US 3215977 A
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Description (OCR text may contain errors)
Nov. 2, 1965 cps.
A. L. W. WILLIAMS ETAL ACOUSTIC TRANSDUCER Filed July 27, 1960 FIGJ INVENTORS ALFRED L.W.W|LL|AMS HALLEY H.HAML|N AT ORNEY United States Patent 3,215,977 ACOUSTIC TRANSDUCER Alfred L. W. Williams, Cleveland, and Halley H. Hamlin, Lyndhurst, Ohio, assignors to Clevite Corporation, a corporation of Ohio Filed July 27, 1960, Ser. No. 45,570 Claims. (Cl. 340-) This invention pertains to an acoustic transducer, particularly for underwater service but not limited thereto.
It has been a problem to obtain an inexpensive underwater transducer operating in the acoustic range from very low frequencies up to and including about 15,000 cycles per second, and which is rugged and which has a substantially fiat frequency response.
It is therefore an object of the present invention to provide a transducer for use in the acoustic range which is inexpensive yet which is very rugged structurally and which has a substantially flat frequency response.
Still another object of the invention is to provide a very stiff rugged acoustic transducer whose resonant frequency is around 40,000 cycles per second where it will not interfere with the. acoustic range, and which has structural features causing adequate amplitude of movement of its pistons in order to obtain a signal of high amplitude even though the transducer appears to be very stiff.
For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.
An aspect of the invention lies in the provision of an acoustic transducer, microphone or speaker, wherein there are two dish-shaped housing sections each of which is a given thickness and is made of a material such that when it is excited at acoustic frequencies in the range up to at least 15,000 cycles per second each housing section moves as a true piston. The two housing sections are connected together face-to-face with the concave sides forming a cavity which contains expander type piezoelectric element means such as a tubular element of iezoelectric ceramic. The piezoelectric element is in driving engagement with both of the housing sections for motion in a direction parallel to the axis of the sections, and its length is related to the depth of the sections so that when the fastening means is pulled up tight there is clearance between the peripheral edges of the housing sections. Thus during a transducing action when the two pistons move toward and away from each other their edges will not engage each other. The stiffness of the fastening means must be low compared to the stiffness of the piezoelectric element means in order to achieve adequate signal output from the transducer yet permit the element to expand and contract without undesirable loading effects, and compliant sealing means seal around the peripheral edges of the two housing sections.
With reference to the single sheet of drawing there is shown in FIGURE 1 a graph showing the flat output versus frequency curve of the device of the present invention.
FIGURE 2 is a front face view of the device.
FIGURE 3 is a rear face view of the device.
FIGURE 4 is a sectional view taken along line 4-4 of FIGURE 2, and
FIGURE 5 is an isometric view of the transducer device.
With reference to FIGURE 1 there is shown a graph illustrating the frequency response of the acoustic transducer of the present invention, in water. It will be seen that the curve is essentially flat to 10,000 cycles per second with a slight tendency at that point to rise, and the curve projected to 15,000 c.p.s. will still be essentially flat.
Patented Nov. 2, 1965 It is this characteristic which, together with adequate signal output, is hard to obtain in an inexpensive yet rugged transducer.
The structure of transducers of the present invention is best shown in FIGURE 4. First and second housing sections 10, 11 are provided. Each is madeof a lightweight, relatively stiff material such as aluminum or plastic, and each has a wall thickness of adequate dimension to insure that as the housing section is vibrated it will vibrate as a unit. That is, it acts as a true piston, not as a diaphragm which breaks up into a variety of vibrating areas. Furthermore, each of the housing sections is preferably dish-shaped thereby to increase its stiffness without a corresponding increase in mass such as would be inherent if the wall thickness were increased. Each of the wall sections preferably is round in face view, as shown in FIGURES 2 and 3, thereby to achieve a high degree of symmetry, and each is thickest at its central areas 12, 13, tapering slightly toward a thinner outside peripheral edge portion 14, 15.
Transducer element means 20 is mounted in engagement with the inside faces of the two housing sections 10, 11 to actuate the housing sections if the transducer is operated as a speaker sending out sound waves, and to be actuated by the housing sections if the device acts as a microphone. In both types of action the housing sections function as cantilever beams. When the transducer is a microphone there is uniform loading on the outside face of each housing section transferred to the piezoelectric element located at the thicker central section area of each section. When the transducer is a speaker expansion and contraction of the piezoelectric element drives each of the housing sections from the thicker central area, causing the housing sections to move as true pistons. Since the housing sections are, in effect, cantilever beams, the stiffness at the peripheral edge does not have to be as great as at the central loading point.
It is desirable to have the piezoelectric element engage the two housing sections at an optimum location along the diameter of the unit, as shown in FIGURE 4. If the housing sections were infinitely stiff it would make no difference whether they were driven from the center or from the peripheral edge. However, even though they are stiff due to their thickness and due to the material from which they are made, they are not infinitely stiff. Consequently, a tubular piezoelectric unit is employed, having a diameter which is about 1/ 3 to 1/2 the diameter of the housing section, and it engages the housing sections at spaced apart locations on the diameter thereof.
Ideally, if the housing walls were of uniform thickness and there was not a center bolt 30 to stiffen the central area the contact locations should be at the l/ 3 and 2/ 3 points along the diameter. However, since it is preferable to make the housing sections thinner near the peripheral edge, and since the mounting bolt 30 does stiffen the central section even though it is made of plastic, the contact areas should be a little less closer to the center than the l/ 3 and 2/3 points along the diameter.
The piezoelectric transducer 20 preferably is a tubular section of polarized ceramic material, for example barium titanate, lead zirconate titanate or other such material. A single tube of the material may extend across the cavity defined by the two dish-shaped housing sections and contact both inside faces in driving engagement, or as shown, two or more separate rings 21, 22 may be stacked together to form the tubular element. The advantage of separate rings lies in the fact that they can be electrically connected together in series, or in parallel, or a larger number of rings may be used and connected in series-parallel. The advantages and disadvantages of such connections are known in the art. The tubes or rings 21, 22 should be electroded, polarized and connected together so that the tube expands and contracts in a direction parallel to its axis. The tube is mounted between and is held by the housing sections so that its axis is parallel to the axes of the dish-shaped housing members.
Fastening means such as bolt 30 extend through both housing sections 10, 11 and through the tubular piezoelectric element 20 to secure the two housing sections together and to apply a compressive bias on the piezoelectric element. The bolt 30 perferably is made of plastic material such as nylon or the like so that its stiffness to expansion and contraction is small compared to the stiffness of the piezoelectric element 20. The low stiffness or high compliance of the bolt compared to the transducer element 20 causes the element 20 to drive the housing sections 10, 11 with higher amplitude than would otherwise be the case if the bolt were made of steel, and facilitates a true piston action of the sections 10, 11. If the bolt were made of steel the centers of the two housing sections 10, 11 would virtually be fixed thereby preventing a close approach to true piston action.
For maximum strength it is advantageous that the fastening means actually bolt the housing halves together.
However, if maximum strength is not an important factor, the fastening means may comprise the tubular piezoelectric element cemented to both housing halves. For example, a thin layer of epoxy resin may be applied between each end of the tubular element means and the housing halves, and if the element means is made up of several rings, as shown, the several rings may be cemented together by epoxy resin. Such a structure has the added advantage that no energy is lost in stretching and compressing a bolt which may be used to connect the unit together.
The bolt 30 holds the two housing sections together and the transducer element 20 limits the inward position of the housing sections. It is important that the length of the piezoelectric element be related to the depth of the two dish-shaped housing sections so that after assembly the peripheral edges 14, of the housing sections do not engage each other, the piezoelectric element holding them apart to establish a gap 35. This is desirable because the element 20 vibrates about its neutral position, expanding and contracting under the influence of an alternating force applied to the housing or an alternating voltage applied to the element. The distance 35 should be just slightly more than the element 20 is expected to contract during its piezoelectric action so that the peripheral edges of the housing sections do not touch. If they do touch substantially true piston action is destroyed.
Sealing means 36 such as a rubber band is applied around the periphery of the housing to keep out dirt and water. The sealing means should be compliant to reduce to a minimum its effect on the vibration of the two housing sections.
While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.
1. An acoustic transducer comprising, in combination, first and second substantially dish-shaped housing sections each of given thickness and formed of material such that when it is excited at acoustic frequencies in the range up to at least '15,000 cycles per second each housing section moves as a piston, tubular piezoelectric ceramic element means of the expander type located between and in driving engagement with both of said dish-shaped housing sections for transducing motion in a direction parallel to the axis of said housing sections, fastener means extending through said tubular element means rigidly fastening said two housing sections together faceto-face defining a cavity containing said piezoelectric element means and with said piezoelectric element means limiting the motion of said housing sections toward each other whereby there is applied to said piezoelectric element means a compressive bias, the length of said piezoelectric element means in the direction of its expansion and contraction being related to the depth of the dishshaped housing sections to establish clearance between the peripheral edges of the two housing sections whereby during the piston motion of the two sections they are free of engagement with each other, the stiffness of said fastener means in tension being low compared to the stiffness of said piezoelectric element means, and compliant sealing means sealing around the peripheral edges of said two housing sections.
2. An acoustic transducer as set forth in claim 1, further characterized by said tubular piezoelectric element means being in contact with both of said housing sections.
3. An acoustic transducer as set forth in claim 2, further characterized by said fastener means comprising a plastic bolt.
4. An acoustic transducer as set forth in claim 3, further characterized by each of said housing sections being formed of aluminum, and by the spacing between the peripheral edges thereof being on the order of .005 or less.
5. An acoustic transducer comprising, in combination, first and second substantially round dish-shaped housing sections each of a given thickness and formed of material such that when it is excited at acoustic frequencies in the range up to at least 15,000 cycles per second each housing section moves as a piston, piezoelectric ceramic element means of the expander type shaped as a tube located between and in driving engagement with both of said dish-shaped housing sections for transducing motion in a direction parallel to the axis of said housing sections and parallel to the axis of said tubular piezoelectric element, fastener means extending through said tubular element fastening said two housing sections together face-to-face defining a cavity containing said piezoelectric element means and with said piezoelectric element means limiting the motion of said housing sections toward each other whereby there is applied to said piezoelectric element means a compressive bias, the length of said piezoelectric element means in the direction of its expansion and contraction being related to the depth of the dish-shaped housing sections to establish clearance between the peripheral edges of the two housing sections whereby during the piston motion of the two sections they are free of engagement with each other, the stiffness of said fastener means in tension being low compared to the stiffness of said piezoelectric element means, and compliant sealing means sealing around the peripheral edges of said two housing sections, said tubular piezoelectric element means having an internal diameter and an external diameter such that it engages the housing sections at locations about one third of the radius out from the said fastener means.
References Cited by the Examiner UNITED STATES PATENTS 1,526,319 2/25 Chubb 340l0 2,138,036 .11/38 KunZe 340l0 2,895,062 7/59 Abbott.
2,947,889 8/ 60 Rich.
CHESTER L. JUSTUS, Primary Examiner.
KATHLEEN H. CLAFFY, Examiner.