Crystal hydrophone

May 9, 1961

2,983,901

LE ROY C. PASLAY ETAL

CRYSTAL HYDROPHONE

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May 9, 1961

2,983,901

LE ROY C. PASLAY ETAL

CRYSTAL HYDROPHONE

Filed April 16, 1945

Sheets-Sheet 2

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United States Patent Office

2,983,901

Patented May 9, 1961 arrangement disclosed herein, the hydrophone may be designed in a compact unit which is economical to manufacture and which possesses a high degree of sensitivity.

One of the objects is the provision of new and improved 6 means within the hydrophone for increasing the strength of the impulses received and applying the increased signals to a crystal microphone element.

Another of the objects of the present invention is to provide a sensitive hydrophone in which the crystal ele10 ment thereof is relatively small.

Another of the objects is the provision of a hydrophone having new and improved means for controlling the frequency band of the signals to which the hydrophone is responsive.

A still further object is the provision of a hydrophone having new and improved means for preventing the Tern sponse of the crystal microphone element thereof to frequencies of less than a predetermined frequency.

Still another object is to provide a hydrophone having 20 new and improved means for causing the hydrophone to be uniformly sensitive to incoming signals throughout a predetermined range of frequencies.

A further object is the provision of a hydrophone for use for various purposes such, for example, as for con25 trolling the operation of marine mines, torpedoes, seismic detecting devices and the like, which will be rugged in construction, economical to manufacture and which possesses all the qualities of reliability and a high degree of sensitivity in service.

Still other objects, advantages and improvements will become apparent from the following description taken in connection with the accompanying drawings, of which:

Fig. 1 is a front view of the device of the present invention in accordance with a preferred embodiment thereof 35 and showing a fragmentary portion of the casing to which the device is secured;

Fig. 2 is a rear view of the device of Fig. 1 with the cover partially broken away;

Fig. 3 is an enlarged end sectional view in elevation of 4° the device;

Fig. 4 is a view partially broken away of the device with the baffle plate removed; and

Fig. 5 is a fragmentary view in section showing the filler plug and the pair of signal conductors extending _4g from the casing of the device.

Referring now to the drawings for a more complete understanding of the invention, on which like numerals of reference are employed to designate like parts throughout the several views and more particularly to Fig. 1 _δθ thereof, there is shown thereon a casing 10 such, for example, as the casing of the mine to which is secured as by a plurality of bolts 11, a hydrophone indicated generally by the numeral 12. The casing 10 is provided with a circular aperture 13 within which the hydrophone 55 *^{s arran}£^ed. The hydrophone includes a circular support 14 having a raised annular portion 15 adapted to be brought into registered engagement with an annular recessed portion 16 within the casing, within which is disposed a gasket 17 of material suitable for the purpose 60 ^sy^cb’ ^^{0Γ ex}^mple, as rubber, thereby to effect a watertight seal between the support 14 and the casing as the bolts 11 are tightened. A plurality of washers 18 are preferably employed between the support and the heads of the bolts 11 to maintain the hydrophone secured to _6g the casing and in water-tight relation therewith.

The support 14 is provided with a circular recessed portion 19 having a lip 21 arranged thereabout and adapted to be engaged by a flexible diaphragm 22 composed of rubber or any of the synthetic varieties thereof thereby to effect a seal between the diaphragm and the support 14 as the diaphragm is clamped thereto by the clamping ring 23 and screws 24. Preferably, though not

2,983,901

CRYSTAL HYDROPHONE

Le Roy C. Paslay, 4012 University Blvd., Dallas, Tex.; Charles B. Brown, 114 Forrester St. SW., Washington, D.C.; and Nelson N. Estes, Austin, Tex.

Filed Apr. 16, 1945, Ser. No. 588,519

Claims. (Cl. 340—10) (Granted under Title 35, U.S. Code (1952), sec. 266)

This invention relates to a hydrophone and more particularly to a hydrophone adapted to receive underwater signals having signal frequencies within a predetermined frequency band and transmit electrical impulses corresponding thereto in which the strength of the received signals is multiplied by the hydrophone during the reception of the signals thereby to effect , a high degree of sensitivity of the hydrophone.

In devices of this character heretofore devised, it has been the usual practice to employ a diaphragm in communication with the surrounding water of substantially the same area as the face of the crystal microphone in which the diaphragm either is in physical contact with the face of the crystal element of the hydrophone or separated therefrom by a pressure transmitting fluid. These hydrophones are adapted to apply substantially the same pressure to the face of the crystal microphone as the pressure of the signals or impulses received by the diaphragm. Such devices have proved not altogether satisfactory under certain conditions of service when employed with underwater ordnance devices, for example, by reason of the fact that the voltage generated by the crystal microphone in response to weak signals received is insufficient to control the electroresponsive mechanism connected thereto.

In the device of the present invention this difficulty has been overcome by employing a diaphragm of considerably greater area than the pressure surface of the crystal microphone and arranging a movable baffle section or diaphragm of substantially rigid construction intermediate the first named diaphragm and crystal microphone, and filling the space between the two diaphragms and about the crystal with a pressure transmitting fluid, a vent or bypass being provided within the baffle plate to permit a flow of fluid therethrough for a puipose which will become more clearly apparent as the description proceeds. The baffle plate is provided preferably with a pressure surface of substantially the same area as the area of the flexible diaphragm and arranged adjacent thereto, and a second surface of reduced area corresponding substantially to the area of the pressure face of the crystal microphone.

The strength of the pressure impulses received by the hydrophone is thus multiplied and the increased pressure is applied to a relatively small crystal microphone, thereby providing an arrangement in which the electrical signal output of the microphone is greatly increased.

The device of the present invention is also adapted to respond to signals within a predetermined frequency band such, for example, as a range of frequencies from 150 cycles to 15,000 cycles per second, the lower limit of the frequency band being controlled by the size of the by-pass or vent within the baffle plate and the density of the pressure transmitting fluid. By employing a -vent of sufficiently smaller diameter, the lower range of frequencies to which the device is responsive may be extended to a predetermined frequency cut-off such, for example, as 5 cycles per second. Furthermore, by employing the

2,983,901 necessarily, there is provided beneath the heads of the screws 24 a plurality of washers 25. The support also includes an annular recessed portion 26 thereby to provide a baffle plate or diaphragm 27 of truncated conical shape, in the form of a frustum of a cone with a base of 5 substantial breadth and a relatively smaller altitude, concentrically arranged within the recessed portion 26 of the support and integrally connected to the support by a thin annular portion 28 of the support whereby the baffle plate 27 is adapted to be actuated in an axial direction 10 in response to impulses of pressure received by the rubber diaphragm 22 by reason of the thin annular portion 28 which supports the baffle plate. There is also arranged within the baffle plate 27 an aperture 29 within which is arranged a bushing or sleeve 31. The sleeve 31 is se- 15 cured to the baffle plate in any convenient manner as by pressing the parts together and provides a by-pass or vent between the inner and outer faces of the baffle plate for the flow of fluid therebetween as pressure impulses are received by the diaphragm 22, the rate of flow of the 20 fluid being controlled by the size of the opening within the sleeve 31.

The support 14 has secured thereto as by the bolts 32, a casing 33, generally circular in shape, and provided with a well 34 within which is arranged a crystal microphone 25 indicated generally by the numeral 35.

A suitable gasket 36 is disposed within complementary annular recessed portions of the support 14 and casing 33 thereby providing an oil-tight joint therebetween when the bolts 32 are drawn tight, the tightness of the joint ³⁰being effectively maintained by a plurality of washers 37 arranged beneath the heads of the bolts 32. The cavity on the inward side of the diaphragm 27 is partly filled by an arcuately shaped member 38 composed of material suitable for the purpose such, for example, as a material ³⁵known in the trade as Corprene, to which a flexible coating of oil resisting compound has been applied. ;

The crystal microphone 35 may be composed of any material suitable for the purpose such, for example, as ammonium dihydrogen phosphate whereby the frequency ⁴³response of the crystal is preferably independent of the temperature of the crystal and the crystals possess the characteristic of being relatively insensitive to variations in humidity.

The crystal elements comprising the microphone com- 45 prise a plurality of laminae of the crystal material secured together in a unitary structure as by a coating of cement applied thereto and by a length of cord 39 wrapped thereabout. The external electrical connections to the crystal are established by a pair of conductors 41 and 42 secured to the pads 43 and 44 respectively whereby alternate crystals are connected to one of the conductors and the remaining crystals are connected to the other one of the conductors. The crystal microphone is cemented to a pair of plates 45 composed of insulating material suitable for the purpose such, for example, as a material known as linen Bakelite, the plates being of sufficient thickness to cause a predetermined pressure to be applied to the crystal when the casing 33 is clamped to the support 14 and no impulses are received by the flexible diaphragm 22.

The conductors 41 and 42 are connected in any suitable manner to a pair of rods 46, each of the rods passing through a bushing 47 arranged within an aperture 48 within the casing 33 substantially as shown. A gasket 65 or gland 49 is also arranged about each of the rods 46 and adapted to be compressed by the bushing 51 and nut 52 sufficiently to form an oil-tight seal between the rod 46 and the casing. An external electrical connection to the rods 46 is established by the pair of conductors 53 secured thereto.

There is also provided within the casing 33 an annular recessed portion 54 adapted to be enclosed by the cover 55 secured to the casing as by a screw 56.

The conductors 53 extend through an insulated bushing arranged within the casing 33 and are preferably formed into a knot 58 within the recessed portion 54 of the casing, as is well known in the electrical art. There is also provided within the casing 33 a filler hole 59, through which the interior cavities of the hydrophone are filled with a fluid such, for example, as a fluid known in the trade as Dow Coming, having a viscosity which varies but slightly with changes in temperature. A sufficient quantity of this fluid is introduced into the cavities of the hydrophone to force the flexible diaphragm 22 outwardly, substantially as shown in Fig. 3.

A convenient method of achieving this result is to reduce the pressure on the outside of the flexible diaphragm 22 during the filling operation although, if desired, the fluid may be inserted into the aforesaid cavities under pressure. The filler hole 59 is sealed by a gasket 61 arranged between a pair of discs 62 and adapted to be compressed by the plug 63 sufficiently to effect an oiltight seal.

The cover 55 is provided with an aperture 64 through which a quantity Of material known as insulating compound is preferably introduced within the annular recessed portion 54 of the casing 33 sufficiently to form an additional seal for the electrical·terminals and filler hole and additionally to secure the conductors 53 to the casing.

By providing a sufficient quantity of fluid within the cavities of the microphone sufficient to effect a considerable distension of the flexible diaphragm 22 from the baffle plate 27 before submersion of the device within the water, the diaphragm is prevented from striking the baffle plate-while impulses of pressure are received by the device while in a submerged condition.

By providing a baffle plate having a rigid truncated cone-shaped central portion, supported by a thin annular section in such a manner that the base of the baffle plate is disposed adjacent the flexible diaphragm 22 and the reduced area of the plate is in abutting relation with the crystal microphone assembly, an arrangement is provided such that a mechanical advantage is gained by exerting the combined pressure on the base of the diaphragm against-the crystal assembly and thus the pressure per unit area applied to the crystal is much greater than the pressure per unit area applied by the incoming signal to the flexible diaphragm 22. Furthermore, by providing a baffle plate or diaphragm 27 comprising a solid central portion and an annular weakened portion at the outer periphery of the diaphragm of relatively small compliance or resistance with respect to the central portion of the diaphragm, the restoring force applied to the central por50 tion of the diaphragm by the aforesaid thin annular section is sufficiently small as to be negligible in comparison with -the force transmitted from the diaphragm to the crystal assembly;

An arrangement is thus provided in which a chamber 55 65 intermediate the base of the baffle plate and the flexible diaphragm 22 is in communication with a chamber 66 on the opposite side of the plate 27 by a duct formed by the sleeve 31 within the solid portion of the baffle plate. When the pressure applied to the outside of the flexible qq diaphragm 22 is increased, the fluid within the chamber 65 is_; additionally compressed, thereby causing a flow of fluid from the chamber 65 through the duct 31 into the chamber 66 and, conversely, when the pressure against the outside of the flexible diaphragm is reduced, the .fluid flows from the chamber 66 into the chamber 65. The combination of the density of fluid, the size of the aperture 31, and the compliance of the member 38 within the chamber 66 causes a resonant condition at a predetermined frequency, which may increase the degree of re70 sponse of the hydrophone to signals of that frequency and produce a sharp reduction of response to signals having frequencies lower than the resonance frequency. This condition is caused by the difference in the instantaneous pressure of the fluid within the chamber 65 on the base of the baffle plate and the pressure of the fluid within the

2,989,901 s chamber 66 against the inside portion of the baffle plate and the active faces of the crystal which are not normally or but slightly pressed otherwise.

The device is particularly adapted for use with underwater ordnance employing acoustic detecting devices in which it is desired to receive signals of relatively weak strength within a predetermined frequency range such, for example, as the range from 150 cycles to 15,000 cycles per second. The device, however, is also well adapted for use with signals having a different range of frequencies such, for example, as the range from 10 to 18 cycles per second, it being merely necessary to employ a bushing within the baffle plate having an aperture therein of smaller size to provide the desired cut-off characteristic of the device to signals of the lower frequency. Conversely, by employing a bushing within the baffle plate having a larger aperture therein, the device is adapted to cut off signals having frequencies of less than a relatively higher frequency. Furthermore, by employing a pressure multiplying baffle plate having a weakened annular section on the periphery thereof, the voltage generated by the crystal assembly is increased in proportion to the ratio of the multiplication factor of the aforesaid baffle plate and thus is made substantially equal to the sensitivity and energy output of a crystal assembly of much greater size and having substantially the same pressure surface area as the exposed area of the flexible diaphragm in communication with the surrounding water.

While the invention has been described with reference to a specific embodiment which gives satisfactory results, it will be understood by those skilled in the art to which the invention appertains, after understanding the invention, that various changes and modifications may be made without departing from the spirit and scope of the invention, and it is our intention, therefore, in the appended claims to cover all such changes and modifications.

The invention herein described may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.