US3715713A

US3715713A - Pressure gradient transducer

Info

Publication number: US3715713A
Application number: US00012565A
Authority: US
Inventors: F Massa
Original assignee: Dynamics Corp of America
Current assignee: MASSA DONALD P COHASSET; Dynamics Corp of America; Massa Products Corp
Priority date: 1970-02-19
Filing date: 1970-02-19
Publication date: 1973-02-06
Anticipated expiration: 1990-02-06

Abstract

A simple tubular housing structure has a vibratile diaphragm sealing each open end. Each of these diaphragms is mechanically connected to a separate vibratile piston. These pistons are rigidly bonded to the opposite sides of a piezoelectric transducer element assembly. These assemblies generate at least two independent voltages when sound pressure impinges upon the diaphragms. The voltages generated by each of these two piezoelectric plates are combined to effectively neutralize each other when sound pressures of the same amplitude and phase are simultaneously applied to each of the two pistons.

Description

Unite States assa [ Feb.6,1973

[54] PRESSURE GRADIENT TRANSDUCER [75] Inventor: Frank Massa, Cohasset, Mass.

[73] Assignee: Massa Division, Dynamics Corporation of America, Hingham, Mass.

22 Filed: Feb. 19, 1970 21 Appl.No.:12,565

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W V 3l0/8.7 51 Int.Cl ..H04r17/00,H0lv 7/00 58 FieldotSearch ..340/l0,8.7

[56] References Cited UNITED STATES PATENTS 3,187,300 6/1965 Brate ..340/10 Primary ExaminerBenjamin A. Borchelt Assistant Examiner-H. A. Birmiel Attorney-Louis Bernat [57] ABSTRACT 11 Claims, 6 Drawing Figures PRESSURE GRADIENT TRANSDUCER This invention relates to an improved pressure gradient transducer, and more particularly to a transducer which may be used as a microphone either in the air or under water.

Reference is made to my earlier U. S. Pat. Nos. 3,354,426 and 3,363,228. Each of these patents shows a pressure gradient transducer of the described type. However, they are much more complicated.

In each of these patents, there is shown a housing having a pair of axially aligned diaphragms sealing opposite ends thereof. A connecting rod extends between opposing sides of these two diaphragms. A piezoelectric assembly includes a bilaminar flexural element having one edge attached to the inside of the housing and another edge attached to the connecting rod.

When the two diaphragms are exposed to a pressure field originating from a sound source lying in a plane perpendicular to the longitudinal axis of the connecting rod, the pressure on each diaphragm is of the same in tensity and phase. Therefore, no differential force results and the output of the transducer is zero. For sounds arriving along an axis removed from the perpendicular plane, a phase shift exists between the sound pressure arriving at each diaphragm. As a result, there is a net differential force which will displace the connecting rod. This rod movement flexes the bilaminar assembly and causes a resulting signal to be generated responsive to excursion of the piezoelectric element.

In the present invention, a pair of axially aligned vibratile pistons are located at opposite ends of a hollow cylindrical housing structure. A piezoelectric element is bonded between the end faces of the opposing piston structures. This element is capable of converting oscillatory compressional forces to electrical signals. The configuration of the piezoelectric transducer element is such that equal voltages are generated in two sections of the piezoelectric element assembly when equal sound pressures of the same phase are imposed on each of the exposed piston surfaces.

An advantage of this arrangement is that the transducers of this invention are relatively simple, both in construction and operation. The inventive transducers are manufactured easily and inexpensively. Nevertheless, they are rugged and reliable in operation, and they provide excellent response characteristics. They have relatively great sensitivity, particularly within the audio frequency range.

According to an important feature of the invention, mechanical connection means are provided between two opposing vibratile diaphragms. These diaphragms sealingly close the opposite ends of a hollow tubular housing. A compressionally sensitive piezoelectric structure inside the housing generates at least two independently equal voltages when equal pressures, of the same phase, simultaneously impinge upon the exposed vibratile diaphragm surfaces.

An object of this invention is to provide means for combining these independently generated voltages to neutralize one to another and deliver a zero voltage when equal pressures, of the same phase, impinge upon the exposed faces of the vibratile piston or diaphragm surfaces.

Another object is to provide new and improved transducers, of the described type, which are simpler and lower in cost than similar prior art devices without sacrifice in performance.

These and other objects, features and advantages will become more fully apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:

FIG. 1 is an end view of a pressure gradient transducer incorporating the invention;

FIG. 2 is a cross-sectional view of the transducer, taken along the line 22 of FIG. 1;

FIG. 3 is a cross-sectional view of another embodiment, showing a tubular piezoelectric element that may be substituted for the tubular piezoelectric element illustrated in FIG. 2;

FIG. 4 is a cross-sectional view of a central portion of atransducer assembly which may be substituted for the corresponding structure shown in FIG. 2;

FIG. 5 is a cross-sectional view of a modified portion of the structure shown in FIG. 4; and

FIG. 6 is a cross-sectional view of still another modification of the inner portion of the transducer assembly.

The major portions of the inventive pressure gradient transducer are a housing 15, a pair of oppositely disposed transducer assemblies l6, 17, a driven piezoelectric element 18, and a waterproof cable 19.

The housing 15 includes tubular structure 20 which encloses a transducer assembly comprising a pair of

tapered pistons

21 and 22. These pistons are rigidly bonded to opposite ends of a piezoelectric cylinder 24. The bonding may be made by an epoxy agent, or by any other suitable means.

A pair of small,

center bosses

25 and 26 are located on the large diameter faces of the two

tapered piston structures

21, 22, respectively. Thin

walled caps

27, 28 enclose the two ends of the housing. Each cap has a center hole in its face, which clears the

bosses

25, 26 to enable concentric alignment between the piston face and the periphery of the cap. Each cap is bonded to the face of an individually associated piston and sealed at its outer periphery to the housing 20. During operation of the transducer, the flat surfaces of the

caps

27 and 28 act as vibratile diaphragms which respond to the action of sound waves striking their exposed outer surfaces.

The transducer and piston element assembly 18, inside the housing 15, comprises a number of

electrode surfaces

30,31, 32 formed on the piezoelectric ceramic cylinder 24. These electrode surfaces may be a pair of fired silver layers applied to the inside and outside walls of the ceramic tube 24. A small uncoated margin is provided on the ceramic cylinder, at each end of the inside electrode 32. The two

electrodes

30 and 31 are symmetrical spaced around the outer periphery of the ceramic cylinder 24. When the piezoelectric ceramic tube 24 is polarized, one polarity of the polarizing voltage is connected to the inside electrode 32 and the two

outside electrodes

30 and 31 are connected together and to the other polarity. The drawing has been marked, by way of example, to show that the electrode 32 is positive and the

electrodes

30, 31 are negative during polarization.

For wiring the piezoelectric ceramic element to the cable 19, an electrical conductor 33 is soldered between the electrode 30 and the center conductor of the coaxial cable 19. Another electrical conductor 34 is soldered between the electrode 31 and the shield of the coaxial cable 19. After the electrical connections are made the end of the coaxial cable 19 is anchored by tape, or other means, (not shown) to the periphery of one of the pistons, such as piston 22.

To complete the assembly, the transducer elements are placed inside the housing 15. The coaxial cable 19 is brought out through a suitable opening in the wall of the housing tube 20. The end caps, or

vibratile diaphragms

27 and 28, are then attached to the piston surfaces and sealed to the open ends of the housing tube 20.

During operation of the transducer, equal voltages are generated in each half of the piezoelectric cylinder 24 when sound arrives along an axis perpendicular to the "axis of the cylinder 20. The voltage appearing across the coaxial cable conductors is the'difference' between these two equal voltages generated in the two

separate electrode sections

30, 31 of the ceramic cylinder 24. Thus, the transducer has a zero sensitivity to sounds arriving in a plane which is perpendicular to the longitudinal axis of the housing. The transducer response is maximum when sounds arrive along the normal axis of the transducer assembly. In this direction there is a maximum phase shift of the pressure gradient appearing at each of the two

diaphragms

27 and 28. Therefore, the directional characteristic of the complete transducer assembly, shown in FIG. 2, is a true cosine pattern, as illustrated in FIG. 9 of U. S. Pat. No. 3,363,228.

The transducer has a maximum sensitivity when the length of the housing 20, along the longitudinal axis of maximum response, is equal to one-half the wavelength of sound at a preferred frequency in the pertinent medium. Below this preferred frequency, the sensitivity falls off, gradually at first and then at the rate of 6 dB per octave. This characteristic in the response of the pressure gradient transducer is described in connection with FIG. 8 ofU. S. Pat. No. 3,363,228.

FIG. 3 illustrates another embodiment of the electrode surfaces on the walls of the piezoelectric ceramic cylinder. Here, there are two

separate electrodes

40 and 41 on the inner surface of the piezoelectric ceramic cylinder 42. Four separate

annular electrodes

43, 44, 45, and 46 are distributed over the outer surface of the ceramic cylinder 42. During polarization of the ceramic element 42, one polarity is connected simultaneously to the

electrodes

40 and 41. The other polarity is simultaneously connected to all of the

external electrodes

43, 44, 45, and 46.

If the ceramic tube 42 (FIG. 3) is substituted for the tube 24 (FIG. 2), the

leads

33 and 34 are connected to the

electrodes

43 and 46. A third electrical conductor 47 connects the two

electrodes

44 and 45 together. The electrode configuration illustrated in FIG. 3 approximately doubles the sensitivity of the structure of FIG. 2.

Another embodiment replaces the piezoelectric structure 18 with the structure which is illustrated in FIG. 4. Here, a modified piston structure 50, 51 is substituted for the

piston structure

21, 22. Two identical

piezoelectric discs

52, 53 are bonded to each other with a conducting cement, such as epoxy with silver granules. The opposite sides of the disc are bonded to the opposing ends of the two piston structures 50, 51. Electrically conductive foils 54, 55 and 56 are placed between the cemented faces to provide for making external connections to the electrode surfaces of the piezoelectric discs.

These electrode surfaces are formed on the flat surfaces of the

discs

52 and 53 prior to the polarization thereof. After polarization, the discs are assembled with the polarity of one disc, 53, in contact with the polarity of the other disc 52. These two surfaces of opposite polarity are commonly connected by foil electrode 56. The two outer electrode surfaces of the

discs

52, 53 are connected together by

foil electrodes

54 and 55. The conductor 33 is connected to the foil electrode 56, and conductor 34 is connected to foil

electrodes

54 and 55, to complete an assembly which is fully substitutable for the assembly of FIG. 2.

FIG. 5 illustrates yet another embodiment for mountand 61 are bonded together with electrically conducting cement. The negative potential surfaces of these two discs are joined together. The outside or positive electrode surfaces of

discs

60 and 61 are mechanically attached, one to each of the two opposing surfaces of pistons 62a, 63a.

Foil electrodes

64 and 65 make electrical contact with the outer electrode surfaces of the

discs

60, 61. For the arrangement of FIG. 5, either the pistons 62a and 63a must be electrically non-conductive or they must be insulated from the electrically conducting foils 64 and 65. This insulation may be accomplished by covering the ends of the pistons 62a, 63a, with a suitable material, such as a thin Bakelite sheet, before attachment to

discs

60, 61.

FIG. 6 shows still another embodiment of a transducer assembly that may be used in place of the structure 18. In this arrangement, the ceramic discs 70 and 71 are separated from each other. An electrically conductive, cylindrical rod 72 is bonded, with electrically conducting cement, between the positive potential face of the piezoelectric disc 71 and the negative potential face of the piezoelectric disc 70.

Foil electrodes

73 and 74 provide electrical contact between the electrode faces of the piezoelectric discs 70, 71 and the ends of the opposing

pistons

75 and 76.

In this FIG. 6 embodiment, it is possible to use electrically conductive pistons. When these pistons are at tached to the piezoelectric discs 70, 71 with an electrically conductive cement, the pistons assume the common potential of the electrode surfaces of the

foil electrodes

73 and 74. An electrical conductor 77 makes an electrical connection between the

foil electrodes

73 and 74. Electrical conductor 34 connects the conductor 77 to the shield of coaxial cable 19. Electrical conductor 33 establishes connection between the common potential rod 72 and the center lead in coaxial cable 19; hence, the

pistons

75 and 76 have the common electrical ground potential appearing on the shield of the coaxial cable 19. Preferably, the cable 19 is fastened to the rod 72 by means of a wrapping, such as tape 78 in order to provide strain relief and prevent a tension on the connecting leads 33 and 34. If desired, it is possible to insulate the

pistons

75 and 76 from this common potential. Either the

pistons

75, 76 are made from nonconductive material; or, an insulating film is interposed at the point of contact between the pistons and the electrode surfaces of the piezoelectric discs.

A further modification of the construction of FIG. 6 reverses the polarity position of the piezoelectric disc 71. The negative electrodes on the two discs 70, 71 are then commonly connected by the rod 72. If this polarity orientation is used, the

pistons

75 and 76 must ing the piezoelectric discs. The piezoelectric discs, W

remain insulated from the electrode surfaces of the piezoelectric discs, to which they are attached. Also, the electrical conductor 77 is not necessary and may be eliminated. The electrical conductor 33 is connected between the center conductor of the coaxial cable 19 and the foil electrode terminal 73. Electrical conductor 34 is connected between the shield of the coaxial cable 19 and the foil electrode terminal 74. This modified arrangement of FIG. 6 is similar to the arrangement illustrated in FIG. 5.

While several embodiments of the invention have been shown and described, it should be understood that various modifications and alternative constructions may be made without departing from the true spirit and scope of the invention. Therefore, the appended claims are intended to cover all equivalent constructions which fall within their true spirit and scope.

I claim:

ll. A pressure gradient electroacoustic transducer comprising a transducer element assembly having a pair of parallel surfaces, separate vibratile diaphragm means coupled to each of said parallel surfaces, means responsive to an application of substantially equal alternating mechanical forces upon said diaphragms and said parallel surfaces for causing said transducer element to generate two separate and substantially equal alternating voltages, and means for connecting said transducer elements for applying said separate and substantially equal alternating voltages in subtractive relationship one to the other, whereby an application of substantially equal mechanical forces upon said diaphragms produces a zero output.

2. The invention of claim 1 wherein said electroacoustic transducer includes a tubular housing with an opening at each end of said tubular housing, said transducer element assembly being positioned centrally within said tubular housing, and said separate vibratile diaphragm means being attached to seal the open ends of said tubular housing.

3. An electroacoustic transducer comprising a tubular housing with an opening at each end of said tubular housing, a transducer element assembly having a pair of parallel surfaces, said transducer element assembly being positioned centrally within said tubular housing, separate vibratile diaphragm means coupled to each of said parallel surfaces, said separate vibratile diaphragm means being attached to seal the open ends of said tubular housing, said transducer element assembly includes a tubular piezoelectric element having electrode surfaces on both the inside and outside peripheral walls of said tubular piezoelectric element, means responsive to an application of substantially equal alternating mechanical forces upon said diaphragms and said parallel surfaces for causing said transducer element to generate two separate and substantially equal alternating voltages, means for applying said separate and substantially equal alternating voltages in subtractive relationship one to the other, whereby an application of substantially equal mechanical forces upon said diaphragms produces a zero output.

4. The invention of claim 3 wherein at least one of said electrode surfaces on one of said peripheral walls of said tubular element is divided into two substantially equal and electrically se arated areas.

5. The invention of 0 arm 4 further characterized in that said voltage applying means is arranged to apply the voltages generated in each of said electrically separated areas in a subtractive relationship of one with respect to the other.

6. The invention of claim 3 further characterized in that the electrode surface on one peripheral wall of said tubular piezoelectric element is divided into two electrically separated areas of substantially equal size and still further characterized in that the electrode surface on the opposite peripheral wall is divided into four separated areas of substantially equal size.

7. The invention of claim 6 further characterized in that the wall of said tubular piezoelectric element is polarized in the same direction throughout the entire length of the tube.

8. The invention of claim 7 further characterized in that four separate voltages are generated between the two separated electrodes on one peripheral wall and the four electrodes on the opposite peripheral wall of the piezoelectric tube, and means for combining the four voltages so that each is subtractive one from the other.

9. The invention of claim 1 further characterized in that said transducer element assembly includes two similar piezoelectric plates, said plates each having two plane parallel surfaces, said piezoelectric plates being polarized so that opposite polarities appear at opposite surfaces, electrodes on each of said plane parallel surfaces, rigid bonding means between one of said electroded surfaces of said first plate and one of said electroded surfaces of said second plate, and separate vibratile diaphragm means rigidly connected to each of the other electroded surfaces of said rigidly bonded plates.

10. The invention of claim 9 further characterized in that said two piezoelectric plates are mechanically bonded together with electrodes of opposite polarity facing each other and still further characterized in that one electrical connection is made to said common facing electrodes and a second electrical connection is made in common to said opposite facing electrodes.

11. The invention is claim 9 further characterized in that said two piezoelectric plates are mechanically bonded together with the electrodes of the same polarity on each plate facing each other and electrically connected one to the other, and separate electrical conductor means connected to each of the other electrode surfaces on each of said plates.

t l i

Claims

1. A pressure gradient electroacoustic transducer comprising a transducer element assembly having a pair of parallel surfaces, separate vibratile diaphragm means coupled to each of said parallel surfaces, means responsive to an application of substantially equal alternating mechanical forces upon said diaphragms and saiD parallel surfaces for causing said transducer element to generate two separate and substantially equal alternating voltages, and means for connecting said transducer elements for applying said separate and substantially equal alternating voltages in subtractive relationship one to the other, whereby an application of substantially equal mechanical forces upon said diaphragms produces a zero output.

4. The invention of claim 3 wherein at least one of said electrode surfaces on one of said peripheral walls of said tubular element is divided into two substantially equal and electrically separated areas.

5. The invention of claim 4 further characterized in that said voltage applying means is arranged to apply the voltages generated in each of said electrically separated areas in a subtractive relationship of one with respect to the other.