US2490236A - Piezoelectric transducer - Google Patents

Description

Dec. 6, 1949 H. B. SHAPER PIEZOELECTRIC TRANSDUCER Filed June 1'7, 1947 IN VEN TOR.

HARRY B. SHRPER direction, or polar axis.

Fatented l 6, 1949 BIEZOELECTRIC TRANSDUCER Harry B. Shaper, Astoria, N. 2., assignor to The Brush Development Company, Cleveland, Ohio, a corporation of Ohio Application June 17, 1947, Serial No. 755,128

7 Claims. 1

This invention pertains to a transducer device utilizing a piezoelectric crystal exhibiting a'hydrostatic efiect.

By the term hydrostatic piezoelectric eiiect is meant the coupling between a pressure applied uniformly to all sides of a piezoelectric crystal body and an electric field created by this pressure in a certain direction in the crystal which is variously called the electric axis, the polar This eiiect is reversible as the hydrostatic crystal, upon being subjected to an electric field parallel to the polar axis, is subject to a change in volume which can be communicated to a surrounding fluid medium in the form of acoustic Waves. Piezoelectric efiects due to hydrostatic pressure are mentioned at page 194 in the book Piezoelectricity by Walter G. Cady, published by the McGraw-l-Iill Book Company in 1946.

The particular advantages of the hydrostatic effect in the construction of microphones and related acoustic instruments have been described, and tourmaline, tartaric acid and cane sugar crystals have been used in piezoelectric pressure g-ages. The use of the crystal tourmaline is severely limited :by the small suppl of natural crystals of useful quality as well as by the comparatively small value of its piezoelectric modulus for hydrostatic pressure, which modulus is 2.4-- coulomb/Newton. The hydrostatic piezoelectric modulus for tartaric acid is about twice that of tourmaline but is still too low for wide-scale practical applications, and the modulus for cane sugar is even lower than that for tourmaline. In the past transducers have been made incorporating the aforementioned hydrostatic crystal materials but they have not utilized the piezoelectric eiiccts of the various crystal materials to the fullest extent possible, and this factor coupled with the low hydrostatic piezoelectric efiect has limited the usefulness of such transducers.

An object of the invention is to obviate the disadvantages of the prior art transducers for hydrostatic piezoelectric crystals.

A further object is to improve on the design of transducers utilizing hydrostatic piezoelectric crystals.

An object of this invention is to provide a transducer whose construction is such that the maximum possible output with any given hydrostatic crystal material is obtainable.

It is an object of this invention to provide a transducer device which utilizes to the fullest extent the hydrostatic piezoelectric eifect.

Another object of the invention is to provide a transducer device particularly adapted for measuring high frequency underwater acoustic vibrations.

In accordance with the invention there is provided the combination of hydrostatic piezoelectric crystal element means supported and enclosed by means comprising material having a given acoustic impedance in effective vibration transmitting. contact with at least one face of the crystal means, and the other faces of the crystal means are in direct vibration transmitting relationship only with material which has substantially the same given acoustic impedance.

Other objects and a fuller understanding of the invention may be had by referring to the specification, drawing and claims.

Fig. 1 of the drawing is an isometric view of a typical hydrostatic piezoelectric crystal; Fig. 2 is an isometric view showing a piezoelectric element obtained from the crystal of Fig. 1; Fig. 3 is an isometric view of a plurality of plates shown in Fig. 2 interconnected to form a hydrostatic body; Fig. 4 is a sectional view through a transducer utilizing .the crystal body of Fig. 3; Fig. 5 is a sectional view taken alon line 55 of Fig. 4; Fig. 6 is a sectional side view of a portion of the transducer shown in Fig. 4; Fig. '7 is an isometric view of a portion of the transducer; and Fig. 8 is a sectional view showing a modified form of the invention.

In the construction of the transducer shown in the drawing any hydrostatic piezoelectric crystal may be utilized. However, it has been found most advantageous to utilize the crystal lithium sulfate monohydrate (Ll2SO4'H2O) particular cuts of which are shown, described and claimed in application Serial No. 755,167 filed concurrently herewith in the name of Hans Jaife and assigned to the same assignee, as that material has a hydrostatic piezoelectric efiect far superior to any material theretofore known.

In Fig. 1 there is shown a typical crystal of lithium sulfate monohydrate. The crystal is of the monoclinic crystal system and the polar crystal class, and may be either a right" or left hand crystal. Illustrated is a right hand crystal, and the axes have been so defined that the sign of the most important piezoelectric modulus dzz is positive.

The crystallographic axes a, b, and c have been chosen on the basis of the unit cell dimensions a% determined by Ziegler (Zeitschrift f. Kristallographie Volume 89, 1932). The coordinate axes X. Y and Z are correlated with the crystallographic axes in accordance with the nomenclature recommended by the Committee on Piezoelectricity f the Institute of Radio Engineers for the monoclinic polar crystal class to which lithium sulfate monohydrate belongs.

Hydrostatic crystal bodies are obtained from the crystal by cutting blanks having faces perpendicular to the Y axis of the crystalline material, as shown by the crystal blank ID in Fig. 1, and plates II are cut from the blank It). It is preferable that this crystal blank I 0 be plate-like as transducers usually use a number of plates cemented together in 'face-to-face relationship to increase the capacity of the transducer, as is well known to the art.

The crystal plate H to be cut from the blank I0 may have any configuration, and if it has straight sides they may be at any desired angle to the X and Z axes of the crystalline material.

The bodies cut from the crystal shown in Fig. l are electroded by applying to those faces which are substantially perpendicular to the Y axis thin layers of graphite, gold, silver, or any other electrically conducting material indicated by reference character l3, as is well known to the art. Leads M are connected to the electrodes to collect the electric charges produced by a polarization parallel to the polar axis or conversely to apply the external field to the crystal body with a component of the field parallel to the Y-axis.

A Y-cut plate of lithium sulfate monohydrate having one pair of edge faces perpendicular to the X axis and another pair perpendicular to the Z axis has a piezoelectic modulus (122 of 16.0- coulomb/Newton for a pressure parallel to the polar axis. For lateral pressures the moduli are, respectively: dz1=4.0 and (in: 1.510 coulomb/Newton. When pressures in all three of these directions are simultaneously applied to the plate, i. e. hydrostatic pressure, the summation of the values of the three moduli expresses the value of the hydrostatic piezoelectric modulus: 16+1.54.0=13.5-10- coulomb/Newton.

Figs. 4 to 7 illustrate a practical transducer design utilizing a plurality of Y-cut plates H of lithium sulfate monohydrate connected together in face-to-face relationship with opposite polarity to form substantially a cube 20of crystalline material. The leads l4 are connected together and leads I5 are connected together so that the several crystal plates II are electrically in parallel. This construction reduces the voltage output and increases the capacity of such a transducer used as a microphone compared to a solid cube of crystalline material, as is known to the art. It is not essential for some ,transducers that the plurality of plates N form a cube but it is advantageous where a non-directional transducer is desired for use in a frequency range where the length of the wave in the surrounding medium is comparable to the greatest dimension of the plurality of plates. These advantages are also known to the art.

The complete assembly of Figure 4 is designed particularly for use in water from low audio frequencies up to about 100,000 cycles per second. The crystal cube 20 is connected by an adhesive such as rubber cement to the top surface of a block 2| of rubber or th like having an acoustic resistance comparable to the acoustic resistance of water. The bottom surface of the rubber block 2| is cemented to the top surface of a cap 22 and the bottom surface of the cap 22 is cemented to a long hollow tube 23. The cap 22 and the tube 23 may be formed of any hard material such as a phenolic condensation product such as that sold under the trade name Bakelite, and any suitable cement may be used to secure them together. The cap 22, shown isometrically in Fig. 7, comprises a base portion with integral upstanding edge portions 24 to the top surface of which the rubber block 2| is cemented. A broad and deep groove 21 extends through the upper portion of the cap 22 leaving a thin section 28'of material at the bottom of the groove through which rivets 29 extend. Lead connection lugs 30 are held to the cap 22 by the rivets 29, and wires 3| from an amplifier are connected to the lugs 30. The lugs 30 extend out of the groove 21 and are soldered to the crystal leads l4, l5 to c0mplete a circuit from the crystal 20 to the amplifier (not shown). The broad groove 21 also permits sound waves to impinge directly onto the back face of the block 2| of rubber through which they pass to the crystal block 20 with minimum energy loss.

The tube 23 which supports the crystal 20 is positioned inside a hollow metal tube 35. Near the upper end of the metal tube 35 there is an externally threaded mounting member 36 which is soldered to the metal tube 35, and the mounting member 36 includes at its upper end a thin ring portion 31 which surrounds the upper end of the metal tube 35 with a small space between them. The lower edge of a rubber cover member 39 is positioned in the space between the ring 31 and the upper end of the metal tube 35, and the ring 3'! is. grooved at 38 to cause the rubber cover to be tightly connected to the tube 35. An internally threaded collar 40, to which is connected a screen 4|, is screwed into engagement with the mounting member 36 to position the screen 4| around the rubber cap 39. The screen is sufficiently stiff to protect the rubber cover 39 and the crystal within from damage. A base member 45 is soldered to the tube 35 near the lower end of the tube to effect a fluid-tight seal. The base member 45 is in the form of an inverted cup havinga broad flat lip. Connected to the tube 23 by screw 41 is a collar, at the upper end of which is a broad circular ring 46. Screws 49 extend through the ring into the base 45, thereby positioning and holding the tube 23 within the metal tube 35. The cap member 53 which is cup-shaped, having a central partition 55, is connected against the broad lip of the base 45, and fluid sealing means 52 are positioned therebettveen in order to prevent liquid from leaking out between the base 45 and the cap 53. A plurality of screws 5| (only one of which is shown) hold the cap tightly against the base 45. Within the hollow 54, formed by the cup in the base 45 and the cupin the cap 53, the crystal lead wires 3| are connected to two wires 56 which extend through the partition 55. Glass seals 51 insulate the wires 56 from the cap 53 and provide a fluid-tight seal around the wires. The partition 55 has a threaded opening through it, and after the transducer has been assembled a fluid, such as castor oil, is put in through the opening 58 to completely fill the volume inside the rubber cap 39, inside the hollow tube 23, between the tube 23 and the tube 35 and in the enclosure 54. A plug 59 is then used to close the opening 58. The aforedescribed assembly constitutes the transducer proper, and to the transducer there is connected a preamplifier housing comprised of a tubular member 60 to ascaaee which is soldered a cap 6| which has through it a broad opening 69. The tube 60 with its connected cap 6| is attached to the cap 53 with sealing means 62 between their meeting surfaces by means of a plurality of screws 63 (only one of which is shown). Within the tube 60 is a relatively large open space within which .there is positioned a preamplifier housing 65. The amplifier is within the housing 65, but the details of the circuit are not shown as it does not constitute part of the present invention. The tube 60 has an internal shoulder 56 against which the base 66 of the preamplifier housing is pressed by the large rubber washer 61. A metal closure member 68 screws into the bottom end of the tube 60 to compress the rubber washer 61 and exert pressure against theshoulder 66. A cable extends out through the rubber washer 61 and through the closure member 68 for connection to other devices such as amplifier means which may be utilized with the transducer.

A modified form of my invention is shown in the transducer device of Fig. 8. It comprises a block 10 of material which has an acoustic impedance substantially equal to the acoustic impedance of the medium in which the transducer is to be utilized. For example, if the transducer is particularly adapted for underwater applications, the material may be a certain type of rubber, lucite, paraffin, or other such material having an acoustic impedance substantially equal to the acoustic impedance of the Water. Particularly illustrated in Fig. 8 is the block of rubber it having a hollow interior adapted to receive the body 20 of hydrostatic crystalline material. The rubber is in close contact with all of the faces of the crystal body for effectively transmitting sound vibrations fromthe crystal body 20 to the rubber material ill and thence out into the exterior medium or from the exterior medium through the rubber 10 to the crystal body 2i) depending upon whether the transducer is being utilized as a speaker or a microphone. The electrical leads 3! are brought out through a hole in the rubber body it which is just sumciently large to accommodate the wires. Sealing means are provided comprising a rubber extension ll integral with the body 70 and extending outwardly therefrom. The leads 3i pass through this extension 7 I and a metallic band 73 or other clamping device is applied around the extension ll, squeezing it into close engagement with the wires 3| to prevent the ingress of water to the crystals 20. If desired a thin film of oil may be applied between the faces of the crystal body 20 and the interior walls of the rubber body 70 to improve the transmission of sound vibrations therebetween. By this construction the rubber body 10 comprising a mounting and a housing for the crystal element 20, and all faces of the crystal element are substantially equally subjected to sound pressures.

While the invention has been described with a certain degree of particularity it is to be understood that changes may be made in the parts and their arrangement without departing from the spirit and scope of the invention as hereafter claimed.

I claim as my invention:

1. In combination; 'multifaced piezoelectric crystal means responsive to alternating acoustic pressures existing in a medium around said crystal means and substantially uniformly applied ing electric signal and conversely for establishing an alternating acoustic pressure in the medi-.

um by vib a ions from all of its faces in accordance with an alternating electric signal applied to said crystal means, and means supporting and enclosing said crystal means comprising material having a given acoustic impedance in effective vibration transmitting contact with at least one face of said crystal means, the other faces of said crystal means being in direct vibration transe mitting relationship only with material having substantially said given acousticbimpedance.

2. The invention set forth in claim 1 characterized by said means for supporting and enclosing said crystal means comprising a body in direct contact with all of the faces of said crystal means. a

3. The invention set forth in claim 1 characterized by said means for supporting and enclosing said crystal means comprising a body having an acoustic impedance substantially equal to the said given impedance in direct contact with one face of said crystal means, supporting means connected to said body,'a cap having an acoustic impedance substantially equal to the said given acoustic impedance connected to said supporting means and enclosing said crystal means, and fluid means having an acoustic impedance substantially equal to said given acoustic impedance within the enclosure formed by said cap and in engagement with faces of said crystal means.

4. A transducer comprising, in combination, a long slender rod, a block of material having a given acoustic impedance mounted on one end of said rod, hydrostatic. piezoelectric crystal element means mounted on said block of material, enclosure means surrounding said crystal means and connected to said long slender rod to provide a fluid-tight enclosure around said crystal means, said enclosure means having an acoustic impedance substantially equal to said given acoustic impedance, and liquid means substantially filling said enclosure, said liquid means having an acoustic impedance substantially equal to said given acoustic impedance.

5. A transducer comprising, in combination, base means including a block of material having a given acoustic impedance, hydrostatically sensitive piezoelectric crystal element means mounted on said block, enclosure means connected to said base means for enclosing said crystal means within a fluid-tight enclosure, and liquid means substantially filling said enclosure means, said enclosure means and said liquid means having an acoustic impedance substantially equal to said given acoustic impedance.

6. A transducer comprising, in combination, mounting means having a passageway opening onto one face of said mounting means, a block of material having a given acoustic impedance mounted on said face of said mounting means whereby said passageway communicates with a face of said block hydrostatically sensitive piezoelectric crystal element means mounted on said block, enclosure means connected to said mounting means for enclosing said crystal means and said block within a fluid-tight enclosure, and. liquid means substantially filling said passageway and said enclosure, said enclosure means and said liquid means having an acoustic impedance substantially equal to said given acoustic impedance.

.7. A transducer for use in a liquid comprising,

in combination, a body having a hollow interior and a passageway therefrom to the outside, a

2,490,236 7 multifaced hydrostatically sensitive piezoelectric REFERENCES CITED crystal element means having lead means mounted within said hollow interior with said body :2; g gg fg are m the in effective vibration transmsitting engagement with all of the faces of said crystal element means, 5 UN TED STATES PATENTS the said lead means extending out through said Number Name Date passageway, and means for sealing said body to 2 384 465 Harrison Sept 11 1945 said lead means to prevent the ingress of mois-. 24o2531 Til L112; June; 1946 ture. 2,415,832 Mason Feb. 18, 1947 HARRY B. SHAPER. 10

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