US2495730A - Underwater sound generator - Google Patents

Description

Jan. 31, 1950 [DE UNDERWATER SOUND GENERATOR 3 Sheets-Sheet 1 Filed Nov. 12 1943 Qu et "Mg Jan. 31, 1950 J. M. lDE

UNDERWATER souun GENERATOR 3 Sheets-Sheet 2 Filed Nov. 12, 1943 I Elli F Jan. 31, 1950 J. M. lDE

UNDERWATER scum) GENERATOR 3 SheetsSheet 3 Filed Nov. 12, 1943 HARD SANDY MUD Z50 CF50 Z0 DISTANCE FROM BOTTOM IN FEET --'-50FT MUD 1500 CPS 5 P C O 0 2 Y H E R U a E R P PRESSURE-GRADIENT HYDROPHONE'.

727 DISTANCE FROM BOTTOM IN FEET gwuvwbo'v JOHN MJDE...

Patented Jan. 31, 1950 UNITED STATES PATENT OFFICE (Granted under the act of March 3, 1883, as amended April 30, 1928; 370 O. G. 757) The present invention relates to underwater sound generators, and has as its main object the provision of an underwater generator of low pitch, high power sound, capable of operating the sound responsive devices of acoustic mines at amply safe distances from the searching craft.

Another object is the provision of a sound generator of the above type capable of. producing in water sound waves in low frequency bands ranging from frequencies in the neighborhood of 400 C. P. S. down to near 20 C. P. S. and capable of being mechanically tuned to different resonant frequencies over a range of substantially an octave in the above range of low frequencies Without change in mechanical structure or interference with the driving connections or working position of the device. I

Another object is the provision of an underwater high-power vibrating diaphragm in which cavitational erosion is reduced to a minimum.

Another object is the provision of a mechanically driven underwater sound emitter in which the predominant vibrating surfaces produce additive pressures. V

A still further object is the provision of a high power source of underwater sound, well adapted for calibration of hydrophones, in the low frequency range of 20 to 100 C. P. S.in open water.

Various other objects and advantages of the invention will become apparent from a perusal of the following specification and the drawings accompanying the same.

In the drawings:

Fig. 1 is a rear plan view of the sound generator. I

Fig. 2 is an edgewise view looking at the left edge of Fig. 1.

Fig. 3 is a section on the line 3-3 of Fig. 1 on an enlarged scale.

Fig. 4 is a section on the line 4-4 of Fig. 3 on a reduced scale and omitting all parts except the unbalanced rotor device per se. a

Fig. 5 is a reproduction of a pair of superposed records made with a pressure-actuated hydrophone. a

Fig. 6 is a reproduction'of a pair of comparison records made with different types of hydrophones.

Fig. 7 is a diagram showing pressure control connections. a a

The device comprises in general a pair of oppositely facing sound-generating surface elements, one a substantially rigid base or supporting element H! in the form of a dished diaphragm preferably of metal such as iron and the other a relatively flexible, resilient diaphragm I I. preferably of steel mounted on the supporting diaphragm l0, together with a driving vibrator element l2 rigidly coupled to the supporting diaphragm III for vibrating the latter. The generator as a whole is of general disk shape, present ing an edgewise aspect having an area several times less than that of thediaphragm H or the rear face of the base H1. The base I0 which as a whole constitutes a rigid diaphragm is dished on the side toward the flexible diaphragm to form a shallow chamber [3, Fig. 3, surrounded by an annular mounting rim l4 onto which the diaphragm is clamped by means of a clamping ring l5 and screw bolts IS. A gasket I! of rubber or other suitable material interposed between the mount.- ing rim l4 and the outer margin of the diaphragm, seals the chamber I3. An outer gasket l8 beneath the clamping ring compensates for irregularities in the opposing surfaces of the ring and diaphragm, avoids metal-to-metal contact and, together with the inner gasket I! per-, mits slight movement of the diaphragm between the mounting rim l4 and the clamping ring l5 while maintaining continuous contact. between the adjoining parts. An annular shoulder 19 formed in the base It! at therear of the mount.- ing rim serves to lock the bolt heads 20 against turning, by engagement with a fiat side of the latter thus facilitating the mounting of the dia'- phragm.

To increase the stiffness of the base element l0 without undue increase in weight, suitable stiffening ribs are provided, one in the form of an outwardly facing channel element 2 I extending diametrically across the rear face of the base and others in the form of radially extending angle members '22, all rigidly secured to the rear face of thebase in any suitable manner as by welding. It will be understood, however, that such stifiening ribs or their equivalent may be 09.5 integrally with the base element.

The vibrator element 12 is of the unbalanced rotor type. The one used in the embodiment of the invention here shown is what is known as .a concrete vibrator, usually employed for the stirring or puddling of wet concrete. It comprises a non-rotary casing 23 containing an unbalanced rotor element 24 journaled at its ends in antifriction bearings 25-46 mounted within the cas ing near the ends of the latter. An unbalanced condition of the rotor is established through the use of a filling 21 of heavy material such as lead, within the tubular rotor element 24 to one side of the axis of rotation. The rotor 24 is driven by a flexible shaft 31 extending away from the vibrator element through a flexible housing or hose 38. The filling 21, thus constitutes a movable weight carried by the supporting diaphragm I0, through the bearings 25-46 and movable toward and away from the base element for imparting vibratory motion to the supporting diaphragm l through inertia of the weight.

Clamping lugs 28-29 and caps 3fl3l with through-bolts 32, firmly fix the vibrator element l2 to the base element 10. Countersunk portions 33 (Fig. 3) 0n the inner side of the base element receive the heads 34 of the bolts, the shanks of which extend through the clamping lugs and caps to the tops of the caps where on their threaded ends they receive clamping nuts 35 and lock nuts 36. A suitable sealing material, not shown, preferably soft solder provides a sealed connection between the bolt heads and the countersunk portions which receive them, and holds the bolts in place during assemblage and clamping of the parts together.

This arrangement of the mounting lugs at the ends of the channel-beam permits the vibrator element to be mounted in line with the beam and partly within the channel, and makes for strength, compactness and symmetry in the structure as a while. To afford eiiicient transmission of driving force directly from the bearings of the vibrator element of the base member, the mounting lugs and clamps are arranged to engage with the vibrator in the planes of the bearings 25 and 25. This arrangement also has the advantage that the clamping of the vibrator element takes place where the casing is braced by the sturdy outer rings of the anti-friction bearings 25-2E, thus permitting a firm, tight clamping of the vibrator without danger of distorting the casing.

To enable the natural frequency of the diaphragm to be varied without variation in mechanical structure or interference with the driving connections, means are provided for adjusting the air pressure in the chamber i3 to different values. This comprises an air coupling element 39 connecting the interior of the chamber 13 with a flexible pipe 40 for the admission and release of air.

Any known or other suitable source of air or other gas under pressure, and valved connection of the pipe 48 therewith, not shown may be used. Because of the small volume of the chamber 33 of the sound generator, there would result substantial reduction in pressure of the confined gas upon even an extremely small leakage, and to offset this a suitable ballast tank is maintained in communication with the pipe it as indicated diagrammatically in Fig. '7. Also as here indicated the pipe 40 is maintained in communication with a pressure gauge and exhaust valve.

It is preferable that the diaphragm ll be preformed into a slightly, outwardly-bulged shape as shown, and it has been found that an advantageous method of accomplishing this is to do so after the diaphragm is clamped in place and by air pressure applied to the interior of the chamber 53 to force the diaphragm outward sufficiently beyond its elastic limit to establish the desired permanent set. This method of shaping the diaphragm has the further advantage of avoiding the use of forming dies and permitting the diaphragm to be drilled for the clamping bolts and clamped in place while in the flat condition. It also avoids necessity for any substantial allowance in the original dimensions of the diaphragm for distortion incident to the forming. Guide hooks 4| secured to the base element ill at the outer ends of the four stiffening ribs 22 serve to hold the apparatus in a supporting frame not shown and which may be of any known or other suitable form capable of slight horizontal displacement to permit horizontal oscillatory movement of the base element.

In use, the apparatus is carried by a mine sweeping ship below the hull, preferably lowered into the water through a well or sea-chest in the ships hull. To reduce resistance to movement through the water, the apparatus is positioned in a vertical plane parallel to the ships keel so as to move edgewise through the water with the movement of the ship. It is placed a distance of several feet below the surface of the water depending upon the frequency of the sound waves to be generated and at least a quarter wave length. With the flexible driving shaft connected to a suitable source of mechanical power and the air pipe connected to a source of air under pressure, the apparatus is ready for use. It is not essential to the working of the device to maintain pressure above that of the hydrostatic pressure of the surrounding water between the diaphragm and the base element, a variation in such pressure'being necessary only for varying the reso nant frequency of the diaphragm. Upon actuation of the vibrator element by rotation of its unbalanced rotor, the substantially rigid or stiff base element I0 is set into vibration without flexing and at an amplitude determined by the ratio of masses of the eccentric weight of the rotor and the remainder of the base'with its rigidly connected accessories. The base element In with its rigidly connected accessories on the one hand, and the effective mass of the flexible diaphragm on the other hand, form a pair of weights coupled together by the compliance of the flexible dia-- phragm. By the term compliance is indicated the quality or state of yielding to bending under stresses within the elastic limit. Such a combination has the name tompilz in German, but there is no single English word for it. The system vibrates with a single degree of freedom in which the displacements of the masses are out of phase. Accordingly at resonance the pressures produced in the surrounding Water by the outer face of the diaphragm II and the outer face of the base element I0, are additive, resulting in a substantially omnidirectional field pattern as from a point source, as distinguished from the figure eight pattern of a dipole source.

Because of the periodic application of force by the vibrator in all directions through a complete circle in a horizontal plane, there is, of course, also an edgewise vibration of the generator as a whole, but due to its general flat-disk form the aspect areas of its two side edges are so small relatively that disturbance therefrom is negligible. Inasmuch as the oppositely facing, disk-like generating surfaces operate in phase opposition and are small in diameter relative to the length of the sound wave generated, the net result is in effect that of a point source of periodic compressional waves, of a frequency equal to the fre quency of rotation of the eccentric weight of the vibrator and harmonics thereof. The output may be characterized as polyphonic, a word here used to designate a fundamental frequency accompanied by harmonics the intensity of which decreases with the order. At resonance, that is with the vibrator operating at the natural or resonant frequency of the diaphragm, the greater portion of the energy radiated as sound comes from the diaphragm. It is obvious that the base element I should be made as light as possible in order to obtain maximum vibration amplitudes from the available force, and that for maximum efficiency th diaphragm should have a natural frequency substantially equal to that at which the vibrator element [2 is driven. With the air pressure behind the diaphragm at substantially that of the hydrostatic pressure of the surrounding water, the natural frequency of a given diaphragm is at its lowest. As the pressure in the chamber I3 is increased, resonance becomes broader owing to damping in the air chamber,

while the natural frequency of tne diaphragm rises owing to stiffness added by the compressed air and to tangential or radial tension resulting from the stretching of the diaphragm. For example, in one practical embodiment of the invention an increase in air pressure from lbs. per square inch to 35 lbs. per square inch increases the resonant frequency from 101 to 120 cycles per second, and in another from 50 to 82 cycles per second.

Because of the sturdy structure and mechanical natur of the device, large driving forces may be transmitted efficiently to the substantially rigid base element I0 and from the latter to the edgeclamped diaphragm in opposite phase relation and with minimum undesired modes of vibration so that the sound pressure wave is almost purely sinusoidal and the stresses are so distributed as to reduce likelihood of mechanical failure to a minimum.

There is thus supplied an important desideraturn is acoustic mine sweeping, which is to deliver at a safe horizontal distance from the mine sweep, more sound than any ship is likely to produce in the same frequency range, at a mine located on the bottom directly beneath such ship.

One practical embodiment using a pro-formed, dished diaphragm inches in diameter and approximately one-eighth inch thick, operating in an octave band of 70 to 140 cycles per second at a depth of about 12 feet from the surface of the water, produced an optimum sound-pressure level of 168 decibels above .0002 dyne per cubic centimeter at a point near bottom about 40 feet below the surface and a horizontal distance of 6 feet from the source.

There is thus provided an ample margin of assurance of the actuation of known types of acoustic mines.

Because of the rapid movement of the vibrating parts, particularly the central region of the diaphragm, cavitation takes place at these portions in marked degree with resultant strong tendency to cavitational erosion of the diaphragm in the central zone. Obvious expedients for preventing or reducing erosion such as the use of a reinforcing plate at the center of the diaphragm or a protective coating of rubber were found to be impracticable. The use of a central reinforcing plate spot welded to the diaphragm was short lived due to concentration of cavitation at the welded spots, While the use of a coating of rubber gave protection against cavitation but for a brief period terminated by failure of the rubber, apparently due to internal heating. According to the present invention a solution of the problem of reduction of cavitational erosion to a minimum is achieved in the use of a diaphragm preferably of stainless steel, preformed to provide a central outward bulge as shown in Fig. 3. Such constructlon has been found to disburse the cavitational zone over a larger area with consequent diffusion of erosion and its rate of progress. It was further discovered that operation at a depth of at least 10 to 12 feet is an important factor in reduction of cavitational erosion.

Due to the fact that pressure waves generated in the water by the propulsion means and other machinery of a ship are periodic, it is possible to devise acoustic mines selectively responsive to such pressure waves, even at extremely low frequencies.

The present invention is particularly useful in the firing of such mines and in investigations of the characteristics of wave energies of the kind to which such mines would be set to respond, because of the extremely low frequency compressional waves which may be produced by a generator of the form here disclosed. Mechanical considerations set the lower limit in the neighborhood of 20 cycles per second. One device using a diaphragm of 30 inches in diameter has been operated at as low as 30 cycles per second.

Investigations looking to the design and installation of acoustic mines, sound detection devices and the like are made possible through utilization of the present invention in determining the acoustical characteristics of the bottom of a water-way that is whether acoustically soft, hard or transitional. It has been found that the reflections of periodic sound waves from the top and. bottom bounding surfaces of a water-way give rise to standing wave patterns beneath a ship-carried sound source and that the sound pressure near the bottom, say within a quarter wave length, may represent a minimum, a maximum or an intermediate position in the pattern depending upon Whether the bottom is acoustically soft, hard or transitional. As will be shown later, these differences may be large.

A method of ascertaining the above-mentioned acoustical characteristics of the bottom of a river, bay or other water-way using the sound generator herein disclosed, comprises the setting up of a sound field beneath a ship by operation of a sound generator carried by the ship and placed in the water beneath the ship, preferably a distance of a quarter-wave length below the surface, and not substantially less. although it may be more. During maintenance of such sound field a sound responsive device is raised vertically from the bottom of the water-way from a point substantially directly below the sound generator to near the surface of the water to ascertain the sound intensity at different vertical distances from the bottom below the ship. Preferably the sound responsive device is in the form of a hydrophone and is raised at a substantially uniform velocity'while the sound pressure level registered thereby is progressively recorded to produce a record of sound intensity plotted against distance from the bottom. It is from an observation of the variations intensity at different vertical distances from the b0ttom thus obtained that the standing wave pattern giving the location of zones of minimum and maximum pressures is ascertained, which conditions indicate the acoustical character of the bottom. 7

Typical records for hard and soft bottom using a pressure actuated hydrophone are shown superposed in Fig. 5 Where the dotted-line curve represents a soft-mud bottom and the solid-line curve a bottom of hard sandy mud. The ordinales indicate decibels above .0002 dyne per square centimeter while the abscissas indicate distance from bottom in feet.

The average pressure level gradients shown by the records are much the same. The initial phases of the standing wave systems are such, however, that a pressure-actuated hydroplione placed on the soft bottom (dotted-line curve) would record 16 decibels lower sound level than the same unit placed on the hard bottom (solidline curve) assuming the same soundL source in both cases.

Such knowledge is important where advantageous placement is desired for pressure-actuated hydrophones, acoustic mines and similar devices. Similar records obtained with a velocity or pressure-gradient hydrophone show a behavior, in the response of such hydrophone, the reverse of that of the pressure actuated hydrophone, that is. maximum response will be obtained from a velocity actuated receiving unit near the bottom when placed substantially di rectly on a soft bottom or a quarter-wave length above a hard bottom. Comparison records coinciding as to frequency and variations in depth but made one with a pressure actuated hydrophone and the other with a pressure-gradient hydrophone, are shown in Fig. 6. Discovery of the above phenomena teaches that advantage can be taken of the phase relations of standing waves by the employment of velocity actuated receiving mechanisms in the design of acoustic mines intended to be actuated by sounds from a ship passing directly thereover, in areas where the bottom is known to be predominantly soft and the mine is to be placed substantially directly on the soft bottom or at the water-mud boundary. Such devices will be less critically dependent upon their position near the bottom than sound-pressure actuated units, and their response will not be seriously weakened. by their being covered or partly covered by the mud or silt of a soft bottom. Because the standing wave patterns of diiferent frequencies manifest the same initial phase near the bottom, and because this holds true for a. considerable hori zontal distance from the sound source, it will be obvious that even with a complex wave-form there will be a maximum velocity response at the bottom. On the other hand, in case of a hard bottom the use of pressure responsive devices is indicated.

It will be clear that knowledge of the acoustical character and concomitant physical condition of the bottom of a given water-way may be obtained from records produced as above described.

A convenient method of interpreting such a record curve to ascertain the acoustical character of the bottom is to compare it with a group of theoretical curves computed from a wide variety of hypothetical acoustical conditions, the acoustical condition of the bottom over which a particular experimental record is made being sub stantially that represented by the theoretical curve most closely matched by the experimental record.

While a single embodiment of the invention has been herein described for the sake of disclosure, it is to be understood that the invention is not limited to such specific embodiment but contemplates all such variants and modifications thereof as fall fairly within the scope of the appended claims.

The invention described herein 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.

What is claimed is:

1. An under-water sound generator comprising a rigid vibrating compressional-wave generating diaphragm rigid throughout its radial extent to and including a rigid peripheral margin so that the entire diaphragm and its peripheral margin will move as a unit, a flexible diaphragm carried solely by the rigid diaphragm, said diaphragms being joined at their peripheral margins with the rigid diaphragm forming the main body portion of the assemblage so that the assemblage as a whole forms a disk shaped structure of relatively narrow edgewise aspect, and means for vibrating the rigid diaphragm as a whole to vibrate the flexible diaphragm carried thereby, together with a body of gas between the diaphragms under a pressure in excess of ambient pressure and in communication with a body of gas at the same super ambient pressure and of several times the volume of the space between the diaphragms, whereby vibration of the rigid diaphragm will cause the flexible diaphragm to vibrate degrees out of phase with the rigid diaphragm.

2. An under-water sound generator comprising in combination a rigid vibrating diaphragm, a relatively flexible diaphragm having inertia and compliance and carried solely by the rigid diaphragm, said diaphragms being secured together at their peripheries with their adjacent faces slightly bowed apart to permit the flexible diaphragm to vibrate to and away from the rigid diaphragm and means for vibrating the said rigid diaphragm as a unit periodically to generate compressional waves radiating from its outer surface, said diaphragms operating as a pair of weights yieldingly coupled together through a compliance provided by the compliance of the flexible diaphragm, whereby the flexible diaphragm is caused to vibrate 180 degrees out of phase with the rigid diaphragm and both diaphragms cooperate to project compressional waves in opposite directions.

3. An under-water sound generator comprising a rigid circular compressional-wave generating diaphragm, relatively flexible diaphragm supported solely by the rigid diaphragm, said diaphragms being secured to each other at their peripheral margins with their adjacent faces spaced apart by a body of gas under pressure to form a pair of weights yieldingly coupled together through a compliance provided by the compliances of the flexible diaphragm and said body of gas, means for vibrating the rigid diaphragm, and means for varying the natural frequency of the flexible diaphragm during operation comprising a source of gas under regulated pressur connected through a flexible pipe with the said body of gas under pressure, whereby the flexible diaphragm may be given various definite natural frequencies each representing a definite wave length in water, and vibration of the rigid diaphragm by the vibrating means at the natural frequency will vibrate the flexible diaphragm 180 degrees out of phase.

4. An under-water sound generator comprising a substantially rigid base element forming a circular diaphragm capable of vibrating to generate compressional waves without flexing, a relatively flexible diaphragm supported solely by 9 the rigid diaphragm, said diaphragms being secured to each other at their peripheral margins with their adjacent faces spaced apart to form a pair of weights coupled together through a compliance provided by the complianc of the flexible diaphragm, and a power driven unbalanced rotor mounted on said rigid diaphragm on an axis parallel to the plane of the latter for vibrating the latter, whereby upon operation of the unbalanced rotor the rigid diaphragm will be vibrated without flexing and said flexible diaphragm will be vibrated 180 degrees out of phase with the rigid diaphragm causing the outer faces of said diaphragms to produce in a surrounding body of water, pressures which are additive, resulting in a substantially omnidirectional field pattern.

JOHN M.

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

Number Number 2 223,356 317,994 374,940

10 UNITED STATES PATENTS Name Date Wood Sept. 29, 1914 Hahnemann Apr. 3, 1923 Slichter June 3, 1924 Bois-Reymond Sept. 9, 1924 Hayes July 27, 1926 Hahnemann Dec. 21, 1926 Behm Nov. 15, 1927' Robbins Oct. 8, 1929 Hahnemann Nov. 12, 1929 Schlenker June 14, 1932 Gifford Dec. 17, 1935 Farrell July 27, 1937 Thienhaus Apr. 26, 1938 Sivian June 21, 1942 FOREIGN PATENTS Country Date Germany June 20, 1910 Germany Nov. 24, 1917 Great Britain June 20, 1932

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