US3718897A - High fidelity underwater misic projector - Google Patents

Abstract

A FERROELECTRIC STACK, POLARIZED FOR AXIAL EXCURSIONS TO TRANSFER SIGNALS REPRESENTATIVE OF ACOUSTIC ENERGY, IS HELD IN COMPRESSION BY CAP MEMBERS MOUNTED ON ITS OPPOSITE ENDS TO TRANSFER RECEIVED OR PROJECTED ACOUSTIC ENERGY THROUGH CIRCUMFERENTIALLY ENCLOSING LONGITUDINALLY RUNNING RIBS. BECAUSE OF THEIR CROSS-SECTIONAL CONFIGURATION AND BECAUSE THEY ARE COUPLED IN A PARTICULAR ANGLE WITH RESPECT TO THE CAP MEMBERS, THE STACK''S AXIAL EXCURSIONS ARE CONVERTED TO MAGNIFIED RADIAL DISPLACEMENTS WHEN THE TRANSDUCER IS USED IN THE TRANSMIT MODE. IN ADDITION, GREATER SENSITIVITY TO IMPINGING ACOUSTICAL ENERGY IS PROVIDED BY THE LARGE RECEIVING SURFACES PRESENTED BY THE OTHER SURFACES OF THE RIBS WHEN THE TRANSDUCER FUNCTIONS IN THE RECEIVE MODE.

Description

Feb. 27, 1973 I F, R, ABBO'TT 3,718,897

HIGH FIDELITY UNDERWATER MUSIC PROJECTOR Filed May 27, 1971 4 SheetsSheet 1 INVENTOR. F RANK R. ABBOTT THOMAS GLENN KEOUGH ERVIN F JOHNSTON ATTORNEYS Feb. 27, 1973 F. R ABBOTT 10H FIDELITY UNDERWATER MUSI C PROJECTOR 4 Sheets-Sheet I Filed May 27, 1971 FIG.30

INVENTOR. FRANK R. ABBOTT FIGB THOMAS GLENN KEOUGH ERVIN E JOHNSTON ATTORNEYS F. R ABBOTT Feb. 27, 1973 HLGH FIDELITY UNDERWATER MUSIC PROJECTOR 4 Sheets-Sheet 5 Filed May 27, 1971 FIG.6

INVENTOR. FRANK R. ABBOTT THOMAS GLENN KEOUGH ERVlN E JOHNSTON ATTORNEYS 1973 F. R ABBOTT 3,718,897

HIGfi FIDELITY UNDERWATER MUSIC PROJECTOR Filed May 27, 1971 4 Sheets-Sheet 4.

ENVENTOR. FRAN K R. ABBOTT THOMAS GLENN KEOUGH ERVIN F. JOHNSTON ATTORNEYS United States Patent 3,718,897 HIGH FIDELITY UNDERWATER MUSIC PROJECTOR Frank R. Abbott, 3953 Wildwood Road, San Diego, Calif. 92107 Filed May 27, 1&71, Ser. No. 147,573 Int. Cl. H0411 13/00 US. Cl. 340-8 R ABSTRACT OF THE DISCLOSURE A ferroelectric stack, polarized for axial excursions to transfer signals representative of acoustic energy, is held in compression by cap members mounted on its opposite ends to transfer received or projected acoustic energy through circumferentially enclosing longitudinally running ribs. Because of their cross-sectional configuration and because they are coupled in a particular angle with respect to the cap members, the stacks axial excursions are converted to magnified radial displacements when the transducer is used in the transmit mode. In addition, greater sensitivity to impinging acoustical energy is provided by the large receiving surfaces presented by the outer surfaces of the ribs when the transducer functions in the receive mode.

STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefore.

BACKGROUND OF THE INVENTION Historically, a favored means of generating sonar waves uses a ferroelectric element, usually a piezoelectric ceramic cylinder, radially vibrating against surrounding water. An alternate version uses axial motion of a ceramic cylinder to vibrate end caps against the water. An immediate limitation of using the element alone or with end caps becomes apparent when it is noted that the specific acoustic impedance of a typical ceramic is about 15 times higher than the acoustic impedance of water, the transmitting medium. In an attempt to match the acoustic impedances, the wall thickness of the radially oscillating ceramic cylinders usually were reduced to about one-fifteenth their diameter to produce an acceptable coupling. However, this procedure fails at frequencies where the diameter is a small fraction of the wave length because at these lower frequencies the acoustic impedance of water becomes lower still. To accommodate these lower frequencies, it naturally follows that the ceramic cylinder walls would have to be made impractically thin and would be intolerably fragile. Another attempt to more efiiciently acoustically couple the ceramics employed a membrane having a cross-sectional appearance identical to a sagging clothesline anchored at its opposite ends and drooped across the circumferential extremes of a ceramic ring. Radial excursions of the ring would greatly magnify axial vibrations of the membrane over a larger area. Another approach relied on coaxially containing a ceramic driving element inside of a shell made up of a pair of opposed trombone-bell shaped elements. The elements increased the area through which acoustical energy is projected when the transducer is used in the transmit mode and also increased the area sensitive to impinging acoustic energy when the transducer is used in the receive mode. However, both of these approaches created large cumbersome and awkward devices which were vulnerable to damage.

10 Claims 3,718,897 Patented Feb. 27, 1973 SUMMARY O THE INVENTION mounted on a separate opposite axial extreme of the element, are rigidly connected to a plurality of sections or ribs arranged in a side-by-side relationship circumferentially disposed about the ferroelectric element. Due to the orientation of connecting means joining the sections to the cap members and the sections cross-sectional configuration, the axial excursions are magnified as the sections are proportionally radially displaced.

The prime object of the invention is to provide a transducer having a superior acoustical coupling to its water medium.

Another object of the invention is to provide a transducer having an increased transmitting and receiving surface ensuring a superior acoustical coupling.

Another object is to provide a transducer having a charging entrained mass improving its frequency response and acoustical coupling characteristics.

Yet another object is to provide a transducer constructed to magnify the axial excursions of its active driving element by its radially displaced projector surfaces when operating in the transmit mode.

A further object is to provide a transducer having a large surface for receiving impinging acoustic energy coupled to an active element in a manner to increase the aggregate incident pressure and thereby provide for greater sensitivity.

A further object is to provide a transducer employing a double-cantilever coupling between its active element and its transmit-receive surfaces for improved response.

Yet another object is to provide a transducer having an omnidirectional response over an extended frequency range.

Still another object is to provide a transducer contracted to maintain its active element in constant compression for minimizing the possibilty of tensile or compressional failure. 7

These and other objects of the invention will become more readilyapparent from the ensuing description when taken with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an isometric view of the invention shown partially in cross section.

FIG. 2 is a side view, shown partially in cross section of a preferred form of the invention.

FIG. 3 is an end view of the invention depicted in FIG. 2, also shown partially in cross section. FIG. 3a is an end view of the transferring means or body member alone.

FIG. 4 is a cross sectional view of the transferring means taken generally along lines 4-4 in FIG. 3a.

FIG. 5a depicts an exaggerated. bending of an adjacent section or rib asit transmits at h.

FIG. 5b depicts an exaggerated bending of an adjacent section or rib as it transmits f FIG. 6 is a cross-sectional view of a modified adjacent section.

FIG. 7 is a cross-sectional view of another modified adjacent section.

FIG. 8 is an end view of the FIG. 7 modification.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, FIG. 1 shows a transducer 10 constructed in accordance with the teachings of the invention operatively connected to remote driving or monitoring circuitry through a pair of terminals a and 10b. A protective boot 11 is formed of a compliantrubber composition having substantially the same density and mechanical impedance as the surrounding water to encase the other transducer elements in a water tight relationship. The interior of the boot is either oil-filled or pressure-compensated rendering the transducer insensitive to ambient pressure changes. Oil filling the boots interior has a tendency to increase the transducers stiffness and raise its frequency response range, so, if less stiffness or a lower frequency response is desired gas pressure compensation is preferred. In either event, the rubber boot is cut off and sealed at its opposite ends by 1 suitable hose clamps, not shown, or the boot extends in a hose-like fashion to contain several transducers for use as either an active or a passive array. r i

In all the disclosed embodiments, each transducer employs as its active element, a coaxially disposed ferroelectric element 15. The ferroelectric element is optionally a stack of ferroelectric cylinders having conductors interposed between adjacent ones and connected in parallel. Irrespective of the exact configuration, the element is fabricated and polarized to provide reciprocal axial travel when driven by a suitable potential source or to provide representative signals when axially deformed. A pair of locator sleeves 15a and 15b are in cluded to aid positioning the stack.

Because of the densities, flexure characteristics, and modulus of the other transducer components, leadzirconate-titanate is preferably chosen for use as the driving element to allow a better internal mechanical coupling as will be elaborated on below. Barium titanate optionally is selected when the design is modified.

Opposite ends of the ferroelectric stack are covered by a separate cap member 16 or 17 serving to hold the stack in compression. Such a compressional holding is required 'when high driving potentials are impressed across the stack due to its inability to withstand selfdestructive, internal tensile forces attendant its outward axial excursions.

In the embodiment depicted in FIGS. 2 through 5, the caps are held in place by a plurality of threaded elongate bolts 20 each reaching through an individual cap hole 19 or 19, equidistantly circumferetially disposed in each respective cap member. The elongate bolts act as members compressing the stack and connecting the cap members to a body member to effect the transfer of acoustic energy to and from the transducer.

Machining the body member from aluminum stock provides an efficient and economical energy transfer vehicle; although, if cost is not controlling or if a higherfrequency response is needed, beryllium, being lighter than and four-times stiffer than aluminum, is more satisfactory.

A pair of annular wedge shaped cuts 26 and 27 are milled from opposite ends to form an outer portion 28 having outer surfaces 28' and 28" for receiving or projectin acoustic energy. Care must be taken that not too much material is removed since a certain degree of stiffness must remain to ensure acceptable sensitivity over a desired frequency range. However, enough material must be removed to accommodate the elongate bolts and to lower the body member's inertia which could preclude high frequency operation. Another design consideration is that upon milling away too much material, the outer portion inside of surfaces 28' and 28 becomes too thin and causes excessive resonance, especially as higher driving frequencies are encountered.

Between the wedged shaped cuts a stem portion 29 cooperates with the outer portion to give the body member a longtiudinal, T-shaped cross-sectional configuration. A longitudinal axial bore 30 reaching across the body member provides a space for housing the ferroelectric s ck a t ha a d ameter s g y n exc ss o the stacks diameter to provide a loose fitting for locator sleeves 15a and 15b and so as not to interfere with the stacks axial excursions. Being thusly milled and bored, an inner portion 31 is formed having its intermost surface roughly defining a cylinder coaxially contained within the cylinder defined by the outer surfaces of the outer portion.

Another machining step necessary to ensure the conversion of the stacks axial excursions to radial displacement of the body member, calls for making longitudinal cuts 32 running the entire length of the body member to segment it into adjacent sections or ribs 33, each including a previously described outer portion, stem portion, and inner portion, see FIGS. 3 and 3a, showing 6 adjacent outer portion-stem portion-inner portion combinations. To facilitate the assembly of the transducer, longitudinal cuts 32 were made by a circular saw which after separating the outer and stem portions of adjacent sections,

,was radially inwardly displaced to cut away most of the inner portion but to leave uncut areas 31a and 31b at the opposite extremes of the inner portions. A body member having its adjacent section or ribs thusly sepa rated is an integral unit and is easier to assemble although the adjacent sections are completely separated in modifications of the invention.

Noting FIGS. 3 and 3a a higher frequency response was enhanced by reaming out sections 29a of the stem portions. This removal of material lowered the ribs central mass and stiffness and enabled a more-rapid response to higher-frequency operation.

With respect to the embodiment of FIGS. 2 through 5, a pair of angled bores 34 are provided midway in the stem portion of each adjacent section and each have threads shaped to mechanically engage threaded elongate bolts 20. Drilling the bores with an incline of approximately 4 with respect to outer surfaces 28 and 28" permits either a distance or pressgre enhancing mechanical coupling between the ferroelectric stack and the body member depending on whether the transducer is operating in the transmit or receive mode of operation.

After both the cap members have been secured to the adjacent sections or ribs via the bolts being screwed into the angled bores, a dual purpose is served. First, the ferroelectric element is held in compression and cannot tear itself apart; and, second, a path is created through the caps and bolts to the adjacent sections which, when oriented in the aforementioned 4 inclination, in theory, should result in a five fold distance or pressure multiplication depending on the transducers operational mode. In addition, the constant compressional force exerted on the ferroelectric element prevents both tensile and compressional failure due to temperature variations. The flexure of metallic adjacent sections: will compensate for the higher rate of thermal expansion in the metals as compared to the ferroelectric stack.

When employing the transducer in the active mode as a projector of acoustic energy, outer surfaces 28' and 28" should be reciprocally radially displaced a distance five times the distance traveled by the fcrroelectric elements axial excursion. Actually in practice, a lesser mag nification is experienced due to internal torsional losses realized as the driving forces are transmitted through the body members. That is to say, for example, across the juncture between the stem portion and the outer portion an internal loss-producing twisting of the metal occurs while effecting the radial displacement of the outer surfaces which obviously diminishes the ideal magnitude of energy transfer.

A high degree of flexibility in determining the transducers frequency-versus-projected-power characteristic when operating in the active mode is afforded by the present design. The mass and the stiffness of the adjacent sections or ribs are selectable to provide the desired charac- Wrislici A greatly g reted excursion. of the body member is depicted in FIG. a which schematically represents the fiexure of outer portion 28 as the transducer proects acoustic energy at a frequency f In a similar manner FIG. 5b shows the same transducer projecting energy at a higher frequency f From a comparison of the two figures it is observed that nodes 35 are nearer the lateral extremes of the outer portion when the lower f is projected. The central area between the nodes undergoes the most effective consistant lateral displacement while the parts of the outer portion 28 outside the nodes go in and out of phase as frequency is raised. To a limited degree, the actual entrained volume-displacement of water associated with lateral displacement of the adjacent sections or ribs decreases with increasing frequency. The acoustic impedance is a function of the specific impedance of the water multiplied by some small power, less than the second power, times the displaced volume of water per stroke. By shaping the adjacent sections to bring the nodes together at a predetermined rate with respect to increasing frequency, a transducer having a nearly linear response over a high frequency range is constructed.

When the transducer functions in the passive mode the broad area presented by outer surfaces 28' and 28" receives impinging acoustic enregy. The total force exerted by the incident pressure wave that is transferred to the ferroelectric element obviously is increased by the broad area and, due to the 4 inclination of the threaded bolts, a mechanical advantage transforms the incident force to a much higher value for deforming the element. Signals representative of the stacks deformation are generated and fed to remote circuitry.

A more efiicient energy transfer between dissimilar materials occurs when their acoustic impedance is matched. This is when the square root of the Youngs modulus times their density multiplied by the areas of transmission equals a value which is constant throughout the system. It naturally follows that such a consideration be taken into account when designing the invention. The square root of Youngs modulus X density of lead zirconate titanate is nearly twice the Youngs modulus of aluminum. This requires that the area of capping members 16 and 17 be equal to twice the axially projected surfaces of the ferroelectric element. However, to efficiently mechanically couple the axial forces laterally across the cap members to the elongate rods, the cross sectional area of the caps is designed to be over twice the total area abutting the end of the ferrolectric stack because the shear modulus of the aluminum caps is roughly /2 its Youngs modulus. For aluminum the shear modulus is thus only about A the Youngs modulus of the titanate and the area should be quadrupled to retain the constant value necessary for the efficient transmission of energy from one medium to another. Similarly, the dimensions of the rods are selected so that their specific impedance-area product equals the constant value.

Being constructed in the disclosed manner permits fabrication of transducers having a diameter of less than one inch. The compactness of this design enables a wide application for eflicient acoustic energy transfer in either the active or passive mode. Because a transducers operating characteristics are also a function of its size, the physical dimensions of a transducer, assembled in accordance with the present invention, are only a matter of choice.

A modification of the aforedescribed configuration is depicted in FIG. 6. In this case a further reduction in the inertial drag of the body member is provided by removing the inner portion (inner portion 31 in the embodiment of FIGS. 2 through 5). Thus, modified adjacent sections 33' are disposed about a ferroelectric stack similar to the first embodiment. However, since there are no uncut areas 31a and 31b to hold the adjacent sections in their side-by-side orientation, this modification has a tendency to twist or come apart especially when it is subjected to abuse.

Another modification of the basic concept of using the double-cantilevered outer surfaces 28' and 28" is presented in FIGS. 7 and 8. In this embodiment the adjacent sections 33" have similar outer and stem portions 28a and 29a as does the embodiment of FIGS. 2 through 5, with the primary exception being that the elongate bolts are dispensed with. In their place short threaded bolts 20a fit into angled threaded bores 34a and maintain a 4 angle with respect to the outer surface of outer portion 28a. To accommodate the short bolts and to provide the 4 inclination, inner portion 310 slants outwardly having inner and outer parallel surfaces with a slot-shaped cut 26a and 27a. Whereas, the first two embodiments had only six adjacent sections 33 or 33 any number of sections 33" may be usde to fabricate a transducer having a greater or lesser diameter. The outer surface of outer portion 28a is laterally curved to cooperate with other adjacent sections to form a cylindrically shaped body memher.

In all the disclosed embodiments the design criteria set forth with respect to the first embodiment is mutually applicable. Furthermore, connecting the cap members to the adjacent section at a 4 inclination is not overly critical since other inclination angles also could be acceptable.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings, and, it is therefore understood that within the scope of the disclosed inventive concept, the invention may be practiced otherwise than as specifically described.

I claim:

1. A transducer of acoustic energy in a water medium comprising:

a ferroelectric element polarized for axial excursions to provide signals representative of said acoustic energy;

a pair of cap inflexible members each mounted on a separate opposite axial extreme of said ferroelectric element having a configuration to ensure the transfer of said axial excursions;

means for transferring said axial excursions representative of said acoustic energy cireumferentially disposed about said ferroelectric element having adjacent sections configured for radial displacement in direct proportion to said axial excursions; and

means connecting said cap members to the transferring means disposed for ensuring the directly proportional said radial displacement.

2. A transducer according to claim 1 in which the connecting means is configured and disposed to hold said ferroelectric element in compression preventing its selfdestruction while ensuring the directly proportional said radial displacement.

3. A transducer according to claim 2 further including:

a compliant casing carried on the outer surface of said transferring means rendering said transducer watertight.

4. A transducer according to claim 3 in which said adjacent sections are arranged in a side-by-side relationship to define a cylindrically shaped said outer surface having longitudinal slots running its length.

5. A transducer of acoustic energy in a water medium comprising:

a ferroelectric element polarized for axial excursions to transfer signals representative of said acoustic ena pair of cap members each mounted on a separate oppo site axial extreme of said ferroelectric element;

means for transferring said acoustic energy circumferentially disposed about said ferroelectric element having adjacent sections arranged in a side-by-side relationship to define a cylindrically shaped outer surface having longitudinal slots running its length, each of said adjacent sections has a T-shaped cross-sectional configuration with its top portion forming a part of 7 said outer surface and with its stem portion reaching inwardly toward said ferroelectric element;

a separate means connecting each of said cap members to each said stern portion of each adjacent section configured and disposed to hold said ferroelectric element in compression preventing its self-destruction for ensuring the directly said proportional radial displacement; and

a compliant casing carried on said outer surface of said transferring means rendering said transducer watertight.

6. A transducer according to claim 5 in which each said connecting means is oriented between its interconnect cap member and stem portion to define a longitudinal angle less than 45 with respect to the direction of said axial excursions and a lateral angle greater than 45 with respect to the direction of radial excursions thereby ensuring improved acoustic coupling between said transducer and said medium.

7. A transducer according to claim 6 in which each said connecting means is an elongate threaded bolt reaching through one of a plurality of holes provided in each said cap member and mechanically cooperating with correspondingly tapped bores in each said stem portion ensuring secure coupling therebetween.

8. A transducer according to claim 6 in which each said adjacent section further includes an inner portion laterally extending in opposite directions from the innermost part of each said stem portion and the extreme opposite ends of each said inner portion are joined to the extreme opposite ends of an adjacent said inner portion giving said transferring means a unitized construction.

9. A transducer according to claim 6 in which each said adjacent section further includes an angled inner portion laterally extending in opposite directions from the innermost part of each said stem portion and reaching toward both said cap members to define said longitudinal angle and said lateral angle and a separate said connecting means joins each said angled inner portion to a separate one of said cap members at its lateral extreme.

10. A transducer according to claim 6 in which lateral bores are provided in each said stem portion further reducing the mass of said transducer to enhance its response.

References Cited UNITED STATES PATENTS 3,539,980 11/1970 Massa, Jr 340-8 R 3,337,842 8/ 1967 Bouyoucos "a..." 340-8 R 2,405,605 8/1946 Goodale Ir. et al. 340-8 LF 3,262,093 7/1966 Junger et al 340-8 R X 3,555,503 1/ 1971 Morris.

BENJAMIN A. BORCHELT, Primary Examiner H. TUDOR, Assistant Examiner US. Cl. X.R. 310-87; 34010

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