US3222462A - Electroacoustic transducer - Google Patents

Electroacoustic transducer Download PDF

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Publication number
US3222462A
US3222462A US237419A US23741962A US3222462A US 3222462 A US3222462 A US 3222462A US 237419 A US237419 A US 237419A US 23741962 A US23741962 A US 23741962A US 3222462 A US3222462 A US 3222462A
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United States
Prior art keywords
electroacoustic transducer
electrostrictive
plate
diaphragm
vibrating plate
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Expired - Lifetime
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US237419A
Inventor
Karmann Richard
Hoffmann Helmut
Brunnert Otto
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Siemens and Halske AG
Siemens AG
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Siemens AG
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    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
    • G10K9/00—Devices in which sound is produced by vibrating a diaphragm or analogous element, e.g. fog horns, vehicle hooters or buzzers
    • G10K9/12—Devices in which sound is produced by vibrating a diaphragm or analogous element, e.g. fog horns, vehicle hooters or buzzers electrically operated
    • G10K9/122—Devices in which sound is produced by vibrating a diaphragm or analogous element, e.g. fog horns, vehicle hooters or buzzers electrically operated using piezoelectric driving means
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
    • H04R17/00—Piezoelectric transducers; Electrostrictive transducers
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
    • H04R7/00—Diaphragms for electromechanical transducers; Cones
    • H04R7/16—Mounting or tensioning of diaphragms or cones
    • H04R7/18—Mounting or tensioning of diaphragms or cones at the periphery
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
    • H04R7/00—Diaphragms for electromechanical transducers; Cones
    • H04R7/16—Mounting or tensioning of diaphragms or cones
    • H04R7/18—Mounting or tensioning of diaphragms or cones at the periphery
    • H04R7/22—Clamping rim of diaphragm or cone against seating

Definitions

  • the present invention relates to an electroacoustic transducer having as a diaphragm a flat vibrating plate which comprises at least one layer of electrostrictive material.
  • Transducers of this type have been known for a long time. However, they have not achieved any importance in actual practice since their sensitivity is too low as compared with the sensitivity of electroacoustic transducers of conventional type. It has already been attempted to increase the sensitivity of such transducers by taking into consideration the form of vibration caused in case of firm clamping of the vibrating plate, by means of a special arrangement of the electrodes, namely, an interruption thereof at the points of change of the direction of curvature of the diaphragm, and in addition, by reducing the thickness of the plate to less than 0.01 millimeter in order better to match the diaphragm to the characteristic impedance of the surrounding acoustic medium, by applying the electrostrictive material by volatilization to a thin support.
  • Such transducers however, likewise have not gained acceptance up to now. In particular, the application of a homogeneous electrostrictive layer in the necessary thickness, by volatilization, produces considerable difficulties from a manufacturing standpoint.
  • the present invention shows how the sensitivity of electrostrictive transducers of the initially noted type can be increased without it being necessary to reduce the thickness of the plate to such an extent that it becomes difficult to manufacture.
  • Theobject of the invention is realized by smoothing the surface of the electrostrictive material which lies below the contact material, and by holding the vibrating plate at the edge between a fixed support and a resilient support.
  • This arrangement is based upon recognition of the fact that it is not sutlicient for the electrostrictive material merely to be ground, particularly in the case of a
  • the scorings produced by the grinding cause a crowding of the lines of force at the notch-like interruptions in the electrostrictive material, upon the polarization of the material, thus resulting in an excessive heating at these points.
  • Upon unlimited polarization current this will lead finally to a disruptive discharge, while with limited polarization current, the voltage breaks down between the electrodes of the electrostrictive material so that it is no longer sufficient to impress a sufliciently high polarization field strength in the rest of the electrostrictive material.
  • sufiicient polarization important, in addition to a homogeneous electrostrictive material and a uniform thickness of the plate, to provide as smooth as possible a surface of the electrostrictive material below the electrodes through which the polarization voltage is applied.
  • the vibrating plate of the transducer of the present invention comprises preferably two smooth contacted disks of electrostrictive material which are about 0.2 millimeter thick and bonded or soldered together.
  • Such disks can be made by grinding and subsequent lapping, from thicker plates, which can be produced in a partic ularly homogeneous, stress-free and crack-free manner. They have the advantage over extremely thin plates of electrostrictive material that they are easier to manufacted States Patent 'ice ture, that the resistance against breakage is greater, that unavoidable small discontinuities, such as inhomogeneity of the material, inclusions of foreign bodies, hair cracks, etc., exert a smaller pcrcentual effect and that the natural vibration lies in the upper part of the transmission range.
  • an increase in sensitivity is obtained not only by the provision of a surface which is as smooth as possible, namely, having a roughness of not more than 5 microns, resulting in increased polarization, but also by the particular nature of the mounting of the vibrating plate.
  • the vibrating plate oscillates like a plate which rests freely on its edge. For a given thickness of the plate, this means an increase in sensitivity by about a factor of 3.2, as compared with the fixed mounting used heretofore in electrostrictive transducers.
  • the lesser stiffness of the resilient clamping also results in better matching to the characteristic impedance of the surrounding acoustic medium.
  • the metal coatings of the electrostrictive disks do not extend up to the edge thereof, but leave an electrode-free edge zone.
  • the form of oscillation which occurs with the mounting of the diaphragm in accordance with the invention, can thereby be taken into consideration. It was found in this connection that the sensitivity is greatest with a diameter of the metal coatings equal to about 0.91 times the diameter of the annular fixed support. A combination of all the aforementioned measures results in a sensitivity which reaches a value corresponding approximately to that of the sensitivity of electroacoustic transducers of the traditional type.
  • FIG. 1 shows part of an electroacoustic transducer which is of interest in connection with the invention
  • FIG. 2 shows a modified mounting for the diaphragm
  • FIG. 3 shows a further modified mounting for the diaphragm.
  • numeral 1 indicates the housing of the electroacoustic transducer which is closed by a cover- 2.
  • the transducer has a plate-shaped diaphragm 3 which comprises two layers 4 and 5 of electrostrictive material and is held between a rigid support 6 and a resilient support 7.
  • the rigid support is made in the form of a protrusion of trapezoidal cross-section in the wall of the housing.
  • the resilient support is made as a ring of resilient material, such as soft rubber, foam, thermoplast, etc.
  • the cross-section of this ring has in this case also a trapezoidal shape. However, it could just as well be rectangular, since it is to consist of such a soft material that it will not interfere with the.
  • the two electrostrictive layers of the diaphragm are in each case metallized on both sides and bonded or soldered together.
  • the two electrostrictive layers are polarized axially in the same direction and the two metal coatings 8 and 9 which lie against each other and the outer coatings 10 and 11 of the diaphragm are in each case connected with each other and connected with an electrical terminal.
  • the metal coatings do not cover the entire surface of the electrostrictive material but only about nine-tenths of the diameter of the annular stationary support. The edge is thus maintained inactive.
  • the stationary support 12 is made in the form of a sharp-angled protrusion with rounded edge in the wall of the housing.
  • the opposite support consists of a resilient metal ring 13 which is made in desired known manner and rests against the housing cover or is fastened to it.
  • the stationary support is made in the form of a non-yieidable metal ring 14 of circular cross-section, while the resilient support is made in the form of a soft rubber ring 15.
  • An electroacoustic transducer having a diaphragm in the form of a vibrating plate, said plate comprising at least one disk of electrostrictive material, a layer of a con- 7 ductive contact material disposed on eachface of the disk,
  • the surface of the electrostrictive material disposed beneath such contact material having a roughness of at most about 5 microns, and means for supporting said vibrating plate at its edge, said means having a stationary supporting element and a resilient supporting element, between whichelements said plate is disposed.
  • An electroacoustie transducer according to claim 1, wherein the disk of electrostrictive material is about 0.2 millimeter thick and is provided with ground and lapped surfaces.
  • An electroacoustic transducer according to claim 1, wherein the vibrating plate consists of two electrostrictive circular disks which are metallized on both sides and bonded together, and wherein the diameter of at least the two outer metal coatings of the vibrating plate is less than thediameter of the stationary support thus leaving a coating-free outer edge.
  • the resilient support is a ring of resilient material, selected from the class of'materials consisting of soft rubber, foam material. thermoplast and the like.

Description

;lU-;5J4 5 XR 3,222,462 J g g; a7;
R- KARMANN ET AL ELECTROACOUSTIC TRANSDUCER Dec. 7, 1965 Filed Sept. 26, 1962 small thickness of the plate.
Claims. in. 179-110 The present invention relates to an electroacoustic transducer having as a diaphragm a flat vibrating plate which comprises at least one layer of electrostrictive material.
Transducers of this type have been known for a long time. However, they have not achieved any importance in actual practice since their sensitivity is too low as compared with the sensitivity of electroacoustic transducers of conventional type. It has already been attempted to increase the sensitivity of such transducers by taking into consideration the form of vibration caused in case of firm clamping of the vibrating plate, by means of a special arrangement of the electrodes, namely, an interruption thereof at the points of change of the direction of curvature of the diaphragm, and in addition, by reducing the thickness of the plate to less than 0.01 millimeter in order better to match the diaphragm to the characteristic impedance of the surrounding acoustic medium, by applying the electrostrictive material by volatilization to a thin support. Such transducers. however, likewise have not gained acceptance up to now. In particular, the application of a homogeneous electrostrictive layer in the necessary thickness, by volatilization, produces considerable difficulties from a manufacturing standpoint.
The present invention shows how the sensitivity of electrostrictive transducers of the initially noted type can be increased without it being necessary to reduce the thickness of the plate to such an extent that it becomes difficult to manufacture.
Theobject of the invention is realized by smoothing the surface of the electrostrictive material which lies below the contact material, and by holding the vibrating plate at the edge between a fixed support and a resilient support.
This arrangement is based upon recognition of the fact that it is not sutlicient for the electrostrictive material merely to be ground, particularly in the case of a The scorings produced by the grinding cause a crowding of the lines of force at the notch-like interruptions in the electrostrictive material, upon the polarization of the material, thus resulting in an excessive heating at these points. Upon unlimited polarization current, this will lead finally to a disruptive discharge, while with limited polarization current, the voltage breaks down between the electrodes of the electrostrictive material so that it is no longer sufficient to impress a sufliciently high polarization field strength in the rest of the electrostrictive material.
It is for sufiicient polarization important, in addition to a homogeneous electrostrictive material and a uniform thickness of the plate, to provide as smooth as possible a surface of the electrostrictive material below the electrodes through which the polarization voltage is applied.
The vibrating plate of the transducer of the present invention comprises preferably two smooth contacted disks of electrostrictive material which are about 0.2 millimeter thick and bonded or soldered together. Such disks can be made by grinding and subsequent lapping, from thicker plates, which can be produced in a partic ularly homogeneous, stress-free and crack-free manner. They have the advantage over extremely thin plates of electrostrictive material that they are easier to manufacted States Patent 'ice ture, that the resistance against breakage is greater, that unavoidable small discontinuities, such as inhomogeneity of the material, inclusions of foreign bodies, hair cracks, etc., exert a smaller pcrcentual effect and that the natural vibration lies in the upper part of the transmission range. While matching to the surrounding acoustic medium is better in the case of extremely thin vibrating plates, the natural vibration lies in the lower part of the transmission range. As a result, the transmission range drops so greatly in the upper part that upon linearization of the frequency response, the increase in sensitivity obtained by the better matching is necessarily lost.
In the arrangement provided by the present invention, an increase in sensitivity is obtained not only by the provision of a surface which is as smooth as possible, namely, having a roughness of not more than 5 microns, resulting in increased polarization, but also by the particular nature of the mounting of the vibrating plate. The
mounting is so constructed that the vibrating plate oscillates like a plate which rests freely on its edge. For a given thickness of the plate, this means an increase in sensitivity by about a factor of 3.2, as compared with the fixed mounting used heretofore in electrostrictive transducers. The lesser stiffness of the resilient clamping also results in better matching to the characteristic impedance of the surrounding acoustic medium.
For the desired increase in sensitivity, it is also ad visable that the metal coatings of the electrostrictive disks do not extend up to the edge thereof, but leave an electrode-free edge zone. The form of oscillation which occurs with the mounting of the diaphragm in accordance with the invention, can thereby be taken into consideration. It was found in this connection that the sensitivity is greatest with a diameter of the metal coatings equal to about 0.91 times the diameter of the annular fixed support. A combination of all the aforementioned measures results in a sensitivity which reaches a value corresponding approximately to that of the sensitivity of electroacoustic transducers of the traditional type.
The invention will be explained in further detail with reference to the accompanying drawing.
FIG. 1 shows part of an electroacoustic transducer which is of interest in connection with the invention;
FIG. 2 shows a modified mounting for the diaphragm; and
FIG. 3 shows a further modified mounting for the diaphragm.
In FIG. 1, numeral 1 indicates the housing of the electroacoustic transducer which is closed by a cover- 2. The transducer has a plate-shaped diaphragm 3 which comprises two layers 4 and 5 of electrostrictive material and is held between a rigid support 6 and a resilient support 7. The rigid support is made in the form of a protrusion of trapezoidal cross-section in the wall of the housing. The resilient support is made as a ring of resilient material, such as soft rubber, foam, thermoplast, etc. The cross-section of this ring has in this case also a trapezoidal shape. However, it could just as well be rectangular, since it is to consist of such a soft material that it will not interfere with the. movement of the diaphragm regardless of its resting surface against the diaphragm. The two electrostrictive layers of the diaphragm are in each case metallized on both sides and bonded or soldered together. In order that the desired movement of the diaphragm be obtained, the two electrostrictive layers are polarized axially in the same direction and the two metal coatings 8 and 9 which lie against each other and the outer coatings 10 and 11 of the diaphragm are in each case connected with each other and connected with an electrical terminal. The metal coatings do not cover the entire surface of the electrostrictive material but only about nine-tenths of the diameter of the annular stationary support. The edge is thus maintained inactive.
In the embodiment shown in FIG. 2, the stationary support 12 is made in the form of a sharp-angled protrusion with rounded edge in the wall of the housing. The opposite support consists of a resilient metal ring 13 which is made in desired known manner and rests against the housing cover or is fastened to it.
In the embodiment shown in FIG. 3, the stationary support is made in the form of a non-yieidable metal ring 14 of circular cross-section, while the resilient support is made in the form of a soft rubber ring 15.
Changes may be made within the scope and spiritof the appended claims which define what is believed to be new and desired to have protected by Letters Patent.
We claim:
, 1. An electroacoustic transducer having a diaphragm in the form of a vibrating plate, said plate comprising at least one disk of electrostrictive material, a layer of a con- 7 ductive contact material disposed on eachface of the disk,
the surface of the electrostrictive material disposed beneath such contact material having a roughness of at most about 5 microns, and means for supporting said vibrating plate at its edge, said means having a stationary supporting element and a resilient supporting element, between whichelements said plate is disposed.
2. An electroacoustie transducer according to claim 1, wherein the disk of electrostrictive material is about 0.2 millimeter thick and is provided with ground and lapped surfaces.
3. An electroacoustic transducer according to claim 1, wherein the vibrating plate consists of two electrostrictive circular disks which are metallized on both sides and bonded together, and wherein the diameter of at least the two outer metal coatings of the vibrating plate is less than thediameter of the stationary support thus leaving a coating-free outer edge.
4. An electroacoustic transducer according to claim 3, wherein the diameter of at least the two outer metal coatings of the vibrating plate is about 0.91 times the diameter of the annular stationary support.
5. An electroacoustic transducer according to claim 4, wherein the stationary support is part of the housing wall in the form of a projection of trapezoidal cross-section.
6. An electroacoustic transducer according to claim 4, wherein the stationary support is part of the housing wall in the form of an acute-angled projection with rounded edg 7. An electroacoustic transducer according to claim 4, wherein the stationary support is a ring of hard material, said ring being disposed upon a ledge of the housing wall.
8. An electroacoustic transducer according to claim 7, wherein the resilient support is a ring of resilient material, selected from the class of'materials consisting of soft rubber, foam material. thermoplast and the like.
9. An electroacoustic transducer according to claim 6, wherein the resilient support is a metal ring having resilient properties.
10. An electroacoustic transducer according to claim 9, wherein said metal ring is fastened to the housing cover.
References Cited by the Examiner Electronic Industries" magazine, February 1959, p. 72. Radio-Electronics magazine, September 1959, pp. 30-31.
ROBERT H. ROSE, Primary Examiner.

Claims (1)

1. AN ELECTROACOUSTIC TRANSDUCER HAVING A DIAPHRAGM IN THE FORM OF A VIBRATING PLATE, SAID PLATE COMPRISING AT LEAST ONE DISK OF ELECTROSTRICTIVE MATERIAL, A LAYER OF A CONDUCTIVE CONTACT MATERIAL DISPOSED ON EACH FACE OF THE DISK, THE SURFACE OF THE ELECTROSTRICTIVE MATERIAL DISPOSED BENEATH SUCH CONTACT MATERIAL HAVING A ROUGHNESS OF AT MOST ABOUT 5 MICRONS, AND MEANS FOR SUPPORTING SAID VIBRATING PLATE AT ITS EDGE, SAID MEANS HAVING A STATIONARY SUPPORTING ELEMENT AND A RESILIENT SUPPORTING ELEMENT, BETWEEN WHICH ELEMENTS SAID PLATE IS DISPOSED.
US237419A 1961-09-29 1962-09-26 Electroacoustic transducer Expired - Lifetime US3222462A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
DES76040A DE1144770B (en) 1961-09-29 1961-09-29 Electroacoustic converter

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US3222462A true US3222462A (en) 1965-12-07

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US237419A Expired - Lifetime US3222462A (en) 1961-09-29 1962-09-26 Electroacoustic transducer

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BE (1) BE623017A (en)
CH (1) CH415759A (en)
DE (1) DE1144770B (en)
GB (1) GB978202A (en)
NL (1) NL283763A (en)

Cited By (18)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3359435A (en) * 1965-05-04 1967-12-19 James E Webb Holder for crystal resonators
US3453458A (en) * 1965-04-19 1969-07-01 Clevite Corp Resonator supporting structure
US3497729A (en) * 1967-01-20 1970-02-24 Us Navy Mount for acoustic transducers
US3573394A (en) * 1967-09-14 1971-04-06 Ind Scient Research Corp Piezoelectric microphone with biasing means
US3638052A (en) * 1969-09-22 1972-01-25 Dynamics Corp America Electroacoustic transducers of the bilaminar flexural vibrating type
US3683129A (en) * 1968-09-30 1972-08-08 Philips Corp Electroacoustic transducer having a diaphragm made of at least one layer of piezoelectric material
US3708702A (en) * 1970-12-02 1973-01-02 Siemens Ag Electroacoustic transducer
US3721840A (en) * 1971-09-14 1973-03-20 Nittan Co Ltd Sound generator
US4156800A (en) * 1974-05-30 1979-05-29 Plessey Handel Und Investments Ag Piezoelectric transducer
US4240002A (en) * 1979-04-02 1980-12-16 Motorola, Inc. Piezoelectric transducer arrangement with integral terminals and housing
US4246447A (en) * 1979-05-29 1981-01-20 Iec Electronics Corporation Piezoelectric transducer drive
FR2462837A1 (en) * 1979-08-02 1981-02-13 Landis & Gyr Ag ULTRASONIC TRANSDUCER
US4373119A (en) * 1980-12-05 1983-02-08 Motorola, Inc. Adjustable transducer
US4475014A (en) * 1982-09-13 1984-10-02 Harman-Motive Inc. Acoustical transducer
FR2583115A1 (en) * 1985-06-10 1986-12-12 Centre Techn Ind Mecanique ELECTROFLUIDIC TRANSDUCER OF THE NOZZLE / PALLET TYPE AND HYDRAULIC SERVOVALVE EQUIPPED WITH SUCH TRANSDUCER
US4708600A (en) * 1986-02-24 1987-11-24 Abujudom Ii David N Piezoelectric fluid pumping apparatus
US4941202A (en) * 1982-09-13 1990-07-10 Sanders Associates, Inc. Multiple segment flextensional transducer shell
US4965483A (en) * 1988-03-17 1990-10-23 Tdk Corporation Piezoelectric buzzer

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE3013788C2 (en) * 1980-04-10 1982-11-25 Siemens AG, 1000 Berlin und 8000 München Piezoceramic disc for a signal generator to generate an acoustic signal
IT216578Z2 (en) * 1989-05-15 1991-09-16 Face Standard Ind PIEZOCERAMIC CAPSULE FOR TELEPHONE DEVICES.

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
None *

Cited By (19)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3453458A (en) * 1965-04-19 1969-07-01 Clevite Corp Resonator supporting structure
US3359435A (en) * 1965-05-04 1967-12-19 James E Webb Holder for crystal resonators
US3497729A (en) * 1967-01-20 1970-02-24 Us Navy Mount for acoustic transducers
US3573394A (en) * 1967-09-14 1971-04-06 Ind Scient Research Corp Piezoelectric microphone with biasing means
US3683129A (en) * 1968-09-30 1972-08-08 Philips Corp Electroacoustic transducer having a diaphragm made of at least one layer of piezoelectric material
US3638052A (en) * 1969-09-22 1972-01-25 Dynamics Corp America Electroacoustic transducers of the bilaminar flexural vibrating type
US3708702A (en) * 1970-12-02 1973-01-02 Siemens Ag Electroacoustic transducer
US3721840A (en) * 1971-09-14 1973-03-20 Nittan Co Ltd Sound generator
US4156800A (en) * 1974-05-30 1979-05-29 Plessey Handel Und Investments Ag Piezoelectric transducer
US4240002A (en) * 1979-04-02 1980-12-16 Motorola, Inc. Piezoelectric transducer arrangement with integral terminals and housing
US4246447A (en) * 1979-05-29 1981-01-20 Iec Electronics Corporation Piezoelectric transducer drive
FR2462837A1 (en) * 1979-08-02 1981-02-13 Landis & Gyr Ag ULTRASONIC TRANSDUCER
US4373119A (en) * 1980-12-05 1983-02-08 Motorola, Inc. Adjustable transducer
US4475014A (en) * 1982-09-13 1984-10-02 Harman-Motive Inc. Acoustical transducer
US4941202A (en) * 1982-09-13 1990-07-10 Sanders Associates, Inc. Multiple segment flextensional transducer shell
FR2583115A1 (en) * 1985-06-10 1986-12-12 Centre Techn Ind Mecanique ELECTROFLUIDIC TRANSDUCER OF THE NOZZLE / PALLET TYPE AND HYDRAULIC SERVOVALVE EQUIPPED WITH SUCH TRANSDUCER
EP0205381A1 (en) * 1985-06-10 1986-12-17 Centre Technique Des Industries Mecaniques Electrofluidic jet/flapper transducer, and servo valve equipped with such a transducer
US4708600A (en) * 1986-02-24 1987-11-24 Abujudom Ii David N Piezoelectric fluid pumping apparatus
US4965483A (en) * 1988-03-17 1990-10-23 Tdk Corporation Piezoelectric buzzer

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CH415759A (en) 1966-06-30
GB978202A (en) 1964-12-16
DE1144770B (en) 1963-03-07
NL283763A (en)
BE623017A (en)

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