US3474403A - Electroacoustic transducer with improved shock resistance - Google Patents

Description

Oct. 21, 1969 F. MASSA ET 3,474,403

ELECTROACOUSTIC TRANSDUCER WITH IMPROVED snocx RESISTANCE Filed June 8, 1966 XWW nited States Patent Ofice 3,474,403 Patented Oct. 21, 1969 3,474,403 ELECTROACOUSTIC TRANSDUCER WITH IMPROVED SHOCK RESISTANCE Frank Massa and Gilbert C. Barrow, Hingham, Mass., as-

signors to Massa Division, Dynamics Corporation of America, Hingham, Mass.

Filed June 8, 1966, Ser. No. 556,820 Int. Cl. H04b 13/00 US. Cl. 340- 16 Claims ABSTRACT (IF THE DISCLOSURE The invention provides a shock resistant, piezoelectric ceramic transducer. More particularly, the transducer assembly includes a plurality of annular ceramic rings separated by and cemented to plates of rigidifying material, such as steel washers. Then, the entire assembly is en closed in a waterproof housing structure which includes a very strong unit enclosed in a waterproof and shock resistant material and filled within an inert dry gas. This construction provides a transducer utilizing an end-to-end compression and precludes damage from high amplitude transient shocks.

This invention relates to an electroacoustic transducer and more particularly to an electroacoustic transducer having improved resistance to mechanical shocks and being otherwise in construction, to be capable of operation at high power levels and to be capable of withstanding high amplitude transient shock pressures, while being readily and economically constructed.

This invention relates generally to transducers wherein a transducer unit is interposed between a vibratile diaphragm member and a support or weight member, the transducer unit including a plurality of transducer elements in end-to-end relation which compress and expand with vibratile movement of the diaphragm member. In operating such transducers, failures of the transducer units have been encountered when the transducer is operated at high power levels and where the transducer is subjected to high amplitude transient shock pressures such as produced by explosions in the vicinity thereof, particularly in underwater operation.

Considerable improvement has been obtained from the use of coupling means such as disclosed in the application of Frank Massa entitled Electroacoustic Transducer With Improved Shock Resistance, Ser. No. 493,806, Oct. 7, 1965. However, it has been found that failure of the units may still occur.

The invention is based in part upon the discovery of causes which give rise to such failures. In particular, we have found that the failure of the transducer units arise from transverse vibrations within the transducer elements, resulting from high power level operation or from high amplitude transient shock pressures applied to the transducer.

According to an important feature of the invention, rigidifying means are associated with the transducer elements to limit vibratory movement thereof in transverse directions, resulting in a marked increase in the resistance of the transducer to shocks and vibrations and permitting operation of the transducer at extremely high power levels.

According to a specific feature of the invention, the rigidifying means are in the form of plate means interposed between the transducer elements and of a material having a Youngs modulus substantially greater than that of the transducer elements. By way of example, the plate means may be of steel, while the transducer elements are of a ceramic piezoelectric material, providing a ratio of the Youngs modulus of the plate means to that of the transducer elements of at least three and in some cases of twenty or more.

Preferably, the plate means are securely bonded to the transducer elements, as by using a suitable cement.

According to another important feature, the transducer elements are ring-shaped while the plate means are preferably in the form of rings of approximately the same size and shape as that of the transducer elements, with holding means extending from the support member centrally through the transducer elements and the plate means to the diaphragm member, to hold the components in assembly. With such elements, a highly efiicient transducer is provided capable of operating at high power levels, while the rigidifying means limits the transverse vibrational movements, to prevent fracture of the transducer elements. The provision of the rigidifying means is particularly advantageous in this construction, because radial resonance effects are encountered with the use of the ring-shaped transducer elements, which would otherwise cause peripheral cracks and destruction of the elements.

A further important feature of the invention is in the provision of coupling means between the diaphragm member and the transducer unit, similar to that disclosed in the above-identified application of Frank Massa, Ser. No. 493,806, such coupling means being characterized in that linear translatory forces are transmitted with negligible attenuation between the transducer unit and the diaphragm member and being further characterized in that flexural vibrations of the diaphragm surface are attenuated by the coupling means to be thereby prevented from reaching the transducer unit.

Still another important feature of the invention is in the incorporation of the assembly within a hollow rigid housing structure, in an arrangement similar to that disclosed in the copending application of Frank Massa entitled Electroacoustic Transducer, Ser. No. 537,899, filed Mar. 28, 1966, now Patent No. 3,328,751. In accordance with this feature, resilient means are disposed between an outer side wall surface of the weight member and an inner side wall surface of the housing structure, to restrain the weight member against transverse vibrations while allowing substantially free movement thereof in a longitudinal direction.

According to a specific feature of the invention, the resilient means includes a silicone rubber material adapted to allow the free sliding movement of the outer side surface of the weight member relative thereto.

Another specific feature is in the provision of a low friction cement on the outer side surface of the weight member.

Still another specific feature is in the provision of a member of a sponge rubber material or the like, between an outer end face of the weight member and in inner face of the housing structure, operative to uncouple the weight member from the housing structure.

This invention contemplates other objects, features and advantages which will become more fully apparent from the following detailed description taken in conjunction with the accompanying drawings which illustrate a preferred embodiment and in which:

FIGURE 1 is a side elevational view of a transducer conducted according to the principles of the invention;

FIGURE 2 is a vertical sectional view on an enlarged scale, taken substantially along 22 of FIGURE 1;

FIGURE 3 is a top plan view of the transducer of FIGURE 1; and

FIGURE 4 is a bottom plan view of the transducer of FIGURE 1.

Reference numeral 10 generally designates a transducer conducted according to the principles of the invention; The transducer comprises a rigid vibratile piston plate or diaphragm member 11 which is preferably made of a material having a high stiifness-to-mass ratio. For example, magnesium and aluminum alloys are some of the materials which may advantageously be used. It is even more advantageous to employ a type of aluminum casting which has a sponge-like structure and which has a density of only a fraction of solid aluminum and it retains a very high stiffness for the piston plate 11 which is necessary in order that it will vibrate without break-up of its surface at the frequency of operation.

A transducer unit generally designated by reference numeral 12 is provided which comprises four cylindrical elements 13-16 in end-to-end relation and having electrode means on the end faces thereof. Preferably, the elements 13-16 may be ceramic cylinders of polarized barium titanate or lead zirconate or any other suitable material which is well known in the art for converting electrical energy to vibrational energy. Advantageously, the elements 13-16 are polarized with faces of like polarity being disposed in abutting relation.

A weight member 18 is secured against the end face of element 13, preferably with a thin insulating washer 19 being provided. At the opposite end of the transducer unit 12, a coupling member 20 is provided, forming an important feature of the transducer, as will be more completely described.

In accordance with this invention, rigidifying means are provided for limiting transverse vibrational movement of the transducer elements 13-16, preferably in the for-m of ring-shaped steel plates or washers 21, 22, 23, 24 and 25, washer 21 being disposed between element 13 and the insulating washer 19, washer 22 being disposed between elements 13 and 14, washer 23 being secured between elements 14 and 15, washer 24 being secured between elements and 16, and washer 25 being secured between element 16 and the coupling member 20. The washers 21-25 are bonded to the elements 13-16 by a suitable cement. The purpose thereof is to provide increased radial stifr'ness for the ring-shaped elements 13-16 so that when a shock wave arrives at the diaphragm member 11, the compressional forces along the axis of the transducer which tend to cause an expansion in the diameter of the elements 13-16 will lessen the magnitude of the radial strain on the elements and thereby prevent fracturing of the elements such as might occur under shock with the rings 21-25 eliminated. It is noteworthy that when a high amplitude transient shock pressure wave is impinged against the diaphragm member 11 it tends to excite radial resonances in the elements 13-16 to cause peripheral cracks. The rigidifying washers 21-25 limit the transverse or radial deformation of the elements 13-16 so as to restrict the radially resonant mode and to protect the elements 13-16 against cracking and failure.

Materials having a high Youngs modulus are preferred for the washers 21-25 such as steels including stainless and high carbon steels. However, it is possible to use other materials including metal alloys and non-metallic materials, provided that the material has a Youngs modulus substantially higher than that of the elements 13-16.

The coupling member serves for transmission of linear translatory forces with negligible attenuation between the transducer unit 12 and the diaphragm member 11 and further serves to cause attenuation of fiexural vibrations of the surface of the diaphragm element 11 to thereby prevent such vibrations from reaching the transducer unit 12. The member 20 is preferably a ring having a triangular cross-sectional shape as shown with a flat surface engaging the washer and with a circular line edge in contact with the surface of the diaphragm member 11. Thus the member 20 cooperates with the washers 21-25 in minimizing vibrations of the elements 13-16 which might otherwise cause fracturing thereof.

To hold the weight member 18, the transducer unit 12 and the vibratile diaphragm member 11 in assembly, securing means are provided which comprise a bolt having a head portion 27 and a shank portion 28 extending through an opening 29 in the weight member 18 with an end portion 30 of the shank portion 28 being threaded into an opening 32 in the diaphragm member 11. A feature of the transducer is in the provision of a plurality of Belleville springs 34 which are disposed between a collar 35 engaged by the bolt head 27 and a base surface of a recess 36 in the weight member 13. The springs 34 are cupshaped washers which behave as springs of relatively high stiffness and when the bolt is tightened, the basic vibrating elements of the assembly are held together with substantially uniform pressures between the end faces of the transducer unit 12 and the weight member 18 and vibratile diaphragm member 11. Without the compression springs, the tension in the bolt would be function of temperature because of differences in thermal expansion of the various materials which make up the complete vibrating structure. With the spring elements 34 compressed by a fractional part of an inch, variations in dimensions of the assembled parts of a few thousandths of an inch will be negligible in determining the applied compressive force to the assembled elements.

A hollow rigid housing structure 38 is provided in surrounding relation to the transducer unit 12 and the weight member 18 with resilient means in the form of an annular gasket 39 being disposed between an open end of the housing structure 38 and a peripheral portion of the vibratile diaphragm member 11. As shown, the gasket 39 is disposed between facing surfaces 41 and 42 of the member 11 and housing structure 38, respectively, the surfaces 41 and 42 being planar surfaces transverse to the axis of vibratile movement of the plate 11. Preferably, the gasket 39 is cemented to the surfaces 41 and 42 with strong waterproof cement such as an epoxy. It will be noted that the gasket 39 compresses and expands with vibratile movement of the plate 11 and when the assembly is placed under water, the water pressure compresses the gasket 39 to a certain degree, to insure an effective seal.

A feature of the transducer construction is that the thickness and stiffness of the gasket 39 is selected such that the resonant frequency, determined by the mass of the housing structure 38 and the compliance of the gasket 39 occurs in a range well below the frequency of operation of the transducer assembly. With this arrangement, the diaphragm 11 will be uncoupled from the housing structure 38 and thus the housing structure 38 can effectively remain stationary to prevent any undesirable radiation of sound from the rear of the transducer assembly. This advantageous construction results in improved efficiency because only useful sound energy is radiated from the front face of the diaphragm 11.

Another feature of the transducer is that the hollow rigid housing structure 38 is tapered, the diameter or transverse dimension thereof at the open end which is adjacent the diaphragm member 11 being substantially greater than the diameter or transverse dimension at the opposite end thereof. This arrangement permits the assembly of a number of transducer assemblies within a minimum circle diameter so that a maximum power density can be achieved with an overall structure which is quite compact.

Preferably, the weight member 18 has an outer surface 43 which has a taper corresponding to that of the wall of the housing structure 38 to be in uniformly spaced facing relation to an inner wall surface 44 of the structure 38, so as to permit the Weight member 18 to have a maximum size relative to the overall size of the assembly.

A very important feature of the invention is in the provision of resilient means 46 between the surfaces 43 and 44, for restraining the weight member 18 against transverse vibrational movement while allowing substantially free movement thereof in a longitudinal direction.

The resilient means 46 is preferably a silicone rubber which is sold under the trade name of Silastic. If this material is used, it may be poured in through a side hole in the housing 38 with the transducer placed with the diaphragm surface upwards, and after filling, the hole in the housing is sealed witha plug 47. When the silicone rubber compound is usedfor the resilient means 46, there is no acoustic loss caused by vibration of the mass or weight member 18. The .reason for this is that the silicone rubber does not stick to the metallic surfaces and the vibrating surface slides freely by the silicone rubber. The presence of the silicone rubber reduces the destructive effect of transverse shock applied to the outer housing.

As another preferred material, amore viscous material such as polyurethane or a soft rubber-like epoxy compound may be used. When using such materials, it is desirable to uncouple the material from the weight member 18 in order to prevent loss in efficiency of the transducer. In order to uncouple the mass or weight member 18, a foam rubber disk 47 is cemented to the rear surface of the weight member 18. Additionally, the outer surface of the weight member 18 is provided with a coating 43 of a low friction material such as Teflon, Mylar film, or the like.

Another feature of the illustrated transducer is in the provision of a coupling transformer 50 which is supported on the outside of an end wall 51 of the housing structure 38, preferably by means of a molded potting compound 52 which encloses the transformer 50. The transformer 50 is of conventional construction, including a threelegged core structure 53 formed of stacked laminations with primary and secondary windings 54 on a center leg of the core 53, the secondary winding being connected to conductors 55 and 56, conductor 55 being connected to washers 21, 23 and 25 and conductor 56 being connected to Washers 22 and 24. It will be understood that suitable electrodes are formed on the end faces of the elements 13-16. The primary winding of the transformer 50 is connected through conductors 57 and 58 to conductors of a waterproof cable 60, the end of the cable 60 being enclosed in the potting compound 52.

To prevent corrosion of the assembly in salt water environments, a cover of waterproof material is bonded to a radiating face 61 of the diaphragm member 11 and completely encloses the housing structure 38 to provide a water-tight seal around the transducer assembly. In particular, a molded rubber cap 62 is provided having an internal contour of the same shape as the contour of the face 61, the internal surface of the cap 62 being bonded to the face 61 by means of a suitable cement, such as an epoxy. Preferably, the cap 62 may be molded directly to the surface of the member 11 by carrying the rubber compound in a mold using the piston or diaphragm member 11 as an insert. The cap 62 has an integral and annular flange portion 63 which embraces a peripheral edge 64 of the member 11 and which embraces an end portion of the housing structure 38.

In accordance with a specific feature of the invention, the cap 62 is generally rectangular, preferably substantially square as shown in FIGURES 3 and 4, and the four corner portions thereof are so formed as to be thinner than the plate 11 and as to define integral mounting tab portions 65, the portions 65 being preferably provided with openings 66 therethrough, for mounting on a rigid support structure.

The rubber cover further comprises a tapered or frustoconical rubber sleeve 68 which surrounds the housing structure 38 and preferably also surrounds the molded potting compound 52 which surrounds the transformer 50. An end portion 69 is preferably disposed within the flange portion 63 of the cap 62, and the outer surface of the housing structure 38 is recessed for this purpose.

In accordance with a further feature of the transducer, the rubber sleeve member 68 is provided with a plurality of peripherally spaced tapered integral wedge portions 70 which serve as additional shock mounts when the transducer is assembled into a frame-like mounting structure. Thus, the mounting structure may include a portion having an internal cylindrical surface for engagement by the tapered wedge portions 70.

The cover for the assembly further includes a cap 71 having an annular flange portion 72 surrounding an end portion of the sleeve 68. The cap 71 additionally has a strain relief extension 73 which embraces the cable 60. It is noted that a plug 74 may be fitted into an opening in the cover 68 after applying the resilient material 46 in the manner as above described.

Still another feature of the transducer is in the prevention of voltage breakdowns within the transducer assembly while permitting application of high voltages to the transducer elements 1316, to obtain high power operation thereof. In accordance with this feature, the plugs 47 and 74 may be removed, to permit the space within the housing structure 38 and around the transducer 12 to be filled with an inert dry gas, after the plate or diaphragm member 11 is secured to the open end of the housing structure 38 through the washer 39. After filling the space with an inert dry gas, the plugs 47 and 74 are inserted in place, after which the transformer 50 and potting compound 52 are installed. Thereafter, the cap 62, sleeve 68 and cap 71 are installed to provide the waterproof cover.

It will be understood that modifications and variations may be effected without departing from the spirit and scope of the novel concepts of this invention.

We claim as our invention:

1. In combination in an electroacoustic transducer, a rigid vibratile diaphragm plate having a high stiffness-tomass ratio and mounted to vibrate without significant breakup of its surface at the frequency of operation, said diaphragm plate member having opposite faces, a support member having an end face, a transducer unit between one of said faces of said diaphragm member and said end face of said support member and comprising a plurality of transducer elements in end-to-end relation to compress and expand with vibratile movement of said diaphragm member toward and away from said support member, and rigidifying means associated with said transducer elements for limiting transverse vibrational movement of said transducer elements responsive to movements of said diaphragm in directions other than toward and away from said support member.

2. In an electroacoustic transducer as defined in claim 1 said rigidifying means comprising plate means interposed between said elements and of a material having a Youngs modulus substantially greater than that of said transducer elements.

3. In an electroacoustic transducer as defined in claim 2, cement means bonding said transducer elements to said plate means.

4. In an electroacoustic transducer as defined in claim 2, said plate means being in the form of steel plates.

5. In an electroacoustic transducer as defined in claim 1, said transducer elements being in the form of piezoelectric ceramic elements.

6. In an electroacoustic transducer as defined in claim 1, said transducer elements being ring-shaped.

7. In an electroacoustic transducer as defined in claim 6, said rigidifying means comprising plate means in the form of rings of approximately the same size and shape as said ring-shaped transducer elements.

8. In an electroacoustic transducer as defined in claim 1, said support member being in the form of a weight member having a relatively large mass, and a hollow rigid housing structure surrounding said transducer unit and said weight member and having an open end adjacent a peripheral portion of said vibratile diaphragm member.

9. In an electroacoustic transducer as defined in claim 8, said Weight member having an outer side wall surface in facing relation to an inner side wall surface of said housing structure, and resilient means between said side wall surfaces for restraining and absorbing transverse vibratory movement of said weight member while allowing substantially free movement in a longitudinal direction.

10. In combination in an electroacoustic transducer, a vibratile diaphragm member having opposite faces, a support member having an end face, a transducer unit between one of said faces of said diaphragm member and said end face of said support member and comprising a plurality of ring-shaped transducer elements in end-to-end relation to compress and expand with vibratile movement of said diaphragm member toward and away from said support member, and rigidifying means associated with said transducer elements for limiting transverse vibrational movement thereof, said rigidifying means comprising plate means in the form of rings of approximately the same size and shape as said ring-shaped transducer elements, holding means extending from said support member to said diaphragm member through said transducer elements and said rigidifying means and acting in tension to hold said transducer unit and said members in assembly.

11. In combination in an electroacoustic transducer, a vibratile diaphragm member having opposite faces, a support member having an end face, a transducer unit between one of said faces of said diaphragm member and said end face of said support member and comprising a plurality of transducer elements in end-to-end relation to compress and expand with vibratile movement of said diaphragm member toward and away from said support member, rigidifying means associated with said transducer elements for limiting transverse vibrational movement thereof, and coupling means between said transducer unit and said one of said end faces of said diaphragm member characterized in that linear translatory forces are transmitted with negligible attenuation between said transducer unit and said diaphragm member and further characterized in that fiexural vibrations of said one of said end faces of said diaphragm member are attenuated by said coupling means and thereby prevented from reaching said transducer unit.

12. In an electroacoustic transducer as defined in claim 11, said coupling means comprising a structure having a flat plane surface on one side thereof attached to said transducer unit and having a narrow line edge on the opposite side thereof in contact with said one of said faces of said diaphragm member.

13. In combination in an electroacoustic transducer, a vibratile diaphragm member having opposite faces, a support member having an end face, said support member being in the form of a weight member having a relatively large mass, a transducer unit between one of said faces of said diaphragm member and said end face of said support member and comprising a plurality of transducer elements in end-to-end relation to compress and expand with vibratile movement of said diaphragm member toward and away from said support member, rigidifying means associated with said transducer elements for limiting transverse vibrational movement thereof, a hollow rigid housing structure surrounding said transducer unit and said weight member and having an open end adjacent a peripheral portion of said vibratile diaphragm member, said weight member having an outer side wall surface in facing relation to an inner side wall surface of said housing structure, and resilient means between said side wall surfaces for restraining and absorbing transverse vibratory movement of said weight member while allowing substantially free movement in a longitudinal direction, said resilient means including a silicone rubber material adapted to restrain said transverse vibratory movements while allowing substantially free sliding movement of said outer side surface of said weight member relative thereto.

14. In combination in an electroacoustic transducer, a vibratile diaphragm member having opposite faces, a support member having an end face, said support member being in the form of a weight member having a relatively large mass, a transducer unit between one of said faces of said diaphragm member and said end face of said support member and comprising a plurality of transducer elements in end-to-end relation to compress and expand with vibratile movement of said diaphragm member toward and away from said support member, rigidifying means associated with said transducer elements for limiting transverse vibrational movement thereof, a hollow rigid housing structure surrounding said transducer unit and said weight member and having an open end adjacent a peripheral portion of said vibratile diaphragm member, said weight member having an outer side wall surface in facing relation to an inner side wall surface of said housing structure, and resilient means between said side wall surfaces for restraining and absorbing transverse vibratory movement of said weight member while allowing substantially free movement in a longitudinal direction, a coating of a low friction material on said outer side surface of said weight member.

15. In combination in an electroacoustic transducer, a vibratile diaphragm member having opposite faces, a support member having an end face, said support member being in the form of a weight member having a relatively large mass, a transducer unit between one of said faces of said diaphragm member and said end face of said support member and comprising a plurality of transducer elements in end-to-end relation to compress and expand with vibratile movement of said diaphragm member toward and away from said support member, rigidifying means associated with said transducer elements for limiting transverse vibrational movement thereof, a hollow rigid housing structure surrounding said transducer unit and said weight member and having an open end adjacent a peripheral portion of said vibratile diaphragm member, said weight member having an outer end face in facing relation to an inner end face of said housing structure, and a member of foam rubber or the like between said outer and inner end faces.

16. In combination in an electroacoustic transducer, a vibratile diaphragm member having opposite faces, a weight member having an end face, a transducer unit between one of said faces of said diaphragm member and said end face of said weight member and comprising a plurality of transducer elements in the form of rings of ceramic piezoelectric material in end-to-end relation to compress and expand with vibratile movement of said diaphragm member relative to said weight member, a plurality of plate means in the form of rings interposed between said transducer elements and having approximately the same size and shape as said transducer elements, said plate means being of a material having a Youngs modulus substantially greater than that of said ceramic piezoelectric transducer elements, cement means bonding said plate means to said transducer elements, holding means extending from said weight member centrally through said transducer elements and said plate means to said diaphragm member, coupling means between said transducer unit and said one of said faces of said diaphragm member characterized in that linear translatory forces are transmitted with negligible attenuation between said transducer unit and said vibratile diaphragm and further characterized in that flexural vibrations of said faces of said diaphragm member are attenuated by said coupling means and thereby prevented from reaching said transducer unit, a hollow rigid housing structure surrounding said transducer unit and said weight member and having an open end adjacent a peripheral portion of said diaphragm member, resilient means interposed between said open end of said housing structure and said peripheral portion of said vibratile plate arranged to flexibly seal said housing structure to said vibratile plate, said weight member having an outer side wall surface in facing relation to an inner side wall surface of said housing structure, and resilient means between said outer and inner side wall surfaces for restraining said Weight member against transverse vibratile movement while allowing substantially free movement thereof in a longitudinal direction.

References Cited UNITED STATES PATENTS 2,405,604 8/1946 Pope 340-10 10 Howatt.

Riesen 3109.1 Massa 340-10 Sims 340-8 Douglas 34010 RODNEY D. BENNETT, Primary Examiner B. L. RIBANDO, Assistant Examiner

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