US3308423A - Electroacoustic transducer - Google Patents

Description

March 7, 1967 F. MASSA, JR 3,308,423

ELECTROACOUSTI C TRANSDUCER Filed Dec. 30, 1963 3 Sheets-Sheet l INVENTOR.

FRANK MA SSA JR. BY 2 March 7, 1967 F. MASSA, JR 3,308,423

ELECTROACOUSTIC TRANSDUCER Filed Dec. 30, 1963 3 Sheets-Sheet 2 m My H! [L IN VENTOR. FRANK MA SSA, JR

March 7, 1967 F. MASSA, JR

ELECTROACOUSTIC TRANSDUCER Filed Dec. 30, 1963 3 Sheets-Sheet 5 INVENTOR. FRANK M45514, JR.

United States Patent Office 3,368,423 Patented Mar. 7, 1967 3,308,423 ELECTROACOUSTHC TRANSDUCER Frank Massa, .lr., Cohasset, Mass assignor, by mesne assignments, to Dynamics Corporation of America, New York, N.Y., a corporation of New York Filed Dec. 30, 1963, Ser. No. 334,203 8 Claims. (Cl. 340-8) This invention relates generally to improvements in electroacoustic transducers, and, more particularly, to a new and improved type of electroacoustic transducer structure for permitting reliable under-water operation in the low and midaudible frequency range.

Those skilled in the art of under-water transducer design will appreciate that severe problems must be overcome to obtain efficient acoustic radiation under water at the lower frequencies. Due to the fact that the wave length of sound in water in the lower audible frequency regions is quite large, it becomes necessary to provide vibrating structures whose radiating area is several square feet or greater if significantly large amounts of acoustic power is to be generated. For example, at 300 cycles per second, a circular piston of about two feet in diameter will vibrate with a total excursion of approximately 0.011 inch displacement in order to radiate one kilowatt of acoustic power. If the piston diameter is reduced to about one foot the required total piston displacement becomes 0.044 inch. These figures are based on the assumption that the vibrating structure is a true piston and is surrounded by an infinite rigid baffle. In the absence of a battle, the required displacement becomes even greater. It is also appreciated by those skilled in the art that a vibrating flat plate having a diameter of a few feet would have to be reasonably thick and heavy in order for it to behave as a rigid piston during its vibratory operation. This is particularly true because of the additional fact that the vibrating plate would be carrying along a relatively heavy water load which for a two foot diameter structure would be approximately 150 pounds.

A practical design of an electroacoustic transducer to operate satisfactorily under the conditions described relates to an electromagnetically driven structure in which the vibrating piston surface is driven through an air gap having sufficient clearance to permit the required amplitude of vibration. In a conventional electromagnetic transducer wherein a rigid housing contains an electromagnetic assembly having a vibratory piston affixed at one end, the transducer would only be capable of operating in relatively shallow water, because the tremendous forces which are exerted on the surface of the piston in deeper water would soon close the air gap and render the transducer substantially inoperative.

Prior art references for preventing air gap closure in conventional electromagnetic transducers have utilized pressure equalization in which the air pressure inside the transducer enclosure is adjusted to correspond to the external water pressure acting on the surface of the diaphragm. This procedure, in addition to being complicated, introduces substantial limitations wherein the air pressure increases in the air gap and the viscosity also increases which in turn interferes with the satisfactory operation of the transducer.

The object of this invention is to improve the design of an under-water transducer in order to provide a vibratory structure which is capable of ellicient generation of acoustic power in relatively deep liquids, such as water.

Another object of this invention is to improve the design of a transducer for operating under water such that the performance characteristics remain virtually unchanged as the depth of submergence is varied.

A still further object of this invention is to provide a simple relatively low-cost electromagnetic transducer design wherein true piston operation is achieved for the vibrating portion of the transducer without resorting to the use of thick massive plates.

An additional object of this invention is to improve the design of an inertia-driven electromagnetic transducer by eliminating the need for electrical conductors between the vibrating external surface of the transducer and the internal flexibly-suspended magnetic assembly.

Another object of this invention is to design a transducer which radiates acoustic power directly from its freely-vibrating surfaces with the analogous efficiency and effectiveness as if the transducer were actually mounted near the center of a very large rigid wall.

Another object of this invention is to improve the linearity of the vibratory displacement of the radiating surface of the transducer.

The novel features which are characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of omration as well as additional objects and advantages thereof will best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a vertical cross-sectional view of an electromagnetic transducer embodying certain novel features of the invention;

FIGURE 2 is a partial vertical sectional view showing a combination of a pair of electromagnetic transducers mounted within a cylindrical baffle for increasing the efiiciency of radiation of acoustic energy;

FIGURE 3 is a crosssectional view showing a modification of the transducer structure of FIGURE 1 in order to achieve an efficient radiation of under-water sound energy without the need for an additional bafiie struc ture;

FIGURE 4 is a plane view taken along line 44 in FIGURE 1, illustrating the armature section of the transducer; and

FIGURE 5 is a perspective view partially in section of the armature section, as shown in FIGURE 4, illustrating the general arrangement of the drive coils and the extended lead terminals.

Referring more specifically to FIGURE 1, the numeral 1i) represents a massive non-magnetic base member having a general cylindrical outer periphery and provided with a fiat surface to which is bonded by a suitable bonding material, such as an epoxy resin cement, a permanent magnet assembly comprising of a plurality of stacks of magnetic laminations 12 interspersed with permanent magnets 14. The permanent magnets 14 are preferably relatively thin rectangular plates of the sintered oxide type which may be efiiciently polarized through the thin dimension, as illustrated by the markings N for north and S for south, representing the polarity arrangement of the various magnets. The assembly of the lamina tion stacks 12 with the magnets 14 is achieved by cementing the various sections together into one composite structure; and then cementing one surface of the composite structure to the base member 10 using a suitable bonding compound, such as an epoxy cement.

A stack of slotted laminations 16 is bonded to one side of a plate member 18, as shown in FIGURE 1. A corresponding number of electrical coils 20 are rigidly bonded by cement within the slots of a lamination stack or armature 16 by a suitable bonding compound after which spring members 22 are mounted between the surfaces of the plate member 18 and the base member 10 by bolts 19 to establish a fixed parallel air gap 24 between the ends of lamination stacks 12 and the mating 3 ends of the lamination stacks 16. The stiffness of the spring members 22 is chosen to provide the desired operating frequency of the assembled transducer and the stiffness range is extremely critical in order to obtain the desired result.

After the assembly of the composite magnetic structure with the spring members 22 and plate member 18 and base member 10, the outer fiat surface of plate member 18 is bonded by a suitable compound, such as an epoxy cement to the inner plane surface of the outer housing 26.

The windings of coils 20 are electrically connected in series with each other, and the end leads of the connected group pass through an opening (not shown) provided in the plate member 18 and housing 26 and are soldered to the terminals 28 which are mounted through an insulating terminal bushing 30, as illustrated in FIG- URE 1. The flexible leads 31 are connected to the corresponding conductors within the electrical cable 32. The cable 32 is molded to a metallic flange 34 which is attached by means of the bolts 36 to a mating member 38, which is recessed and sealed into the housing structure 26, as shown. A high strength bonding compound, such as an epoxy cement, may be used as a sealant between the mating member 36 and the housing 26. The mating surfaces of the flange 34 and the mating member 38 may be sealed with a gasket or an O-ring. The inner terminal assembly or insulating bushing 30 is mounted within a counterbored portion of housing 26. This construction prevents the entry of sea water into the working portion of the transducer assembly which would cause damage to the cable. The coil leads 20 which are connected to the terminals 28 pass through an openin in the housing 26 then potted and sealed with a rigid cast bonding compound, such as an epoxy resin cement, to consolidate the windings and prevent their vibration when the transducer is operating. To complete the transducer, a complementary housing 27 is joined to the housing structure 26 and bonded and cemented at the contacting surface 40 to provide a composite convex hollow outer housing.

The basic electromagnetic structure described up to this point is similar to the transducer structure described and shown in more complete detail in the copending US. patent application Serial No. 164,010, filed January 3, 1962, by Frank Massa, entitled, Electroacoustic Transducer. The operation of the transducer, as described in the aforesaid application, takes place when the passage of alternating current through the coils 20 causes alternating magnetic forces at the air gap 26 which sets up rela tive vibration between the suspended internal portion of the transducer and the outer shell-like housing. The generally convex hollow outer housing provides a rigid vibrating surface which can be much lighter in structure than the fiat vibrating surfaces and further which can resist higher pressures as are encountered in deep water. This invention is not concerned with the permanent magnet electromagnetic drive system thus far described. The invention is concerned primarily with improvements in this design, as will be outlined hereinbelow.

By attaching the drive coils 20 to the outer vibrating portion of the transducer assembly, it is possible to eliminate the need for flexible lead terminals for operating the transducer such as are required in the design shown in US. patent application Serial No. 164,010, filed January 3, 1962, in which the current coils are shown attached to the inner vibrating portion of the assembly. The change in construction shown in the present invention eliminates the risk of failure of the vibrating leads during operation of the transducer.

Another improvement shown in the present design includes the use of a ring member 42 which is fastened by suitable screws 44 to a recessed machined annular surface provided in the base member 1%). The purpose of the ring 42 is to dynamically balance the vibrating structure which is achieved by operating the assembly before attaching housing portion 26, by measuring the relative amplitudes of vibration at various positions around the periphery of the ring 42. These measurements may be made by attaching several accelerometers to the face of the ring member 42 and noting the relative displacements as measured by the several instruments.

The attachment of the accelerometers and the measure ment of the amplitudes of vibration is not illustrated in the drawing since this procedure is routine standard practice and does not form any part of this invention.

After reading the relative displacements at various points about the periphery of the vibration structure, an adjustment is made by removing material from the ring 42 near the region where the displacement measurement is higher than average. Other methods of adding or subtracting material may be employed by plating or mechanically affixing weights. When the proper adjustment of the weight distribution of the ring 42 is reached, the amplitudes of vibration at various positions about the periphery of the vibrating structure will be very nearly equal and the system will be dynamically balanced. The transducer structure will now operate with vibratory mo tion which is relatively free of twisting modes of 'viorlav tion thereby establishing a true linear displacement and a corresponding true simple harmonic motion of the transducer body.

Another improvement in the design of the transducer results from the use of the disc member 46 which is bolted near its periphery to the inside ring surface of the complementary housing 27 by means of screws 48, as shown. After the disc member 46 is attached to the complementary housing structure 27, the complementary housing structure 27 is bonded to the housing structure 26 at the peripheral region 4%, as previously described. Through a lower opening 50, a bolt 52 is fastened to the base member 1 1 and a suitable spacer 54 is placed between the disc member 46 and the base member 14) so that when the bolt 52 is tightened, the position of the base member 19 is radially secured with respect to the assembled outer housing structure of the transducer.

The disc member 46 is rigid in a plane at right angles to the normal axis of the transducer and is flexible along the axis of vibration of the transducer. In this way the disc member 46 serves to increase the ruggedness of the transducer assembly so that it may withstand severe shock without damage as might otherwise occur if the massive base member 10 were unsupported at its free over-hanging end. After the bolt 52 is secured, the opening 59 is sealed by a rigid plug 56 which is bonded to the complementary housing 27 by an epoxy resin cement or some other suitable cement.

An outer protective covering 58 of rubber or other suitable pliable material is preferably molded to the outer surface of the transducer to prevent corrosion of the exposed surface. The pliable covering 58 will serve to provide shear compliance to permit unrestricted oscillation of the transducer when it is placed in contact with a fixed surface such as the wall of a baffle structure.

The operation of the completed transducer is as follows: the permanent magnets 14 establish a flux density in the air gap 24 as is well known by one skilled in the art. When an alternating current passes through the coils 20, the air gap flux is modulated accordingly; and. if the frequency of the current corresponds with the mechanical resonant frequency of the transducer, relatively large displacements will occur between the massive inertial base member 10 and the unitary outer housing structure 26 and 27. The general spherical shape of the transducer design will permit satisfactory operation in relatively deep water because the water pressure will be resisted by the housing shape. The housing is preferably fabricated from a relatively light material, such as aluminum, magnesium or beryllium alloy compositions and the base member 10 is preferably fabricated from relatively heavy material, such as bronze or'non-magnetic stainless steel,

By this choice of materials it is possible to achieve relatively large excursions of the outer radiating surface of the transducer while the inertial base member vibrates at a lower amplitude.

If the transducer illustrated in FIGURE 1 is submerged in water, the entire housing structure will oscillate as an unitary structure. Unless the transducer is placed in a battle to prevent circulation from the front to the rear portion of the oscillating structure, the acoustic coupling to the water will be relatively poor; and a relatively small amount of acoustic power will be generated for large displacements of the transducer.

FIGURE 2 illustrates an arrangement of a bafiie structure which will permit a relatively high efficient acoustic radiation from an under-water transducer illustrated in FIGURE 1. A cylindrical pipe 661 which has a clearance diameter to receive the transducers 62 is fitted with a number of mounting brackets 64 which are secured to the wall of the pipe 69 by suitable bolts 66 or any other equivalent fastening means. To the exposed faces of the brackets 64 is provided a resilient pad 6? which makes contact with the outer surface of transducers 62 when they are assembled in position. Before assembling the transducers 62, a gas-filled bladder 70 or pressure release system is mounted inside the cylinder pipe (it) by means of brackets '72, as illustrated. The gas may be air, nitrogen or any other gaseous medium. The rubber bladder 711 may be made from two rubber sheets bonded between two metallic rings 74 which, in turn, are fastened to the brackets 76 by means of the screws 78. A suitable inlet valve 80 is provided in the wall of the rubber bladder 70 through which air may be pumped into the bladder to expand it to the desired volume. A protective sealing cap 82 is provided to seal the valve stem and protect it from under water corrosion. A pair of the novel transducers 62 are placed one in each open end of the cylinder 60, and are held in place by attaching the additional bracket members 84 which are secured to the cylinder wall 60 by means of the bolts 86. Rubber pads 88 are provided on the faces of the brackets 84 to contain the transducers between resilient movement. The two transducers are electrically connected so that they vibrate in phase; that is, both transducers move axially outward and inward together as they vibrate at the ends of the cylinder.

During the operation of the batfied structure shown in FIGURE 2 acoustic radiation takes place only from the outer exposed area of each transducer 62. The gas-filled bladder 7t) provides a suitable pressure release to permit free vibration of the two transducers insofar as the inner unexposed surfaces are concerned. The cylindrical bathe prevents any interference from the vibrations of the transducer surfaces facing toward the center of the battle. The arrangement shown in FIGURE 2 effectively couples the radiating exposed transducer surfaces to the water as if an infinite wall were present in a plane bisecting the axis of the cylinder 60.

A mounting frame 90 may be welded to one end of the cylinder 60, as illustrated, and a mounting flange 92 may be provided for suspending the transducer from a supporting cable, if desired. The transducer electrical cable 94 is shown passing through a protective sleeve 96 which is welded to the outer surface of the cylinder 60. The second transducer cable 98 is secured together with the free end portion of transducer cable 94 by means of a clamp or a tape 100. Bracket members 102 may be welded around the periphery of the tubular cylinder 60 to be used as pedestal mountings if the transducer assembly is to be fastened to a flat surface such as the deck of a submarine.

FIGURE 3 shows a preferred modification of the transducer structure in which eflicient under-water radiation may take place without the necessity of a special bafile. FIGURE 3 shows two permanent magnet assemblies each driving one-half of the transducer outer surfaces. The inertial massive base member 110 to which is attached the corresponding permanent magnet and lamination assembly 112 which serves the same functions as was described in connection with the structure shown in FIG- URE 1. The armature members 114 with the drive coils 116 are mounted to upper plate members 118 by means of the springs 120 to complete an electromagnetic assembly similar to the electromagnetic assembly described in FIGURE 1. The upper plates 118 which are attached to the housing members 122 in the same way that plate 18 was attached to housing 26 in FIGURE 1, and the cables 124 are provided with the same fittings and the same terminal arrangements as was described for the structure in FIGURE 1. Basically, as thus far described, FIGURE 3 comprises a pair of transducer assemblies identical to the structure shown in FIGURE 1, with the exception that one portionrof the housing enclosure 26, shown in FIGURE 1, is deleted in FIGURE 3. To complete the assembly of the transducer in FIGURE 3, a bellows type spring member 126 is bonded or welded to connect and to seal the two housing portions 122.

During the operation of the transducer of FiGURE 3, each housing member 122 is driven in phase at the resonant frequency determined by the stiffness of the spring members 120. The bellows type spring 126 has sufficient radial stiffness to resist the water pressure at the depth of operation, and to permit the opposite ends of the assembled ransducer to move together in deep water. The amount of static deflection is determined by the stiffness of the bellows spring 126. For maximum efficiency, it is preferable to adjust the stiffness of the spring member 126 so that it is at resonance with the radiating mass of the transducer ends at their resonant frequency of operation. In this manner the internal mass members Will remain virtually stationary and the maximum displacement of the radiating surfaces 122 may be achieved for a given air gap displacement in the transducer. This preferred stiffness of the spring member 126 must also take into account the added mass of the water load which is carried into oscillation by the housing members 122.

The composite transducer structure will also operate fairly satisfactorily if the stiffness of the spring member 126 is kept relatively low and pliable so that it will not impede the motion of the radiating faces 122 at their normal operating frequency. Under this condition, however, motion of the internal mass member 110 will be relatively similar to the motion displayed by the base member 10 during the operation of the transducer described in FIG- URE 1.

By virtue of the design in FIGURE 3, it is evident that both ends of the vibrating structure will vibrate in phase and therefore initiate sound energy in the water without circulatory motion from one surface to the other as occurs from the oscillating structure in FIGURE 1. In other words, the configuration of FIGURE 3 provides within itself the equivalent of an infinite battle in a plane at right angles to the axis of vibration of the composite structure. The efficiency of radiation of the composite structure of FIGURE 3 will be equivalent to the efiiciency of radiation of the bafiled transducer combination illustrated in FIGURE 2.

Although a few specific examples have been chosen to illustrate the basic principles of the invention, it will be obvious to those skilled in the art that numerous departures may be made from the details shown; and therefore it is contemplated that the invention shall not be limited except insofar as is made necessary by the prior art and by the spirit of the appended claims.

What is claimed and desired to be protected by Letters Patent of the United States is:

1. The improvement of an alternating electroacoustic transducer comprising a sealed housing structure, including two vibratile end portions separated by a compliant member, a massive first magnetic means adapted to be spaced from said first end portion for translatory vibration relative thereto, a massive second magnetic means adapted to be spaced from said second end portion for translatory vibration relative thereto, a third magnetic means rigidly secured to said first end portion and positioned in operable relation to said first magnetic means, a fourth magnetic means secured to said second end portion and positioned in operable relation to said second magnetic means, alternating current coil generating means attached to each of said third and said fourth magnetic means and operatively associated with each of said magnetic means, terminal means for supplying alternating electrical current to said coil means, frequency determining spring elements attached between said first and third magnetic means and frequency determining spring elements attached between said second and said fourth magnetic means to hold said attached pairs of magnetic means in operable relationship to each other whereby translatory vibration of said first and said second end portion of said sealed housing structure will be of the same phase whenever alternating current is applied to the current coils.

2. The invention set forth in claim 1 characterized in that the compliance of the compliant member between said vibratile end portions is of a magnitude which is at resonance with the effective vibratory mass associated with each vibratile end portion within the frequency band of operation of said transducer.

3. The invention set forth in claim 1 further characterized in that said alternating current coil means are rigidly attached to said third and said fourth magnetic means.

4. In combination in an electroacoustic transducer adaptable for generating sound under water, a first transducer means comprising an inertia driven sealed housing which executes translatory vibration as a Whole upon being activated by its driving power, a second similar transducer means, a tubular member, means associated near each end of said tubular member for flexibly supporting said transducer means within said tubular means, and a volume of material contained within said tubular member characterized in that its bulk modulus of elasticity is less than fifty percent of the bulk modulus of elasticity of water.

5. The invention set forth in claim 4 further characterized in that said volume of material comprises a water impervious flexible enclosure containing a gas.

6. The invention set forth in claim 4 further characterized in that each transducer means is operated in synchronous phase relationship such that each end portion of the housing structure exposed to the open ends of said tubular means moves simultaneously outward and inward with reference to the center of said tubular means.

7. The improvements of an inertia driven electroacoustic transducer which includes a sealed outer housing portion adapted for executing oscillatory vibrations, a massive inertial portion flexibly mounted inside said sealed housing portion and adapted for executing oscillatory vibrations along the same axis of vibration of said sealed housing portion, means for dynamically balancing said vibratile inner mass portion whereby all points on the surface of said vibratile inertia portion will execute linear vibrations parallel to the axis of vibration of said outer sealed housing structure.

8. The invention set forth in claim 7 further characterized in that said dynamic balancing means includes weight members which may be attached at various regions near the periphery of said inertial mass.

References Cited by the Examiner UNITED STATES PATENTS 2,962,695 11/1960 Harris 340-10 3,048,815 8/1962 Thurston et al. 3408 3,219,969 11/1965 Snavely 340 12 X 3,274,538 9/ 1966 Snavely 3408 RODNEY D. BENNETT, Primary Examiner.

I. P. MORRIS, Assistant Examiner.

Claims (1)

1. THE IMPROVEMENT OF AN ALTERNATING ELECTROACOUSTIC TRANSDUCER COMPRISING A SEALED HOUSING STRUCTURE, INCLUDING TWO VIBRATILE END PORTIONS SEPARATED BY A COMPLAINT MEMBER, A MASSIVE FIRST MAGNETIC MEANS ADAPTED TO BE SPACED FROM SAID FIRST END PORTION FOR TRANSLATORY VIBRATION RELATIVE THERETO, A MASSIVE SECOND MAGNETIC MEANS ADAPTED TO BE SPACED FROM SAID SECOND END PORTION FOR TRANSLATORY VIBRATION RELATIVE THERETO, A THIRD MAGNETIC MEANS RIGIDLY SECURED TO SAID FIRST END PORTION AND POSITIONED IN OPERABLE RELATION TO SAID FIRST MAGNETIC MEANS, A FOURTH MAGNETIC MEANS SECURED TO SAID SECOND END PORTION AND POSITIONED IN OPERABLE RELATION TO SAID SECOND MAGNETIC MEANS, ALTERNATING CURRENT COIL GENERATING MEANS ATTACHED TO EACH OF SAID THIRD AND SAID FOURTH MAGNETIC MEANS AND OPERATIVELY ASSOCIATED WITH EACH OF SAID MAGNETIC MEANS, TERMINAL MEANS FOR SUPPLYING ALTERNATING ELECTRICAL CURRENT TO SAID COIL MEANS, FREQUENCY DETERMINING SPRING ELEMENTS ATTACHED BETWEEN SAID FIRST AND THIRD MAGNETIC MEANS AND FREQUENCY DETERMINING SPRING ELEMENTS ATTACHED BETWEEN SAID SECOND AND SAID FOURTH MAGNETIC MEANS TO HOLD SAID ATTACHED PAIRS OF MAGNETIC MEANS IN OPERABLE RELATIONSHIP TO EACH OTHER WHEREBY TRANSLATORY VIBRATION OF SAID FIRST AND SAID SECOND END PORTION OF SAID SEALED HOUSING STRUCTURE WILL BE OF THE SAME PHASE WHENEVER ALTERNATING CURRENT IS APPLIED TO THE CURRENT COILS.

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