US2497680A

US2497680A - Piezoelectric crystal unit

Info

Publication number: US2497680A
Application number: US519069A
Authority: US
Inventors: Massa Frank
Original assignee: Brush Development Co
Current assignee: Brush Development Co
Priority date: 1942-03-26
Filing date: 1944-01-19
Publication date: 1950-02-14
Anticipated expiration: 1967-02-14
Also published as: GB600248A

Description

Feb. i4, E950 F. MASSA 2,497,680

PIEzoELEcTRIc CRYSTAL UNIT Original Filed March 26, 1942 2 Sheets-Sheet 1 mvEN-rox. fa 4M maar ATTORNEY.

Feb. 14, 1950 F, MASSA 2,497,680

PIEZOELECTRIC CRYSTAL UNIT Original Filed March 26, 1942 2 Sheets-Sheet 2 34 e5 sur4 IN VEN TUR FRAN/r Massa A r rnHNEy Patented Feb. 14, 1950 PIEZOELECTRIC CRYSTAL UNIT Frank Massa, Cleveland Heights, Ohio, assigner to The Brush Development Company, Cleveland, Ohio, a corporation of Ohio Original application March 26, 1942, Serial No. 436,304. Divided and this application January 19, 1944, Serial No. 519,069

9 Claims.

This is a division of my application Serial Number 436,304, now abandoned.

My invention pertains to piezoelectric crystals and piezoelectric devices.

An object of my invention is to provide a piezoelectric crystal assembly capable of being used with greater efciency.

Another object of my invention is to provide la. piezoelectric crystal assembly which will not is simultaneously subjected to alternating pres- Figure 1 is a sectional plan view of a device utilizing the invention taken along line I-I of Figure 2.

Figure 2 is a sectional side view taken along line 2-2 of Figure l.

Figure 3 is an enlarged isometric view of a `piezoelectric crystal assembly of the expander type embodying the invention.

Figure 4 is an isometric view of another embodiment of my invention showing it as used with twister multiplate crystal means.

Figure 5 is an isometric view showing another form of my invention in use with a clamped bender crystal element.

Figure 6 is an isometric view showing my invention applied to a freely supported bender crystal element.

Figure 'l is an enlarged view showing another manner in which my expander piezoelectric unit may be used.

Figure 8 is a sectional plan view of a modification taken along line 8-9 of Figure 9, and

Figure 9 is a. sectional side view taken along line 9-9 of Figure 8.

In the use of piezoelectric crystals such, for example, as quartz, Rochelle salt or the like for submarine microphone and speaker work, it is desirable to fill the crystal housing with oil, such as castor oil, in order to seal the housing against in gress of humidity. The piezoelectric crystals used, lf they are Rochelle salt, have the characteristic that four faces move with piezoelectric efl'ect. For a given applied polarity, two of the faces move toward each other and the other two faces move away from each other and there is substantially no change in the volume of the crystal. Accordingly, in a speaker having an oil lled chamber containing operating piezoelectric crystals in which the oil is in contact with all four faces which operate piezoelectrically, there may be substantially no change in fluid pressure; and in a microphone if the oil surrounds the crystal completely, any pressure change on one pair of crystal faces will generate a, voltage, and the same pressure change on the other pair of faces will generate a cancelling voltage thus greatly reducing the sensitivity of the device. A single crystal or multiple separated crystals completely surrounded by oil may be inoperative particularly in the region of frequencies in. which the wave length is large compared to the size of the crystal. n

To eliminate or reduce the transfer of undesired alternating pressures to certain of the piezoelectrically active faces of the crystal, I have found that it is desirable that there exist adjacent to these faces, a material or a structure that is relatively compressible compared to the crystal material. This substantially eliminates alternating pressures on the certain crystal sides but applies the static pressure, thereby maintaining the sensitivity of the device at a higher level than would be obtained if a differential static pressure were present on only two of the faces of the crystal.

The use of oil inside the transducer housing establishes under high water pressures an internal pressure on the housing which enables the transducer to withstand higher Water pressures. In other words, the housing being iuid lled is in effect substantially solid and can withstand high external pressures.

With reference to Figure 1 my invention comprises a plurality of crystal units I Il positioned in a housing of a speaker or microphone adapted to operate under water. Each crystal unit is made of a slab II of piezoelectrically active material such as Rochelle salt, an electrode I2 at one end of the slab, an electrode I3 at the other end of the slab, and two pieces of sheet cork I4, or other such material. A very suitable material has been found to be a'molded composition of cork and rubber. The slab of Rochelle salt I I and the electrodes I2 and I3 are adapted to be a reduction in output.

connected into an electric circuit in the wellknown manner, and are operable when subjected to an alternating current to establish au alterhating mechanical pressure. The units are also operable when subjected to an alternating mechanical pressure to establish an alternating electric potential. Thus the units may be utilized as motors such as in a loudspeaker, or as gcncrators such as in a microphone.

My invention is applicable to both motor and generator devices but because it has already found use as a generator device for underwater microphones, I will describe it in detail with reference to such a device. It is to be understood, however, that it is not limited to such microphone devices. 'Y Y The microphone which is operated by one or more crystal units I is filled with a fluid such as castor oil in order to prevent the ingress of moisturewhich might harm the crystal slabs and in order to enable it to more easily withstand high pressure from being submerged in water.

When a crystal speaker device is subjected to an alternating current, the piezoelectric crystal oi the Rochelle salt type has the property of vibrating in two directions. Accordingly, there are four vibrating faces. In the usual air filled speaker the relative movement between any two vibrating parallel faces is transmitted to a diaphragm and utilized to send out pressure waves, and the vibration of the other two parallel faces does not interfere with the signal. In a substantially incompressible fluid such as oil, however, the vibration of the other two faces may cause considerable distortion in the directional pattern of the transmitted signal, and may cause It is the nature of Rochelle salt type piezoelectric crystals that at any instant, upon being subjected to an alternating electric signal, one pair of parallel faces will tend to establish an increase in pressure to the surrounding fluid, while the other pair of parallel faces will tend to establish a decrease in pressure to the surrounding fluid. When the surrounding fluid is oil which readily transmits the slightest variation in pressure, the oil coupling of all four vibrating faces to the external transmitting uid (such as water) causes loss of low frequencies and loss of desired directional characteristics.

I obviate these undesirable characteristics by fastening decoupling means to one pair of parallel vibrating faces to decouple substantially those faces from the oil in the speaker, thereby substantially preventing the transmission of a signal from those faces. The decoupling means may be a cork sheet I4, such as is shown in Figures l, 2, and 3.

When a crystal microphone device of the oil filled type is submerged in water and subjected to alternating pressures due to audio or superaudio waves, the piezoelectric crystal of the Rochelle salt type has the property of being vibrated in two directions, and has two faces perpendicular to each of the two directions. Accordingly, there are four vibrating faces. In the usual air filled microphone the relative movement between any two vibrating parallel faces is utilized to establish an alternating potential, and the vibration of the other two parallel faces does not interfere with the alternating potential. When the microphone is filled with a substantially incompressible fluid such as oil, however, and a pressure hits all four crystal faces, the vibration of two faces generates a voltage, and

the vibration of the other two faces generates a cancelling voltage. This is due to the oil transmitting substantially equal pressures to all four vibrating faces and materially reduces the sensitivity of the microphone. It is the nature of Rochelle salt type piezoelectric crystals that at any instant. upon being subjected to a pressure on one pair of parallel faces, the crystal will tend to establish a potential of a first polarity. However, when that same instantaneous pressure is applied to the other pair of parallel faces the crystal will tend to establish a potential of a polarity opposite the first polarity. When the microphone operates in a fluid such as oil which transmits pressures substantially instantaneously and equally in all directions, the instantaneous pressure on all four piezoelectrically active faces may cause loss of signal especially in the low frequency range, and loss of the directional pattern in the high frequency range.

The microphone shown in Figures l and 2 is comprised of a metal base portion which has two threaded openings 2| and 22. The base portion 20 may be circular and has an upstanding peripheral edge portion 23, and an annular shoulder 24 near its center. Positioned against the base 2li and against the shoulder 24 is a contact ring 25, and connected to the contact ring 25 by means of rivets 2B is an inside contact strip 2l which has a plurality of contact lingers 28, one for each crystal unit. Connected to the upstanding edge 23 of the base portion 2U by means of rivets 29 is a somewhat similar outside contact strip 30 having a plurality of contact fingers 3| one for each crystal unit IU. The plurality of crystal units ill are positioned with one of the piezoelectric vibrating faces against the base 20, and with electrode I3 against an outside contact finger 3|, and with electrode l2 against an inside contact finger 28.

A cable composed of two Wires 38 and 3l is brought into the microphone through the threaded opening 22, and waterproof connection means 38 are provided for preventing the ingress of moisture around the cable 35. The wires 36 and 3l are connected at one end to an amplifier, and wire 36 is connected at its other end to inner contact strip 21 by means of one of the rivets 28. The wire 31 is connected at its other end to the base 20 of the microphone by means of a screw 39. The electrical path may, therefore, be traced from wire 31 through the base 20 to the upstanding edge 23, to rivets 29, to contactor strip 30, and through contact fingers 3|, to the electrode vI3 of each crystal unit I0. The contact ring 25 as shown is made of an insulating material, or it may be made of metal and insulated from the base 20 to prevent a shunt circuit.

The waterproof connection means 38 is comprised of a cable clamp bushing 40, a washer 4|, and a cable clamp nut 42. The nut 42 threads into the threaded opening 22 and presses the washer 4I against the bushing 40 to effect the watertight seal. A nut 43 may be provided on a threaded extension of the base 2|! for attaching the microphone to a support, and a layer of cork 34 or other vibration absorbing material may be applied to the bottom of the microphone (or speaker) to isolate it from vibrations in the support.

After the microphone has been assembled a rubber cover 44 having a downwardly extending edge portion 45 is placed over the top of the crystal units I0 with the edge portion 45 outside of the upwardly extending edge portion 23 of the metal base 2li. A clamping ring 46 is placed around the outside of the edge portion 45 of the cover s and is pulled tight and clamped by means of a clamp el.

After the cover t@ has been connected to the upstanding edge 23 to prevent the ingress of moisture, the air within the speaker is evacuated and the air is replaced by castor oil admitted through the threaded opening 2|. This is preferablydone in a position with the rubber cover 44 downward, and by using a vacuum pump. When all of the air is out and oil has replaced it, a threaded plug @8 is used to close the opening 2l.

Upon an alternating pressure signal of one potential being applied to two parallel and piezoelectrically active faces of a crystal in a microphone, a voltage is generated. When the same pressure is applied to the other two parallel and piezoelectrically active faces a voltage is generated. If these pressures are applied at the same time the two voltages tend to cancel each other thereby reducing the signal output. Except for the presence of the cork decoupling means I4 on one pair of moving faces the uid within the microphone would apply substantially equal pressures on all four piezoelectrically active faces at substantially the same instant. This is not desirable. Accordingly, I provide decoupling means such as the cork I4 to prevent the pressures (both positive and negative) from being applied to more than one pair of piezoelectrically active faces. "Piezoelectrically active faces may be defined as those faces which move as a result of an applied eld, and also those faces which, when force is applied, cause a piezoelectric charge to develop in the crystal electrodes.

Figure 3 is an enlarged view of one of the expander crystals made according to my invention, and shows the cork I6 connected to two of the four faces which vibrate piezoelectrically when the crystal ls subjected to alternating mechanical pressures, or when the crystal is subjected to alternating potentials. The electrode I3 is shown connected to one of the uncorked faces and another electrode I2 (not shown-in Figure 3) is connected to the uncorked face which is parallel to the face upon which electrode I3 is placed.

For operation as a microphone, the alternating pressure is applied to two parallel faces which are piezoelectrocally active, that is to two faces which cause the generation of an electromotive force upon being moved toward or away from each other, and the voltage appears between the electrodes I2, I3. The other two parallel faces which are piezoelectrially active are decoupled from the surrounding medium by a rubber impregnated cork material I.

For operation as a speaker, the alternating voltage is applied to the two electrodes I2, I 3 thereby causing the two pairs of piezoelectrically active faces to move. One pair of piezoelectrically active faces is covered with the rubber impregnated cork I and is therefore uncoupled from the surrounding medium. The other pair of piezoelectrically active faces is not covered with the rubber impregnated cork I, and accordingly is closely coupled to the surrounding medium. When an alternating voltage is applied to the electrodes I2, I3, the uncovered faces transmit an alternating signal pressure to the surrounding uid, and the covering on the other faces serves to prevent the vibrations from sending a signal into the surrounding fluid.

The lines II. 2 2. and 3-3 illustrate the direction of the crystallographic axes of the unit.

If the line I-I is the (or A) axis, then line 2-2 may be the y (or B) axis and line 3-3 may be the z (or C) axis. If the line I-I is the y axis. then lines 2-2 and 3-3 become the other two axes and the piezoelectrically active faces are at 4.5 degrees with respect to them. If line I-I is the z axis, then lines 2-2 and 3-3 represent the other two axes of the crystal. In any cut of crystal of th-e Rochelle salt type, the piezoelectrically active faces are at 45 degrees with respect to two of the axes. The arrow indicates one direction of expansion and contraction of the crystal, and arrow 5 indicates the other direction of expansion and contraction. It is desired that the amplitude of movement of the crystal faces in direction d be as small as possible, and that the amplitude of movement in direction 8 be as large as possible.

Figure 4 illustrates a twister multiplate flexing element comprised of crystal plates 'I5 and 16, leads TI, and the usual electrodes. The operation of a twister is such that, a voltage will be generated when quadrants A and B tend to move upwardly and quadrants C and D tend to move downwardly, and vice versa. Accordingly, I apply pressure decoupling means on the top surface of the crystal unit in quadrants A and B, and on the bottom surface in quadrants C and D. An alternating wave striking the unit shown in Figure 4 will cause the crystals to twist lin a `direction which will generate a voltage. For a more detailed discussion of the operation of a twister reference may be had to C. B. Sawyers United States Patent Re. 20,680.

Figure 5 illustrates my invention applied to a bender crystal. The crystal unit is comprised of two slabs of Rochelle salt type crystal 1B to which are connected two electrodes 19 and two leads 80. The crystal unit is adapted to be xed at one end as by a clamp 8| connected to a xed base 82. Decoupling means I are applied to one face only of the crystal. When an alternating pressure wave strikes both sides of the crystal, it will be primarily eiective only on the side without the decoupling means, thereby causing the crystal to bend and generate a voltage.

Figure 6 illustrates my invention applied to an unclamped bender element similar to the element shown in Figure 5. In this case three decoupling means I Il are used, one of which is applied over substantially the center portion of one side of the element and the other two of which are applied over the end portions of the other side of the element. If the bender element is suspended in a fluid such as oil, an alternating pressure wave will cause it to bend back and forth thereby generating a voltage.

The muitiplate flexing unit used in the devices shown in Figures 5 and 6 is comprised of two plates 'I8 of piezoelectrically active crystal material cemented or otherwise held together face to face. Electrodes I9 are applied to their thin edges and leads are connected to the electrodes and are adapted to be connected into a signal circuit. Upon an alternating voltage being applied to the electrodes the crystal will bend according to the arrow, and upon an alternating pressure being applied to a fluid medium surrounding the crystal units they will tend to bend according to the arrows and will generate a voltage which will appear across the leads 83.

The action of the unit shown in Figure 4 is more fully described in the C. B. Sawyer Patent Re. 20,213. The action oi the units shown in Figures 5 and 6 is somewhat similar to the action in Figure 4 but differs in that a simple bending action takes place instead of a compound bending action. The Sawyer multiplate element, in' exaggerated terms, may be said to assume a saddle shape whereas the unit shown in Figs. 5, 6 bends primarily in only one direction at any one lnstant. Accordingly, less internal restraint to operation is evidenced bythe new unit.

Figure 7 illustrates another manner in which a plurality of crystal slabs I I may be put together. In this case the crystal slabs are arranged in a line and have a layer of cork |4 in between each crystal. Electrodes I2 and I3 are provided on a pair of parallel faces, and crystal faces 5| and 52 are adapted to receiye the pressure caused by a signal sent through. a medium which contacts either one or both faces 5|, 52. The crystal units shown in Figure '7 are adapted to be positioned in a directional microphone.

Figures 8 and 9 illustrate another form of my invention. The device is somewhat similar to the device shown in Figures 1 and 2 but will be described in connection with a speaker although it is to be understood that it could also be a microphone. l

The loudspeaker comprises a base portion 85 having a raised outside edge portion 86. Positioned between the edges 86, and resting against the base 85, there is a, circular spider 92 which has a plurality of openings into which a plurality of electroded crystals fit. The crystals 88 are arranged in a ring with one of the two unelectroded narrow faces 89 or 90 in engagement with the base 85 and the other unelectroded narrow face extending slightly above the top of the spider 92. The spider 92 is made of some material such as a molded mixture of rubber and cork, or other such material which will serve to decouple the broad faces of the crystal units'88 from the oil in the speaker. A cover 9| is connected at its ydownwardly extending outside edges to the raised edge portion 88 of the base by means of a clamping ring 93 and a clamp 94 to establish a seal. A. threaded opening is provided in the base 85 through which all of the air in the speaker may be withdrawn and replaced by a fluid such as oil which will transmit fluid pressure to the crystals 88 but which will not affect the crystalline substance. The threaded plug 95 seals the oil within the speaker. A cable 96 comprised of two

wires

91 and 98 is brought into the speaker through sealingmeans 99, and wire 98 is electrically connected to the base 85 of the speaker by means of the screw |00. The wire 81 is connected to a spring contactor strip I I. The contactor strip fits in a slit |02 in the top surface of the spider 92 and electrically engages each of the electrodes |03 on the inner ends of all of the crystals 88. The outer crystal electrodes |04 each engage one of a plurality of U-shaped spring clips |05. Each clip has two legs and one of the legs is in engagement with a crystal electrode |04 and the other leg is in engagement with the raised edge portion of the base 85. Accordingly, an electric circuit is established from wire 91 to the spring contactor strip I0 I, to the inner end of each of the crystals 88 through the crystals 88 to each of the U-shaped clips |05, thence to the speaker edge 86 and base 85, and then to the wire 98. Upon an alternating potential being applied across the crystals 88, the crystals will expand and contract in a direction substantially normal to the base 85, and in a direction substantially normal to the planes of the large crystal faces. The expansion in the latter direction will not materially affect the pressure in the surrounding tluid as the two large crystal faces are in engagement with the spider and accordingly 5 are decoupled from the iiuid. The expansion and contraction in the direction normal to the base 85 will cause an alternating pressure to be established in the oil unit within the speaker, and the cover 9| will transmit the pressures to the water outside the speaker. The waterproof cable connection means 99 may be the same type as used in ,Figure 1, and cork insulating means 34 may be positioned on the bottom of the speaker if it is'so desired to isolate any transmission through the speaker housing.

From my description, it will be seen that in a fluid filled transducer the operating characteristics may be improved by providing certain of the crystal faces with an adjacent compressible material. If the compressible material contains trapped air such as in sponge rubber, cork, or the like, then the sensitivity of the device at low frequencies may be substantially the sensitivity of an air filled unit. The structure may be oil filled but the decoupling means prevents sound pressures from being transmitted to all of the piezoelectrically active faces, especially at the lower frequencies. Because of the decoupling means each unit should be more sensitive to lower frequencies, the directional characteristics should be sharper, and the characteristics should be more uniform among various units of the same type which are made by mass production methods. Further, because the decoupling means is effective to transmit direct hydrostatic pressure such as that caused by the device being submerged in Water, it permits the use of the devices at greater depths without the loss of sensitivity which would arise if the hydrostatic pressure were only present on one or two of the piezoelectrically active faces.

In summation, it will be seen that my invention can be applied to piezoelectric crystals of all types; that is, for example, to Rochelle salt, quartz, tourmaline, etc., and that it can be applied to various cuts of crystals of each type. For example, in the Rochelle salt type the invention can be applied to z, y, or z cut crystals (or A, B, C) or to crystals which are cut at various angles with respect to the axes. v

A continuation-impart application Serial Number 669,761, issued Dec. 14, 1948, as Patent No. 2,456,294, has been led'directed to the crystal unit claimed in this application mounted in a transducer with a particularly advantageous orientation.

Although I have described my invention with a certain degree of particularity, it is to be understood that the present disclosure has been made 00 only by way of example and that numerous changes in the details of construction and the combination and arrangements of parts may be made without departing from the spirit and scope of the inventionas hereinafter claimed.

I claim as my invention:

1. A piezoelectric crystal unit adapted to oper- 4' mass of the said crystal and being adapted to be connected in an electric circuit, and means having a lower acoustic impedance than the liquid directly connected to and substantially covering two of said four piezoelectrically vibrating faces to substantially decouple the said two faces from the high acoustic impedance of the liquid.

2. As an article of manufacture, a piezoelectricl crystal element of the expander type characterized by two pair of faces that move due'to piezoelectric action when said element is properly subjected to an alternating electrostatic eld, at least a pair of sheet-like bodies having lower acoustic impedance than saidelement, and means connecting said pair of bodies in intimate and direct contact with one pair only of said faces that move due to piezoelectric action, said sheet-like bodies substantially covering said pair of faces and decoupling them from a surrounding medium when the acoustic impedance of said bodies in lower than the acoustic impedance of the said medium.

3. As an article of manufacture, a piezoelectric crystal element of the multiplate'flexing type characterized by a plurality of faces that move differentially when said element is subjected to alternating pressures, at least two of said faces being adapted to be closely directly coupled to a wave transmitting liquid medium, first vibration decoupling means directly against and covering a portion only of one of said closely coupled faces, and second vibration decoupling means directly against and covering a portion only of another one of said closely coupled faces whereby upon a vibrational wave being present in said liquid medium said faces which are partially covered by said decoupling means are subjected to a differential pressure action.

4. As an article of manufacture, a piezoelectric crystal element of the expander Rochelle salt type having high acoustic impedance and having six faces of which four tend to vibrate due to piezoelectric acion when the said element is properly subjected to an alternating electrostatic field, electrode means on the two of said six faces which do not tend to vibrate due to piezoelectric action, and body means having a substantially lower acoustic impedance than said crystal element lying against and substantially covering two of said four faces which tend to vibrate due to piezoelectric action.

5. An article of manufacture as set forth in claim 4 in which the six faces of said crystal element deine three pairs of parallel faces, said electrode means being on one of said pairs of parallel faces, and sali body means lying against and substantially covering one of said pairs of parallel faces which tend to vibrate due to piezoelectric action.

6. As an article of manufacture, a piezoelectric crystal characterized by a plurality of faces that tend to vibrate when said crystal is4 properly subjected to an alternating signal, electrode means on faces of said crystal which do not tend to vibrate, a sheet of vibration absorbing material connected directly against at least one but not all of said faces which tend to vibrate, said sheet of vibration absorbing material being of suicient thickness to absorb substantially all of the vibratory energy radiated by the said face against which it is connected.

7. As an article of manufacture, a piezoelectri crystal having a plurality of faces from which vibrational energy is radiated when said element is properly subjected to an alternating electrical -eld, electrode means on faces of said crystal which do not tend to vibrate, sheet means of vibration absorbing material, and adhesive means holding said sheet` means directly against at least one but not all of said plurality of crystal faces from which vibrational energy is radiated, said sheet means being substantially co-extensive with said face against which it is held and being of suicient thickness substantially to prevent'radiation of Vibrational energy from said. at least one covered face.

8. As an article of manufacture, a multiplate flexing piezoelectric element of the twister type, sheet means of vibration absorbing material connected to one face of said element at two diagonally opposite quadrants, and sheet means of vibration absorbing material connected to the other face of said element at the other two diagonally opposite quadrants, said sheet means being of sufficient thickness substantially to prevent external vibrational energy from being imposed on said covered quadrants and substantially to prevent vibrational energy derived from the element from being transmitted from said covered quadrants.

9. In combination, a piezo-electric crystal having a front face and a rear face and adapted to be vibrated, and means for cutting off the transmission of vibrations from predetermined areas of the front face, said means comprising vibration absorptive material in the path of vibrations from the predetermined areas.

' FRANK MASSA.

REFERENCES CITED The following references are of record in the le 'of this patent:

UNITED STATES PATENTS Number Name Date Re. 20,680 Sawyer Mar. 29, 1938 1,973,673 Rice Sept. 11, .1934 2,270,902 Rubissow Jan. 27, 1942 2,283,285 Pohlman May 19, 1942 2,406,767 Hayes Sept. 3, 1946