JPH0344269B2 - - Google Patents

Description

BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to electroacoustic transducers used in sonar devices, and more particularly to electroacoustic transducers used in sonar devices, and more particularly to electroacoustic transducers for receiving multiple sonar beams using collimated acoustic lenses. The present invention relates to an electroacoustic transducer that can be used.

Description of the Prior Art Sonar devices are e.g. Jack Kritz published in 1966.
and Seymour D. Lerner, U.S. Patent No.
No. 3,257,638, Dotsupura Navigation System, uses a narrow beam of sound energy projected from a ship in certain desired directions and receives reflected energy from these directions. Conventionally these beams are propagated. or by vibrating a voltage disk having a large diameter compared to the wavelength of the sound waves to be received. If multiple beams are used, the converter assembly must be enlarged to accommodate the large number of necessary elements. Prior art multi-beam converters are difficult to install, especially on small vessels, and the large gate valves and strong required structural support members increase installation costs. Accordingly, there is a need for a relatively compact multi-beam conversion device that facilitates installation and reduces associated costs.

Prior application SN354973 filed on March 5, 1982 by the inventor of the present invention "Multiple Beam Lens Transducer For
Sonar Systems) describes a compact device that transmits and receives multiple sonar beams. The acoustic lens directs an incident plane wave in a desired direction onto an electroacoustic transducer disposed in a spherical shell segment centered on the focal region of the lens associated with the incident beam. The electroacoustic transducer transmits a spherical wave which is converted by an acoustic lens into a plane wave appearing in a desired direction.

The manufacture of this transducer involves a certain degree of difficulty and expense resulting from the need to fabricate piezoceramic crystal elements in the form of spherical shell segments.

SUMMARY OF THE INVENTION It is an object of the present invention to maintain a compact construction of a multi-beam lens transducer for a sonar device while eliminating the need for an electroacoustic transducer in the form of spherical shell segments.

A sonar conversion device embodying the principles of the invention includes means for converting an incident planar sound wave into a convergent sound wave at a focal region of its focal plane. Plane waves incident in different predetermined directions are focused on different focal regions. Sound waves emitted from the focal region are converted into plane sound waves emitted in these predetermined directions. Receiving and transmitting sound waves using an electroacoustic transducer having a flat surface. Means are provided for providing focused sound waves that are in phase in the plane of the transducer. The sound waves emitted by the plane of the converting device are converted in a predetermined direction as plane waves into a diverging beam emitted from the device according to the invention.

A preferred embodiment of the present invention includes a double concave acoustic lens that focuses plane waves incident from a plurality of predetermined directions onto a plurality of focal areas in corresponding relation to the direction of incidence. A silicone rubber medium is bonded to the inner surface of the lens. The low speed of sound in the rubber results in a short focal length and reduces the depth of the assembly. An acoustic lens having a spherical surface is positioned to collimate the focused sound waves to generate plane waves at the planes of the three piezoelectric ceramic crystal transducers. An epoxy alignment is positioned between the collimating lens and the plane of the transformer. The matching section converts the low acoustic impedance of the collimating lens to a higher value to impart advantageous electrical properties to the crystal's electrical terminals when measured at the crystal's electrical terminals. An aluminum backplate is positioned behind the converter. The backplate provides both structural strength and heat transfer to the crystal. The planes of the transducer are positioned to receive and transmit beams each tilted 15 degrees from the central axis of the double concave lens.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the invention will now be described with reference to the accompanying drawings.

The present invention relates to a method of constructing a multi-beam conversion device with one aperture in the form of an acoustic lens providing the required aperture-to-wavelength ratio. A sound wave diagram showing the focusing effect of an acoustic lens is shown in FIG. Parallel lines of the incident plane wave 10 propagating in the aqueous medium 11 impinge on the acoustic lens 12 . In order to focus the incident plane wave, the acoustic lens 12 has a double concave surface and is made of a medium in which the speed of sound is greater than that of the water 11 and the adjacent medium 13. The focusing effect is caused by the beam first bending away from the normal to the surface of the low index lens as it enters the lens, and then towards the normal as it exits the lens. The incident plane wave 10 is therefore focused onto a focal point 14 by the lens configured in this way. Conversely, 14 point sources illuminating the lens 12 with sound waves project plane waves as indicated by parallel lines 10. A property of a lens constructed in this manner is the unique correspondence between the direction of plane wave incidence and the associated focal point in the focal plane of the lens. Collimated beams entering from different directions are arranged to have different foci. For example, a plane wave indicated by parallel lines 15 is focused at a second focal point 16 . Thus, there are a number of such focal points at the focal plane 16a of the lens 12, each focal point defining a different beam direction for reception or projection of sound waves. It is therefore possible to transmit and receive a sound beam using a large number of miniature electroacoustic transducers placed at different focal points, such that the beam width is characterized by the lens diameter.

The main obstacles to realizing such a configuration are:
This means that small converters cannot operate at large power levels. The sound intensity (watts/unit area) in the medium 13 in the vicinity of the transducer is high due to the small transducer surface area, resulting in cavitation and collapse of the medium. Furthermore, the heat dissipation caused by converter losses is limited to a small converter surface and generates high temperatures when large power is applied. The present invention uses a large transducer with a much larger surface area placed in front of the focal point. An electroacoustic transducer 17 having a plane 18 for receiving and transmitting sound waves has a focal point 14 and a lens 12
is placed between. A lens 19 disposed between the electroacoustic transducer 17 and the lens 12 provides in-phase sound waves to the surface 18 of the electroacoustic transducer 17 . Lens 19 accomplishes this by refracting the converging lines and orienting them perpendicular to plane 18 of electroacoustic transducer 17. The material making up the lens 19 has a sound velocity greater than the sound velocity of the medium 13 and preferably has a specific acoustic impedance close to that of the medium 13 to minimize unwanted reflections. have. By means of the above, almost all the acoustic energy received by the lens 12 can be used to convert into electrical energy by the electroacoustic transducer 17. Conversely, when transmitting, the transducer 17 in cooperation with the lens 19 projects the acoustic beam as if the focal point 14 were the source. The advantage provided by this configuration is that small changes in focal spot position do not significantly change performance. This is because all sound waves are subsequently intercepted by the transducer 17 with only slight phase-shift interference. With a small transducer element placed directly at the focus, small changes in focus position can produce large changes in captured energy.
Another advantage is that the device does not have to extend behind the lens 12 into the medium 13 to the focal plane, reducing the overall depth of the device.

The curvature of the lens 19 necessary to provide in-phase sound waves to the plane 18 of the electroacoustic transducer 17 can be determined with reference to FIG. The sound wave rays 20 directed toward the focal point 14 impinge on the surface of the lens 19 . If there is no lens 19, the sound wave rays 20
is the focal point 1 through the medium 13 whose propagation speed is C _M
Proceed a distance R relative to 4. When lens 19 is present, acoustic ray 20 travels a distance X relative to the Y-axis 21 drawn through focal point 14 through the lens medium with a propagation velocity of C _L . For all sound wave rays refracted by lens 19 to arrive in phase at Y-axis 21, the propagation time _{t n} that would have been experienced by each sound wave number in medium 13 if the lens were not present. teeth,
The time it takes for the sound ray to traverse the lens material
It must be equal to t _L plus an additive constant K.

Therefore, t _n =R/C _n =t _L +K=X/C _L +K Therefore, (R/C _n ) ² = (X/C _L +K) ² = (1/C _L (X+C _L K)
^{) 2By the Pythagorean theorem R 2} _{= _ _ _} ^{_ _ _} _{_} C _L ) ² (X+C _L K) ² This is the well-known formula for a conic section with eccentricity equal to C _n /C _L and directrix equal to C _L K.
Since the material of lens 19 has a higher propagation velocity than medium 13, C _n /C _L is less than 1 and the curve is elliptical.

The elliptical shape of lens 19 can be approximated by a sphere whose radius is chosen to best fit over the region.

A representative design implementing the invention is shown in FIG. Diameter 17.14cm (6.75 inches), center thickness 0.95cm
(0.376 inches), with an inner radius of 18.2 cm (7.18 inches) and an outer radius of 60.5 cm (23.82 inches), a synthetic foam solid lens 25 is in contact with water on its outer surface and bonded to a silicone rubber medium 26 on its inner surface. There is. The configuration in the figure is the central axis 2 of each lens 25.
Gives three transmit or receive beams oriented 7 to 15 degrees apart underwater. The low speed of sound in the rubber results in a short focal length 28 of 10.91 inches, thus reducing the depth of the assembly. Diagonal 29 is 33 degrees.

Each of the three piezoceramic crystals, such as 30, has a beam receiving and transmitting plane, and each crystal has a diameter of 2.5 inches. Each crystal starts from the central axis 27 of the lens 25.
They are placed 10.5 degrees apart. Each crystal is 122K
It has a thickness such that it resonates at Hz, and is coupled to a metal support member 31. A collimating lens such as 32, constructed of the same synthetic foam material as lens 25, collimates the acoustic rays of the focused beam to the plane of the crystal. The ellipsoidal surface of each collimating lens is approximated by a spherical surface of radius 5.46 cm (2.15 inches) centered 4.7 cm (1.85 inches) in front of the focal point, such as 33. A plastic matching section, such as 34, preferably constructed of epoxy, is inserted between each crystal and its associated collimating lens. Each matching part has a diameter
6.35 cm (2.5 inches), quarter wavelength (in this example, quarter wavelength, 0.53 cm (0.21 inch))
has a thickness equal to an odd multiple of . The matching section provides advantageous electrical properties when measured at the electrical terminals of the crystal by converting it to a high value to provide the crystal with a low acoustic impedance. The matching section creates an acoustic impedance match between the crystal and the collimating lens. Essentially, the alignment section has two purposes. That is, the engagement portion widens the bandwidth and increases the efficiency of the transducer (see "The Effect of Bucking and Matching on the Performance of Piezoelectric Ceramic Transducers").
Backing and Matching on the Performance
of Piezoelectric Ceramic Transducers)
George Kossoff, IEEETransactions on
Sonics and Ultrasonics, Volume SU−13, No.
1, see March 1966). The face of each crystal opposite the receiving face is coated with a metal, preferably 6.35 cm (25 inches) in diameter and having a thickness that is an integer multiple of a half wavelength (in this case 2.59 cm (1.02 inches)). A back plate such as No. 35 is provided, which is made of aluminum. The backplate provides both structural strength and heat transfer to the crystal and is essentially transparent at operating frequencies. Transparency, a negligible effect on wave transmission, follows the standard sound transmission coefficient equation for waves crossing two boundaries (e.g. Fundamentals
of Acoustics), pp. 149-153, Kinsler, Frey,
(see Wiley, 1950). If only heat conduction from the back plate is desired, the plate may be made thinner.

The backplate may alternatively be positioned in contact with the receiving and transmitting surfaces of the crystal. A matching section may then be used between the back plate and the collimating lens to provide acoustic impedance matching between the plate and the lens.

Since the collimating lens is configured such that the sound rays traversing the medium are in phase with the Y-axis 21 (see Figure 2), the sound rays are necessarily in phase with the Y-axis 21 in the medium.
is in phase with any line parallel to . Therefore, the sound wave rays emerging immediately from the plane of the lens are in phase and remain so as they pass through a medium of uniform thickness on their way to the plane of the crystal.

Although preferred embodiments of the invention have been described, the language used is intended to be illustrative and not limiting, and it is understood that changes may be made within the scope of the claims without departing from the true scope and spirit of the invention in broad terms. I hope you understand that it is possible.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a double concave acoustic lens, an electroacoustic transducer having a flat surface, and a collimated acoustic lens disposed between them, with a sound wave diagram superimposed to show the action of the lens. FIG. 2 is a schematic illustration of a sound wave impinging on a collimated acoustic lens used to calculate the curvature of the lens, and FIG. 3 is a cross-sectional view of a preferred embodiment of the invention. In the figure, 10...incident plane wave, 11...water, 12
...acoustic lens, 14, 16...focal point, 17...
Electroacoustic transducer, 18...plane, 19...lens.

Claims (1)

[Claims] 1. In a multi-beam lens conversion device equipped with a collimation device for a sonar device, an incident plane sound wave is converted into a sound wave that converges in the focal region of its focal plane, so that plane waves incident in different predetermined directions are lens means having a central axis for converging on different focal areas and converting sound waves emitted from said focal areas into planar sound waves emitted from said lens means in said predetermined direction; and a flat surface for receiving and transmitting sound waves. a plurality of electroacoustic transducers; applying sound wave rays of the same phase to the plane of the electroacoustic transducer; the sonic rays having a focusing action of the lens means on the plane waves incident on the lens means in the predetermined direction; a sound wave emitted by the plane of the electroacoustic transducer, which is converted by the lens means into the plane sound wave emitted in the predetermined direction; and in-phase means for converting into. 2. In the device according to claim 1, the lens means includes a double concave acoustic lens made of a material having an acoustic propagation velocity higher than that of water, and a combination of the lens and the in-phase means. an acoustic propagation medium having an acoustic propagation velocity less than the acoustic propagation velocity of the lens material positioned therebetween. 3. The device according to claim 2, wherein the in-phase means substantially focuses the sound wave rays generated by the focusing action of the lens means on the plane waves incident on the lens means in the predetermined direction. 3. A plurality of collimated acoustic lenses collimated to a plane of the electroacoustic transducer, the substantially collimated acoustic ray being substantially perpendicular to the plane of the electroacoustic transducer. 4. The apparatus of claim 3, wherein the in-phase means is further positioned between the electro-acoustic transducer and the collimating acoustic lens so as to The device as described above, characterized in that it comprises matching means for providing acoustic impedance matching at. 5. The apparatus of claim 4, further comprising a back plate positioned adjacent a face of the transducer opposite the plane to transmit acoustic signals, transfer heat and provide structural strength. The above device characterized in that it comprises means. 6. The apparatus of claim 3, wherein said in-phase means is further positioned between said electro-acoustic transducer and said collimated acoustic lens to transmit acoustic signals, transfer heat and improve structural strength. and matching means positioned between the window means and the collimating acoustic lens to provide acoustic impedance matching between the window means and the collimating acoustic lens. The above device. 7. In the device according to claim 3, the collimating acoustic lens has an elliptical surface facing the double concave acoustic lens and a plane facing and parallel to the plane of the electroacoustic transducer. The above-mentioned device is characterized by comprising: 8. In the device according to claim 3, the collimating acoustic lens includes a spherical surface facing the double concave acoustic lens, and a flat surface facing and parallel to the flat surface of the electroacoustic transducer. The above-mentioned device is characterized in that: 9. In the device according to claim 4, the collimating acoustic lens has an elliptical surface facing the double concave acoustic lens and a plane facing and parallel to the plane of the electroacoustic transducer. The above-mentioned device is characterized by comprising: 10 In the device according to claim 4, the collimating acoustic lens includes a spherical surface facing the double concave acoustic lens, and a flat surface facing and parallel to the flat surface of the electroacoustic transducer. The above-mentioned device is characterized in that: 11. In the device according to claim 7 or 4, the electroacoustic transducer has a cylindrical shape, and the matching means converts 1/1/2 of the incident sound wave.
The device described above has a cylindrical shape having a thickness that is an odd multiple of four wavelengths. 12. The device according to claim 5, wherein the back plate means comprises a cylindrical body having a thickness that is an integral multiple of a half wavelength of the incident sound wave. 13. In the device according to claim 1, each of the electroacoustic transducers is a piezoelectric ceramic crystal, and each crystal is inclined by 10.5 degrees from the central axis of the double concave acoustic lens. The above device is characterized by: