du boîtier s'étendant longitudinalement suivant l'axe 7. Un troisième moyen consiste à placer en surface de la gaine absorbante un réseau diffractant. Un quatrième moyen consiste à placer une gaine réfléchissante autour de la gaine absorbante et du réseau diffractant de façon à augmenter la marche des ondes de cisaillement converties en ondes de compression dans le milieu absorbant.The invention can adapt to any type of sonars with a slight modification of the outer envelope of the transducer. The invention applies in particular to Janus-Helmholtz type transducers, as shown diagrammatically in section FIG. 6. The Janus-Helmholtz transducer comprises two piezo-acoustic motors respectively 1 and 21 aligned along the same axis 7 and fixed on a contermass central 5. Each piezoelectric motor 1, 21 is connected to a roof 4, 24 by a prestressing rod. The two flags 4, 24 are thus located at opposite ends on the axis 7 of the device. A housing 8, respectively 28 surrounds each motor-horn subassembly 1 and 4, respectively 21 and 24. The counterweight is fixed by a metal plate on the one hand to the housing 8 and on the other hand to the housing 28. The inner cavity each housing 8, 28 is filled with a fluid. In a manner analogous to the invention described above in connection with a Tonpilz transducer, the housings 8 and 28 can be modified so that they comprise means of attenuation of acoustic waves emitted and / or received in directions transverse to the acoustic axis 7. One or more wave attenuation means may be applied in a direction transverse to the housing of each of the two coaxial resonators. The first means consists in using casings 8 and 28 with a thickness greater than λ, and preferably equal to 2λ or 3λ. A second means consists in fixing an absorbent sheath on a wall of the housing extending longitudinally along the axis 7. A third means consists in placing on the surface of the absorbent sheath a diffractive grating. A fourth means consists in placing a reflective sheath around the absorbent sheath and the diffracting grating so as to increase the shear waves that are converted into compression waves in the absorbing medium.
Le transducteur Janus-Helmholtz pourvu de ces moyens d'atténuation des ondes acoustiques transverses à l'axe acoustique 7 présente une directivité améliorée. L'invention trouvera une application particulièrement avantageuse dans les antennes sonar. La figure 7 représente schématiquement une antenne sonar vue de face. L'antenne comprend une pluralité de transducteurs. Sur l'exemple de la figure 7 quatre pavillons de transducteurs de type Tonpilz sont alignés dans un même boîtier 8. La figure 7 représente une gaine absorbante disposée sur un des côtés du sonar. Des portions de gaine absorbante peuvent être disposées sur les autres côtés du boîtier qui s'étendent longitudinalement suivant l'axe 7 des pavillons 4 des transducteurs. La gaine absorbante est placée sur une paroi du boîtier dont l'épaisseur est supérieure à λ dans une direction d'émission des lobes secondaires. Comme indiqué en dessous du sonar sur une vue en coupe agrandie, la gaine absorbante 17 coopère avantageusement avec un milieu réfléchissant 18, et un réseau diffractant 18. Les moyens d'absorption peuvent comprendre des éléments séparés sur des côtés externes du boitier, ou une gaine continue sur la périphérie du boîtier dans un plan perpendiculaire à l'axe acoustique.The Janus-Helmholtz transducer provided with these acoustic wave attenuation means transverse to the acoustic axis 7 has an improved directivity. The invention will find a particularly advantageous application in sonar antennas. Figure 7 schematically shows a sonar antenna seen from the front. The antenna comprises a plurality of transducers. In the example of FIG. 7, four flags of Tonpilz transducers are aligned in the same casing 8. FIG. 7 represents an absorbent sheath disposed on one of the sides of the sonar. Absorbent sheath portions may be disposed on the other sides of the housing which extend longitudinally along the axis 7 of the flags 4 of the transducers. The absorbent sheath is placed on a wall of the housing whose thickness is greater than λ in a direction of emission of the side lobes. As indicated below the sonar in an enlarged sectional view, the absorbent sheath 17 advantageously cooperates with a reflecting medium 18, and a diffractive grating 18. The absorption means may comprise separate elements on external sides of the housing, or a continuous sheath on the periphery of the housing in a plane perpendicular to the acoustic axis.
L'invention permet ainsi de supprimer les lobes secondaires d'une antenne sonar formée d'un ensemble de transducteurs ayant sensiblement le même axe acoustique. L'invention permet d'améliorer considérablement la directivité d'une telle antenne sonar ainsi que sa réjection arrière.The invention thus makes it possible to eliminate the secondary lobes of a sonar antenna formed of a set of transducers having substantially the same acoustic axis. The invention makes it possible to considerably improve the directivity of such a sonar antenna as well as its rear rejection.
L'invention s'applique également aux transducteurs piezo-électriques de technologie dite « sciée » ou type céramique collée utilisées dans les sondes échographiques médicales ou lame quart d'onde (« Diagnostic Ultrasound Imaging », éd. Elsevier , Thomas L. Szabo). The invention also applies to piezo-electric transducers of "sawed" or glue-type technology used in medical ultrasound or quarter-wave plate probes ("Diagnostic Ultrasound Imaging", Elsevier, Thomas L. Szabo). .

Claims

REVENDICATIONS
1 . Transducteur d'ondes acoustiques comprenant :1. Transducer of acoustic waves comprising:
- au moins un moteur électro-acoustique (1 , 21 ), - un pavillon (4, 24) ayant une paroi interne et une paroi externe,at least one electro-acoustic motor (1, 21), a horn (4, 24) having an inner wall and an outer wall,
- une contremasse (S)5 et- a counterweight (S) 5 and
- un boîtier creux (8, 28) ayant une paroi interne et une paroi externe et au moins une ouverture acoustique,a hollow casing (8, 28) having an inner wall and an outer wall and at least one acoustic opening,
- ledit moteur (1 , 21 ) étant relié d'une part au pavillon (4, 24) et d'autre part à la concernasse (5) suivant un axe (7), ledit moteur (1 , 21 ) étant apte à exciter le pavillon (4, 24) autour d'au moins une fréquence de résonance acoustique f,- said motor (1, 21) being connected on the one hand to the horn (4, 24) and on the other hand to the concernasse (5) along an axis (7), said motor (1, 21) being able to excite the horn (4, 24) around at least one acoustic resonance frequency f,
- ledit boîtier (8, 28) étant relié à la contremasse (5) et entourant le moteur (1 , 21 ) et le pavillon (4, 24), la paroi externe du pavillon étant placée en face d'une ouverture acoustique du boîtier (8, 28), et l'espace entre la paroi interne du boîtier (8, 28) et la paroi interne du pavillon formant une cavité (9) comprenant un fluide (10), caractérisé en ce que ledit transducteur comprend des moyens acoustiques d'atténuation solidaires d'une paroi externe du boîtier (8, 28) pour atténuer les ondes acoustiques en émission et/ou en réception à ia fréquence f dans au moins une direction transverse à l'axe (7).- said housing (8, 28) being connected to the counterweight (5) and surrounding the motor (1, 21) and the horn (4, 24), the outer wall of the horn being placed opposite an acoustic opening of the housing (8, 28), and the space between the inner wall of the housing (8, 28) and the inner wall of the roof forming a cavity (9) comprising a fluid (10), characterized in that said transducer comprises acoustic means of integral attenuation of an outer wall of the housing (8, 28) for attenuating acoustic waves in transmission and / or reception frequency f ia in at least one direction transverse to the axis (7).
2. Transducteur selon la revendication 1 caractérisé en ce que le boîtier a une paroi s'étendant longitudinalement selon l'axe (7) et d'épaisseur E, ladite épaisseur E étant supérieure à la longueur d'onde acoustique λ correspondant à la fréquence f dans le boîtier de manière à absorber une partie des ondes acoustiques à la fréquence f dans au moins une direction transverse à l'axe (7).2. Transducer according to claim 1 characterized in that the housing has a wall extending longitudinally along the axis (7) and of thickness E, said thickness E being greater than the acoustic wavelength λ corresponding to the frequency f in the housing so as to absorb a portion of the acoustic waves at the frequency f in at least one direction transverse to the axis (7).
3. Transducteur selon la revendication 2 caractérisé en ce que lesdits moyens d'atténuation comprennent une gaine absorbante (17) fixée sur une paroi externe du boîtier (8, 28) et apte à absorber des ondes acoustiques à la fréquence f dans au moins une direction iransverse à l'axe (7). 3. Transducer according to claim 2 characterized in that said attenuation means comprise an absorbent sheath (17) fixed on an outer wall of the housing (8, 28) and able to absorb acoustic waves at the frequency f in at least one direction iransverse to the axis (7).
4. Transducteur selon la revendication 3 caractérisé en ce que lesdits moyens d'atténuation comprennent en outre un réseau diffractant (19) entourant la gaine absorbante (17), ledit réseau (19) étant apte à diffracter des ondes acoustiques dans la bande passante du transducteur et des moyens de suspension aptes à amortir le couplage d'ondes acoustiques entre le réseau diffractant (19) et la gaine absorbante (17). 4. Transducer according to claim 3 characterized in that said attenuation means further comprises a diffractive grating (19) surrounding the absorbent sheath (17), said array (19) being able to diffract acoustic waves in the bandwidth of the transducer and suspension means adapted to damp the coupling of acoustic waves between the diffractive grating (19) and the absorbent sheath (17).
5. Transducteur selon la revendication 4 caractérisé en ce que lesdits moyens de d'atténuation comprennent en outre une gaine réfléchissante (18) autour du réseau diffractant (19) et des moyens de suspension aptes à amortir le couplage d'ondes acoustiques entre la gaine réfléchissante (18) et la gaine absorbante (17). 5. Transducer according to claim 4 characterized in that said attenuation means further comprise a reflective sheath (18) around the diffractive grating (19) and suspension means capable of damping the coupling of acoustic waves between the sheath. reflective (18) and absorbent sheath (17).
6. Transducteur selon la revendication 5 caractérisé en ce que la gaine réfléchissante (18) est en aluminium, la gaine absorbante (17) est en résine polymère ou en mousse syntactique, et les moyens de suspension en polymère viscoélastique.6. Transducer according to claim 5 characterized in that the reflective sheath (18) is aluminum, the absorbent sheath (17) is polymeric resin or syntactic foam, and the viscoelastic polymer suspension means.
7. Transducteur selon l'une des revendications 5 à 6 caractérisé en ce que la gaine réfléchissante (18) est de forme extérieure bombée de manière à atténuer une partie des ondes acoustiques émises et/ou reçues dans des directions transverses à l'axe (7).7. Transducer according to one of claims 5 to 6 characterized in that the reflective sheath (18) is of convex outer shape so as to attenuate a part of the acoustic waves emitted and / or received in directions transverse to the axis ( 7).
8. Transducteur selon l'une des revendications 1 à 7 caractérisé en ce que le transducteur est un transducteur de type Tonpilz, comprenant un moteur (1 ) piézoélectrique de forme allongée, ledit moteur (1 ) comprenant un empilage de composants piezo-électriques et des électrodes (3), l'empilage étant relié selon un axe (7) de symétrie par une extrémité au pavillon (4) et par l'autre extrémité à la contremasse (5).8. Transducer according to one of claims 1 to 7 characterized in that the transducer is a Tonpilz-type transducer, comprising an elongated piezoelectric motor (1), said motor (1) comprising a stack of piezoelectric components and electrodes (3), the stack being connected along an axis (7) of symmetry by one end to the horn (4) and the other end to the counterweight (5).
9. Transducteur selon l'une des revendications 1 à 7 caractérisé en ce que le transducteur est un transducteur de type Janus-Helmholtz, comprenant deux moteurs (1 , 21 ) piezo-électriques de forme allongées dont les axes sont alignés, chaque moteur (1 , 21 ) comprenant un empilage de composants piezo-électriques et des électrodes, l'empilage étant relié selon un axe de symétrie par une extrémité à un pavillon (4, 24) et par l'autre extrémité à une contremasse (5) centrale commune aux deux moteurs (1 , 21 ), ledit transducteur comprenant deux boîtiers (8, 28) entourant chaque sous- ensemble moteur-pavillon.9. Transducer according to one of claims 1 to 7 characterized in that the transducer is a Janus-Helmholtz type transducer, comprising two elongated piezoelectric motors (1, 21) whose axes are aligned, each motor ( 1, 21) comprising a stack of piezoelectric components and electrodes, the stack being connected along an axis of symmetry at one end to a horn (4, 24) and at the other end to a central counterweight (5). common to both motors (1, 21), said transducer comprising two housings (8, 28) surrounding each engine-flag subassembly.
10. Antenne sonar comprenant une pluralité de transducteurs selon l'une des revendications 1 à 9, lesdits transducteurs étant placés dans un boîtier (8) commun. 10. Antenna sonar comprising a plurality of transducers according to one of claims 1 to 9, said transducers being placed in a housing (8) common.
EP09805787.0A 2008-12-23 2009-12-23 Acoustic wave transducer and sonar antenna with improved directivity Active EP2367640B1 (en)

Applications Claiming Priority (2)

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FR0859015A FR2940579B1 (en) 2008-12-23 2008-12-23 ACOUSTIC WAVE TRANSDUCER AND SONAR ANTENNA OF ENHANCED DIRECTIVITY.
PCT/FR2009/052682 WO2010072984A1 (en) 2008-12-23 2009-12-23 Acoustic wave transducer and sonar antenna with improved directivity

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EP2367640B1 EP2367640B1 (en) 2019-01-09

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WO2010072984A1 (en) 2010-07-01
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JP2012513718A (en) 2012-06-14
EP2367640B1 (en) 2019-01-09
US8780674B2 (en) 2014-07-15
FR2940579A1 (en) 2010-06-25
JP5504276B2 (en) 2014-05-28

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