Acoustic Transducer

Abstract

A hydrophone is provided by a piezoelectric shear element fastened between two masses in various configurations. Sound waves impinge on and accelerate one mass. The acceleration of the one mass and the inertia of the other mass set up shearing forces in the piezoelectric shear element. The piezoelectric shear element produces a voltage proportional to the shearing forces.

Description

BACKGROUND OF THE INVENTION

My invention relates to an improved electroacoustic transducer, and particularly to such a transducer for use in liquids.

Electroacoustic transducers are used in liquids for converting applied electrical energy or signals into acoustical energy or sound for transmission through the liquid, and for converting acoustical energy or sound in the liquid into electrical energy or signals for use in an electrical circuit. When operated in the latter mode, such transducers are commonly referred to as hydrophones. Hydrophones are extensively used in water for detecting acoustical energy which may be relatively weak and which may have frequencies covering a relatively wide band. Hydrophones are also used to detect the direction of the source of acoustical energy with respect to some reference direction.

Because of the many uses and applications of hydrophones, there is always a need and desire for better hydrophones, especially those which have directional characteristics. Directional information may be obtained by utilizing two or more normally nondirectional hydrophones or transducers in spaced arrays. Other hydrophones, such as the velocity ribbon type, have directional characteristics because of the nature of their construction. The spaced array uses pressure sensitive hydrophones which are spaced so that the pressure gradient of the acoustic signal and its direction relative to the source may be determined. Generally, four such pressure sensitive hydrophones are positioned in a circular array 90.degree. apart. The two pressure sensitive hydrophones along one axis provide a voltage indicative of the pressure gradient along the one axis, and the two pressure sensitive hydrophones along the other axis provide a voltage indicative of the pressure gradient along the other axis. This spacing technique can be used with many types of hydrophones; however, at relatively low acoustic frequencies, the pressure gradients are relatively small so that the hydrophones must be spaced relatively far apart in order to provide a usable sensitivity. The velocity ribbon type of hydrophone utilizes a metallic strip which is positioned in a magnetic field. An impinging acoustic wave causes movement of the strip with the result that it produces a voltage in the magnetic field. Two such ribbons may be oriented at 90.degree. with respect to each other so as to provide a directional hydrophone. However, this type of hydrophone is relatively expensive and is not as rugged and reliable as may be required in certain applications.

Accordingly, an object of my invention is to provide an improved sound transducer particularly for use in water.

Another object of my invention is to provide an improved acoustic transducer or hydrophone for producing electrical signals in response to sound energy that covers a relatively wide band of frequencies, the electrical signals having characteristics indicative of the characteristics of the sound energy.

Another object of my invention is to provide an improved acoustic transducer that is relatively rugged and strong, and that is capable of functioning under various marine and environmental conditions.

Another object of my invention is to provide an improved hydrophone which has directional characteristics, and which is easily packaged and deployed.

Another object of my invention is to provide a transducer device which is not only easily fabricated, but which also has directional characteristics that are not dependent upon the operational frequency or upon the matching of the sensitivity of separate transducers as in spaced directional arrays.

Some of the prior art electroacoustic transducers or hydrophones are sensitive to rotational velocities. Such velocities may cause erroneous output signals to be produced, or may distort or ruin the desired output signal so that a useless or erroneous signal is provided.

Accordingly, another object of my invention is to provide an improved hydrophone which is relatively insensitive to rotational velocities about its center of mass.

SUMMARY OF THE INVENTION

Briefly, these and other objects are achieved in accordance with my invention by a hydrophone having at least one piezoelectric shear type element fastened to two masses. One of the masses is arranged so that it is exposed to sound energy which is to be detected. This sound energy strikes or impinges on the one mass and accelerates it so that a shearing force is set up in the piezoelectric shear element because of the inertia of the other mass. This shearing force causes the piezoelectric shear element to create a voltage indicative of the shearing force. This voltage may be derived by electrical connections to the usual piezoelectric surface electrodes or to the two masses, where the masses act as such electrodes or are electrically connected to such electrodes. In a preferred embodiment, the one mass is a hollow cylinder that is closed at its ends by a top and a bottom. The other mass is a solid cylinder positioned coaxially within the hollow cylinder. A first piezoelectric shear element is fastened between one end of the solid cylinder and the top, and a second piezoelectric shear element is fastened between the other end of the solid cylinder and the bottom. The poling direction or axis of greatest sensitivity of the first piezoelectric shear element is preferably positioned at 90.degree. with respect to the poling direction or axis of greatest sensitivity of the second piezoelectric shear element so that the hydrophone can respond to signals from all directions. Two or more piezoelectric shear elements may be fastened between one end of the solid cylinder and the top, and two or more piezoelectric shear elements may be fastened between the other end of the solid cylinder and the bottom. The piezoelectric shear elements may be connected so that the hydrophone does not produce signals in response to rotational motion about its center of mass, but does produce signals in response to impinging sound waves.

BRIEF DESCRIPTION OF THE DRAWING

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the claims. The structure and operation of my invention, together with further objects and advantages, may be better understood from the following description given in connection with the accompanying drawing, in which:

FIG. 1 shows a side elevational view of a simplified embodiment of my invention;

FIG. 2 shows a top plan view of the simplified embodiment of FIG. 1;

FIG. 3a shows a longitudinal cross-sectional view of a preferred embodiment of a hydrophone constructed in accordance with my invention;

FIG. 3b shows a simplified equivalent electrical circuit of the hydrophone of FIG. 3a;

FIG. 4a shows a longitudinal cross-sectional view of another preferred embodiment of a hydrophone constructed in accordance with my invention;

FIG. 4b shows a simplified equivalent electrical circuit of the hydrophone of FIG. 4a;

FIG. 5 shows a typical output response curve with respect to frequency of the hydrophone of FIGS. 4a and 4b;

FIG. 6a shows an exploded perspective view of a hydrophone constructed in accordance with my invention that is relatively insensitive to rotational motion;

FIG. 6b shows a simplified equivalent electrical circuit of the hydrophone of FIG. 6a;

FIG. 7a shows an exaggerated cross-sectional view of the hydrophone of FIG. 6a when subjected to rotational motion about its center of mass;

FIG. 7b shows a simplified equivalent electrical circuit of a portion of the hydrophone under the conditions shown in FIG. 7a;

FIG. 8a shows an exaggerated cross-sectional view of the hydrophone of FIG. 6a when subjected to an impinging sound wave; and

FIG. 8b shows a simplified equivalent electrical circuit of a portion of the hydrophone of FIG. 8a.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 show side elevation and top plan views of a simplified representation of a hydrophone constructed in accordance with my invention. Although other configurations of the piezoelectric element may be used, my hydrophone as shown in FIG. 1 comprises a piezoelectric shear type element 10 in the configuration of a plate or layer which is connected or fastened to a mass M.sub.1 and a mass M.sub.2. The piezoelectric element 10 may be of any suitable piezoelectric material having the desired shear properties. Examples of such materials are: lead zirconate, lead titanate, sodium niobate, potassium niobate, or crystalline quartz. As known in the art, these materials, when properly polarized, produce voltages at their outer electrodes in response to shearing force set up in the material. The materials have a poling direction or a direction of greatest sensitivity as indicated by the arrow in FIG. 1 and in FIG. 2. This poling direction is the direction along which a given shearing force causes the piezoelectric element 10 to produce the greatest voltage at its electrodes. One such piezoelectric element 10 is commercially available from the Clevite Corporation of Bedford, Ohio, under the designation PTZ-5. With reference to FIG. 1, if a force F is applied to the mass M.sub.1 at an angle .theta. (Theta) with respect to the poling direction, the mass M.sub.1 and the mass M.sub.2 are accelerated in a direction parallel to the poling direction by a force F cos .theta.. The acceleration of the mass M.sub.1 and the inertia of the mass M.sub.2 introduces a shearing force M.sub.2 (F cos .theta.) /(M.sub.1 +M.sub.2) along the poling direction. With respect to FIG. 2, if the force F is at an angle .phi. (Phi) with respect to the poling direction, the mass M.sub.1 and the mass M.sub.2 are accelerated in a direction parallel to the poling direction by a force F cos .phi.. With respect to FIGS. 1 and 2, the acceleration of the mass M.sub.1 and the inertia of the mass M.sub.2 introduces a net shearing force M.sub.2 /(M.sub.1 + M.sub.2) (F cos .theta. cos .phi.) along the poling direction. Thus, the shear piezoelectric element 10 produces a voltage in response to a force F in almost all directions relative to the poling direction. The magnitude of this voltage is increased as the ratio M.sub.2 /(M.sub.1 +M.sub.2) is increased. Since the effective force parallel to the poling direction of the shear piezoelectric layer 10 varies as a function of the cosine of the angle of the applied force with respect to the poling direction, the output of the piezoelectric element 10 becomes very small as either the angle .theta. or the angle .phi. approach 90.degree.. Conversely, the output of the piezoelectric element 10 becomes its greatest when both the angle .theta. and the angle .phi. approach zero degrees.

The output voltage V produced by a piezoelectric element 10 may be determined by the following equation:

In this equation, g.sub.15 is the piezoelectric shear constant in

t is the thickness in centimeters of the shear element, M.sub.2 is the mass of the mass that is the last to be accelerated, F is the component of force acting on mass M.sub.1, .theta. and .phi. are the angles at which the force F is applied relative to the poling direction, A is the area of the shear element in square centimeters, N is the number of shear elements, and M.sub.1 is the mass of the mass receiving the applied force F (i.e., the first mass to be accelerated). Calculations for a typical piezoelectric element that is 1.3 centimeters long, 103 centimeters wide, and 0.845 centimeter thick indicate that such an element has a sensitivity of -120 db. (reference to 1 volt per microbar) for an applied force of 0.132 dyne. Such forces are typically produced by relatively weak underwater acoustic signals or waves. Hence, the output voltage of a piezoelectric shear element is practical for available acoustic forces, and the sensitivity of the element may be easily calculated.

FIG. 3a shows a longitudinal cross section of a hydrophone constructed in accordance with my invention and which uses the principles outlined in connection with FIGS. 1 and 2. In FIG. 3a, I provide an inner, solid metallic cylinder 12 which serves as the mass M.sub.2 or FIG. 1. This cylinder 12 is positioned concentrically along a longitudinal axis 14. At the upper end of the cylinder 12 I provide a piezoelectric shear element 16 which may have a left-right poling direction in the plane of FIG. 3a. For convenience, this is referred to as the north-south (N-S) direction. At the lower end of the cylinder 12 I provide a piezoelectric shear element 18 which may have a poling direction perpendicular to the plane of FIG. 3a. For convenience, this is referred to as the east-west (E-W) direction. These elements 16, 18 are fastened to the inner inertial mass 12 by any suitable electrically conductive securing means such as a conducting epoxy. A circular metallic upper plate 20 is fastened to the outer face of the element 16, and is positioned concentrically about the axis 14. A similar circular metallic lower plate 22 is fastened to the outer face of the element 18, and is positioned concentrically with respect to the axis 14. The upper and lower plates 20, 22 are electrically connected to the shear elements 16, 18 respectively by any suitable electrically conductive securing means such as a conducting epoxy. The hydrophone is closed by a hollow, metallic cylinder 24 which is positioned between and fastened to the upper and lower plates 20, 22 so that it is concentric with respect to the axis 14. The cylinder 24 is electrically insulated from the plates 20, 22 as indicated by the insulation layers 26, 27, which may be a nonconducting epoxy. The hollow cylinder 24 and the plates 20, 22 act or serve as the mass M.sub.1 of FIG. 1. The inner cylinder or mass 12 is electrically connected to the cylinder 24 by a connection 30 which preferably is a soft or pliable material that does not transmit mechanical vibrations between the inner cylinder 12 and the outer cylinder 24. A first terminal 32 is connected to the upper plate 20, a common or second terminal 34 is connected to the cylinder 24, and a third terminal 36 is connected to the lower plate 22. The piezoelectric elements 16, 18 have their respective poling directions lying in parallel planes but positioned at right angles relative to each other. Thus, the hydrophone shown in FIG. 3a is sensitive to signals or sound waves for 360.degree. around the axis 14, and is also sensitive to sound waves which reach the hydrophone at an angle less than 90.degree. with respect to the axis 14.

FIG. 3b shows a simplified equivalent electrical circuit of the hydrophone in FIG. 3a. The same reference numerals have been used for the circuit elements in FIG. 3b which correspond to the parts in FIG. 3a. In FIG. 3b, it will be seen that voltages or signals from the upper piezoelectric element 16 may be derived from the terminals 32, 34, and that voltages or signals from the lower piezoelectric element 18 may be derived from the terminals 34, 36. In FIG. 3b, the capacitors shown by the dotted lines between the terminals 32, 34 and between the terminals 34, 36 represent the capacitance resulting from the upper plate 20 with respect to the cylinder 24 and the cylinder 12 and their associated dielectrics; and likewise, the lower plate 22 with respect to the cylinder 24 and the cylinder 12 and their associated dielectrics. The hydrophone shown in FIG. 3a was constructed and tested. The inner mass or cylinder 12 was constructed of type metal having a diameter of 1.75 inches, and a height of 3.36 inches. The upper and lower plates 20, 22 were constructed of aluminum, having a diameter of 3.5 inches and a thickness of 0.0625 inch. The cylindrical shell or hollow cylinder 24 was also constructed of aluminum having an outside diameter of 3.5 inches, a wall thickness of 0.0625 inch, and a height of 3.5 inches. The cylinder 24 was joined to the plates 20, 22 by a nonconducting epoxy having a thickness of approximately 0.03125 inch at the layers 26, 27. The shear piezoelectric layers 16, 18 were Clevite PZT-5 having a surface dimension of 0.5 by 0.5 inch, and a thickness of approximately 0.1 inch. These layers 16, 18 were fastened to the inner mass or cylinder 12 and to the upper and lower plates 20, 22 respectively by a conducting type of epoxy. The outer surfaces of the assembled hydrophone were covered with a nonconducting epoxy for insulation. The completed hydrophone was tested up to approximately 6,000 cycles per second. At approximately 2,300 cycles per second, the hydrophone exhibited a self resonance.

FIG. 4a shows another hydrophone in longitudinal cross section. This hydrophone comprises an inner inertial mass or cylinder having two solid metallic cylinders 41, 42 joined by an insulating or nonconducting layer 43 and concentrically positioned on a longitudinal axis 40. A piezoelectric shear type element 45 is fastened to and electrically connected to the upper surface of the cylinder or mass 41, and a piezoelectric shear-type element 46 is fastened to and electrically connected to the lower surface of the cylinder or mass 42. As explained in connection with FIG. 3a, the upper layer 45 has a N-S poling direction in the plane of the paper of FIG. 4a, and the lower layer 46 has an E-W poling direction perpendicular to the plane of the paper. Upper and lower circular metallic plates 48, 50 are concentrically positioned on the axis 40, and are respectively fastened to and electrically connected to the piezoelectric elements 45, 46. The hydrophone is closed or surrounded by a concentric, hollow, cylindrical metallic cylinder 51 which is electrically connected to the plates 48, 50. Electrical signals are derived from a terminal 54 connected to the solid cylinder 41, a common terminal 55 connected to the hollow cylinder 51 or one of the plates 48, 50, and a terminal 56 electrically connected to the other solid cylinder 42. The outer surfaces of this hydrophone need not be insulated from the surrounding liquid transmission medium, but may be so insulated by a suitable material if desired. Acoustic signals which reach the outer cylinder or mass 51 cause shearing forces to be set up in the piezoelectric elements 45, 46, so that electrical signals are produced at the terminals 54, 55, 56. FIG. 4b shows a simplified equivalent electrical circuit of the hydrophone of FIG. 4a. The same reference numerals have been used for the circuit elements of FIG. 4b which correspond to the parts in FIG. 4a. In FIG. 4b, the capacitor indicated by dotted lines and connected between the two inner masses or cylinders 41, 42 represents that capacitance resulting from the area between the two masses 41, 42 and the associated dielectric of the insulating layer 43. The coupling or interaction between the N-S and E-W circuits which results from this capacitance may be reduced if a conducting layer is positioned midway between the outer boundaries of the insulating layer 43 and connected to the common terminal 55.

The hydrophone of FIG. 4a was constructed and tested. The inner cylinders or masses 41, 42 were constructed of type metal having a diameter of 1.75 inches. The cylinders 41, 42 were joined by the insulation layer 43 having a diameter of 1.75 inches and a thickness of approximately 0.0625 inch. The combined length of the joined cylinders 41, 42 was 3.0625 inches. The piezoelectric elements 45, 46 were made of Clevite PZT-5 having a surface area of 0.5 by 0.5 inch and a thickness of 0.25 inch, and were fastened by solder. The upper and lower plates 48, 50 were constructed of brass having a diameter of 4 inches and a thickness of 0.0625 inch. The outer cylinder or mass 51 was also constructed of brass and had an inside diameter of 4.0 inches, a wall thickness of 0.0625 inch, and a height of 3.8125 inches to allow for solder thickness. The cylinder 51 and plates 48, 50 were soldered together. This hydrophone was tested with the outer cylinder and end plates uninsulated from the surrounding liquid transmission medium. Its response with frequency is shown in FIG. 5. It will be noted that the hydrophone of FIG. 4a exhibits a resonance at approximately 2,300 cycles per second. However, the response of this hydrophone is useful and of good quality down to frequencies as low as 20 cycles per second. Generally, the frequency response is a linearly varying one. The directional response of the hydrophone of FIG. 4a, as well as the hydrophone of FIG. 3a, generally followed the cosine function mentioned earlier so that their directional patterns have the familiar FIG. 8 response. Both hydrophones exhibited a resonance around 2,300 cycles per second. This resonance may, if desired, be damped or otherwise controlled by the use of materials for the inner and/or outer masses which inherently exhibit the desired resonant characteristics and/or by the addition of suitable damping material to the masses. In addition, the resonant frequency may be shifted, if desired, by changing those design parameters which affect the resonant frequency of the masses.

The hydrophones of FIGS. 3a and 4a were constructed with the idea that their respective axes 14, 40 would be oriented in a vertical direction. Thus, sound traveling through water in a generally horizontal direction would strike or impinge on the hydrophones at a right angle to these axes 14, 40. If these hydrophones rotate about a horizontal axis passing through their center of mass, the acceleration resulting from this rotation causes the piezoelectric elements to produce a spurious output signal. FIG. 6a shows, in an exploded perspective view, a hydrophone in accordance with my invention which reduces the signals resulting from this rotation about a horizontal axis passing through the center of mass. The hydrophone comprises an inner mass 60 to which piezoelectric shear type elements 61, 61a are fastened and poled in an east-west (E-W) direction. Outer or additional piezoelectric shear type elements 62, 62a poled in a north-south (N-S) direction are respectively fastened to the outer faces of the east-west elements 61, 61a. The poling directions lie in parallel planes. End plates 65, 65a are fastened to the outer faces of the north-south elements 62, 62a respectively, and the outer mass is completed by an outer cylindrical shell 66. A simplified equivalent electrical circuit of this hydrophone is shown in FIG. 6b, where circuit elements in FIG. 6b are given the same reference numerals as their corresponding parts in FIG. 6a. A common terminal 68 is connected to the junction of the elements 61, 62 and to the junction of the elements 61a and 62a. An east-west terminal 67 is connected to the inner mass or cylinder 60. A north-south terminal 69 is connected to the end plates 65, 65a and/or the outer mass or cylinder 66. If the hydrophone of FIG. 6a is rotated about its center of mass 70 in a clockwise direction as shown by the arrows 71, 72 in FIG. 7a, the east-west piezoelectric elements 61, 61a will produce the voltage polarities shown in FIG. 7a at their respective electrodes. FIG. 7a also shows in exaggerated form the direction in which the piezoelectric elements 61, 61a, 62, 62a are moved by the external clockwise rotational force about the center of mass 70. This movement causes the voltages of the elements 61, 61a to have a positive polarity at their upper electrodes, and a negative polarity at their lower electrodes. When these voltages are combined at the terminals 67, 68 as shown in the partial circuit diagram of FIG. 7b, it will be seen that no voltage exists between the terminals 67, 68. In a similar manner, no voltage would be produced by a counterclockwise rotation about the center of mass 70. Thus, rotational motion or acceleration about the center of mass 70 produces no net output signal in the combined piezoelectric elements. A similar condition holds for the north-south elements 62, 62a, but these polarities have not been shown in order to keep FIGS. 7a and 7b relatively simple. However, if an acoustic signal or wave strikes the hydrophone as indicated by the arrow 80 in FIG. 8a, the piezoelectric elements take the position indicated in exaggerated form in FIG. 8a. In this case, the inner electrodes of the east-west shear piezoelectric elements 61, 61a are negative, and the outer electrodes are positive. When these voltages are combined at the terminals 67, 68, a voltage is produced as shown in the partial circuit diagram of FIG. 8b. A voltage is also produced by the north-south layers 62, 62a when a north-south signal is received. Thus, the use of two piezoelectric shear type elements for each of the two directions, namely east-west and north-south, compensates for any rotational acceleration about a horizontal axis passing through the center of mass.

It will thus be seen that my invention provides an improved hydrophone which can sense sound waves in a liquid such as sea water, and which can be constructed in a relatively rugged and compact structure. My hydrophone can be compensated for rotation about horizontal axes by the use of two shear piezoelectric elements. Rotation about the control vertical axis can be compensated with shear piezoelectric elements on that axis or symmetrically located about that axis. My hydrophone is relatively insensitive to vibrations that compress the piezoelectric shear-type elements, or that set up resonant and symmetrical vibrations that are in opposition relative to the longitudinal axis of the hydrophone. My hydrophone also provides an improved response over a wide band of frequencies. While I have shown specific embodiments of my hydrophone, persons skilled in the art will appreciate that modifications may be made. For example, the inner and outer masses may be constructed of other materials such as plastic, and may have other shapes such as spherical. The inner and/or outer masses may be covered with material for damping and/or insulation. Other electrical circuits and electrode configurations for combining the output voltages of the piezoelectric elements may also be used. The piezoelectric shear-type elements may have various configurations such as a circular shape rather than a square or rectangular shape as shown and described. In addition more than one piezoelectric element may be used at each end of the inner mass for a given poling direction, if additional strength or holding structure is desired between the inner mass and the outer mass, or if different voltage output levels or impedances are desired. Additional piezoelectric elements may be used at each end or at one end of the inner mass to provide two or more poling directions. Where the additional elements are used, they may be stacked or may be in a side-by-side configuration. The outer mass of plates and cylinders need not be supported by the piezoelectric elements alone, but can be supported to some extent by a mechanical construction between the inner mass and the outer mass, if such construction does not reduce the shearing forces below the necessary level. And finally, other fastening means, such as welding or mechanical fasteners, may be used. Therefore, while by invention has been described with reference to particular embodiments, it is to be understood that modifications may be made without departing from the spirit of the invention or from the scope of the claims.

Claims

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An improved electroacoustical transducer for detecting sound energy in a liquid transmission medium comprising: a first cylindrical mass having a peripheral surface in acoustic contact with said transmission medium and arranged to be accelerated in directions substantially at right angles to the longitudinal axis of said cylindrical mass by impingement of said sound energy upon said surface, the planes of said surface being parallel to said longitudinal axis, a first piezoelectric shear type element having a first shearing portion secured to said first mass for accelerating said element in response to acceleration of said first mass, said element having a poling direction substantially at right angles to the longitudinal axis of said first cylindrical mass, a second mass shielded from said sound energy and fastened to a second shearing portion of said piezoelectric element and providing inertia so that a shearing force is set up in said piezoelectric element in response to acceleration of said first mass, and electrodes coupled to said piezoelectric element for deriving an electrical signal therefrom; said second mass being surrounded by said first mass and so positioned that it is isolated from direct acceleration by said sound energy; further comprising a second piezoelectric shear type element fastened to said first and second masses, said first and second piezoelectric elements having poling directions disposed in parallel planes and oriented at substantially 90.degree. with respect to each other.

2. An improved electroacoustical transducer for detecting sound energy in a liquid transmission medium comprising: a first mass having a peripheral surface in acoustic contact with said transmission medium and arranged to be accelerated in directions substantially at right angles to the longitudinal axis of said mass by impingement of said sound energy upon said surface, the planes of said surface being parallel to said longitudinal axis, a first piezoelectric shear type element having a first shearing portion secured to said first mass for accelerating said element in response to acceleration of said first mass, said element having a poling direction substantially at right angles to the longitudinal axis of said first mass, a second mass shielded from said sound energy and fastened to a second shearing portion of said piezoelectric element and providing inertia so that a shearing force is set up in said piezoelectric element in response to acceleration of said first mass, electrodes coupled to said piezoelectric element for deriving an electrical signal therefrom, and further comprising a second piezoelectric shear type element fastened to said first and second masses, said first and second piezoelectric elements having poling directions in parallel planes and oriented at different angles with respect to each other.

3. An improved hydrophone comprising: a first mass of material, a first piezoelectric shear-type element having one active surface fastened to said first mass, a second mass of material fastened to another active surface of said piezoelectric element, said first mass surrounding at least a portion of said second mass and said piezoelectric element so that impinging sound energy accelerates said first mass only to set up stresses in said piezoelectric element and so that said piezoelectric element produces a voltage which varies as a function of said stresses, and a second piezoelectric shear-type element having one active surface fastened to said first mass and having another active surface fastened to said second mass so that impinging sound energy accelerates said first mass to set up stresses in said second piezoelectric element, and so that said second piezoelectric element produces a voltage which varies as a function of said stresses, said first and second piezoelectric elements have their poling directions lying in parallel planes but extending at right angles with respect to each other.

4. An improved hydrophone comprising: a cylindrically shaped inner mass concentrically positioned along a longitudinal axis, a first piezoelectric shear-type element having one active face operatively fastened to one end of said inner mass and symmetrically positioned relative to said longitudinal axis, a second piezoelectric shear-type element having one active face operatively fastened to the other end of said inner mass and symmetrically positioned relative to said longitudinal axis, said first and second piezoelectric elements having poling directions disposed in planes that are substantially parallel to each other, a hollow outer mass positioned around said inner mass and said first and second piezoelectric elements and spaced from said inner mass, said outer mass being symmetrically positioned relative to said longitudinal axis and being fastened to respective second active faces of said first and second piezoelectric elements so that acceleration of said outer mass in a direction lateral to said axis creates shearing stresses in said first and second piezoelectric elements, means connected to said first and second piezoelectric elements deriving electrical signals therefrom, third and fourth piezoelectric shear-type elements respectively operatively secured between said first and second piezoelectric elements and said outer mass, said third and fourth piezoelectric elements having poling directions disposed in planes parallel to the planes of the poling direction of said first and second piezoelectric elements extending in a first direction and those of said third and fourth piezoelectric elements extending in a second direction.

5. An improved hydrophone comprising: a cylindrically shaped inner mass concentrically positioned along a longitudinal axis, a first piezoelectric shear-type element having one active face operatively fastened to one end of said inner mass and symmetrically positioned relative to said longitudinal axis, a second piezoelectric shear-type element having one active face operatively fastened to the other end of said inner mass and symmetrically positioned relative to said longitudinal axis, a hollow outer mass positioned around said inner mass and said first and second piezoelectric elements and spaced from said inner mass, said outer mass being symmetrically positioned relative to said longitudinal axis and being fastened to respective second active faces of said first and second piezoelectric elements so that acceleration of said outer mass in a direction lateral to said axis creates shearing stresses in said first and second piezoelectric elements, means connected to said first and second piezoelectric elements deriving electrical signals therefrom, wherein said first and second piezoelectric elements having poling directions that lie in parallel planes but that are positioned at an angle with respect to each other.