US2473971A - Underwater transducer - Google Patents

Description

D. E. Ross UNDERWATER TRANSDUGER IFiled Feb. 2s, 1944 June `21, 1949.

Patented `lune 21, 1949 UNITED STATES if!vv NT OFFICE UNDERWATER TRANSDUCER Application February 25, 1944, Serial No. 523,887

1 Claim. 1

This invention relates to fan-underwater transducer, and itis an improvement on-the invention set forth in application Ser. No. 129,640, led March `8, 1937, patented January 27, 1948, No. 2,434,926.

For many years past, `the ,problem of case radiation and visolation Ahas vbeen one of great importance in the construction of underwater electro-mechanical transducer. Because of the manner of use, rigid construction is necessary and this has been largely responsible for causing such radiation. Particularly in the construction of .piezoelectric crystal units. where the crystals are lmounted on backing plates of relatively large mass, vibrations have been transmitted to the Vtransducer housing and in turn into the water. The result in any particulartype vis a large loss in energy and eiiiciency in the case of a radiator, and a poor signal to noise ratio in a receiver. It is thus one of the objects of '.theinvention to provide a transducer of simple .construction in which the motor assembly, composed of the vibrating elements and Aany associated backing plate, is insulated and isolated from the transducer housing.

The problem kma-y be better understood by considering an ideal Atransducer (radiator) as a piston operating in a xed,rimmovable balile. This analogy `is well known in the art, and the overall response of such a unit isdue .entirely to the ac- `tion of the piston. However, 4in practice, it is found that units only approach this ideal and case or housing radiation produces an additional pressure in the sound field over and above that produced by the piston. In other Words, every actual unit, to some extent, exhibits an overall radiation pattern which is due to both vibration of the elements (the piston) and vibration of the case. Itis desirable to reduce the case radiation as Amuch as possible so that the unit produces a `pattern, with its major and minor lobes, which approaches as nearly as possible that produced vby the ideal unit, It islthe object `of =the invention to produce a unit which has a radiation pattern which very closely approaches this ideal by isolating the case or housing from the vibrating ele ments. The resulting response curve is thus largely undisturbed by the Yvibrations of the case which are normally present. This argument -applies with equal force to receivers byvirtue of the principle of reciprocity.

The solution to the-above stated problem has become of greater importance as the requirements for `directive transducershave ybecome more exacting. -A`s used here; 1-directivity for fthe unit refers not only to the width of the major lobe, but to the ratio of the response level in a certain desired direction to such level Ain fother directions in which high response is undesirable.

In order that such directivty may be predicted and achieved, means must 'be provided toy reduce such response in all but the desired direction (or directions) and it is one of the lprimary objects of the invention to provide such means. Experiments have shown that this is possible and that the response in such desired directions may -loe made to exceed by over 50 db. the response in other directions.

The invention consists primarily in supporting the motor assembly with `an insulating material of low acoustic impedance in such a manner that such material entirely surrounds such assembly, except in the directions in which high response is desired.

It has likewise been found -that the undesirable effects produced by water noise, reverberation and cavitation (particularly at high speeds) are greatly reduced and that the sensitivity :is correspondingly increased.

An additional advantage of this invention is its ease of construction and assembly, particularly as compared with other types of Vunits which provide an air chamber adjacent the backing plate( Not only is the chamber unnecessary, but only one side of the backing plate need be nished or machined in the present construction.

And an even further advantage of the :present invention is that since no bo1ts,.supports,..etc.,.ar.e used to mount the motor assembly, acoustic paths, through which vibrations might travel from the motor to the housing, and vice versa, are eliminated.

In the drawings:

Figure l is a side View, partially in section.

Figure 2 is a front view, partially in section.

As has been stated, the invention consists .primarily in means for isolating and insulating the motor assembly from the case .or housing. Any material used for this .purpose must be .characterized by low acoustic impedance whereby sonic energy is reflected from, rather than .transmitted by, it. For this purpose, cork and .Corprene (a mixture of cork and neoprene, prepared by the Armstrong Cork Works) have been foundto ibe very acceptable. Among other material which satisfy this criterion are cork composition materials, Foamglas (a patented material manufactured by the Pittsburgh Corning Corp), bre board, sponge rubber (with non-communicative Acells), and -interspaced wire mesh -All yoirsu'ch 3 materials provide poor acoustic impedance matching with metal, piezoelectric crystals, castor oil, and other materials which go to make up the transducer.

The invention may be used with a variety of transducer units and the particular type disclosed in the drawings is not intended as the only type of unit claimed. It has been chosen for illustrative purposes, while the invention claimed comprises any such use of insulating means.

The piezoelectric crystal transducer illustrated in the drawings comprises a housing, generally designated l, which may be of a metal such as steel, meehanite, bronze, brass, etc. It may be of any desired shape, although the one illustrated is square. The housing l is formed with a large central recess bounded by walls 3 and a back 4. Centrally of the back Il is an upstanding circular ilange 5 which is adapted to receive and support a stufng box 5 by means of bolts l. The adjacent portions of the back fi of the housing and the stuiing box 6 are recessed to form a terminal box 8 in which cable connections may be made. This is made watertight by a rubber gasket seal 9. The stuffing box is internally threaded at its outer end to receive a threaded nut it* which bears on a rubber gasket Il.

Centrally of the ange 5 is a hole extending through the back #i of the housing. A small plate i2 is adapted to nt over this hole and is held in place by bolts E3. This plate supports two conducting lead pins it, by means of insulators l and a rubber washer Il'.

Cable i8 enters the terminal box S through nut i6 and is there soldered to the lead pins i4, I5, which are in turn connected to crystal leads inside the housing l, as will be described. It is thus noted that cable connections may be made or changed within the terminal box 8 without dismantling the entire' unit resulting in emptying and refilling it with oil. The box 8 is maintained water and air tight by means of gasket H, seal 9 and washer Il.

Four bosses i are positioned externally of and at the corners of the back 4 of the housing and are internally threaded to receive mounting studs, not shown.

In the unit illustrated the motor assembly, generally designated 2B, comprises a bank of piezoelectric crystals '2| forming a at vibrating face, and a backing plate 22, to which the crystals are secured. This backing plate 22 may be of metal (such as steel or meehanite), Bakelite, glass, Lucite, concrete, plaster-of-Paris, porcelainized metal, ceramic material, Transite (a patented material sold by Johns Manville Co.) or any other suitable material with desirable characteristics. On two of the edges of the backing plate 22, supports 23 are mounted by means of bolts 24, and these, together with small pieces of Corprene used as spacers between the crystals, serve to hold the crystals in fixed position with respect to the backing plate 22.

No supports arey used on the other edges of the backing plate 22, but the plate itself extends slightly beyond the crystals. This provides a space in which the electrodes 25 connected to the crystals 2| may be brought out and soldered to lead wires 21.

The motor assembly 20 is designed to fit into the recess of housing l, but is formed so that its overall dimensions are from 1/2" to 11/2" less than the dimensions of the recess. Before the motor 20 is placed in position, the recess is lined with layers of cork, Corprene or any of the other described suitable materials. Thus, four sheets 28 are snugly itted against the sides of the recess, and another sheet 29 is placed on the bottom of the recess in such a manner that it is held snugly in place by the side sheets 28. The motor likewise fits snugly into this lined recess and two strips 38 of the same insulating material are positioned along the outer or upper edges of supports 23 and are supported by sheets 28 and supports 23. It bears directly against a small flange on the upper end of walls 3, which ilange also helps to keep sheets 2@ in position. The housing is closed by a metal frame' 32 to which has been vulcanized a relatively thick sound transparent window 33 formed of a material, such as rubber, whose acoustic impedance does not diier substantially from that of water. The frame is held in position by bolts 3A and the closure is made water and oil tight by a rubber gasket seal 35 of the same type as seal This gasket seal is of particular interest since it consists simply of a rectangular grooved recess formed in the flange, and approximately 1/8 wide' and .068" deep. An 1/8 neoprene or rubber circular rod is positioned therein to forni an exceptionally tight seal.

The electrodes are soldered to lead wires 27, which extend through holes in sheets 23. Grooves 36 are formed in the adjacent walls 3, which grooves extend down the walls and across the back t, and carry the lead wires 2l into a position where they are connected to pins ill, i5.

The remaining space within the recess is filled with oil so that the whole of the motor assembly Eil is immersed in it. Such oil should preferably have, as does castor oil, an acoustic impedance substantially that of water at the teinperature of use.

The particular unit just described illustrates the general principle of the invention. The motor assembly 2li is entirely surrounded by the insulating sheets 28 and 29, except for that portion facing the sound transparent window 33. The sonic insulation produced by this construction between the motor and case is very great and sharp directivity is thus obtained in a direction normal to window 33. In transducers where sensitivity is desired in two or more directions, the same principle is followed and the insulating layers are positioned to entirely surround the :fnotor or motors, except in the directions where high response is desirable.

This construction so isolates and insulates the motor assembly 2d from the housing i that vibrations normally transmitted between the motor assembly and housing are largely eliminated and greatly increased overall eiciencies are obtained. This is accomplished not only by the insulating characteristics of the sheets 23, 29 but also because the motor assembly is entirely supported by this material. No bolts, braces, or supports normally used to mount the motor assembly are present and cannot, therefore, act as sonic conductors between the motor and housing. Even ordinary rubber, which has sometimes been used to support the motor, acts as a reasonably good conductor when under compression and is affected by high frequencies. This is not true for the materials prescribed for use with the present invention.

Having thus described my invention, l claim;

A transducer for propagating sound in iuid, said transducer comprising a housing having a recess open at one end, a metal frame closing said recess, said frame having an opening, nonmetallic material closing said opening thereby forming a sound transparent window, said nonmetallic material having an acoustic impedance substantially equal to the fluid in which the sound is to be propagated from the transducer, means securing said frame to said housing Jo form a uid-tight enclosure therefor whereby said transducer can be immersed in the iluid, a unitary motor assembly within the housing facing the window, said motor assembly including a back plate on the opposite side from the window and end support members secured to the back plate, and layers of a cork and rubber mixture having a low acoustic impedance lning said recess and completely surrounding said i motor assembly except at the position facing said Window, said layers being snugly tted between the back plate and support members and the housing to form the sole support for said motor unit assembly to position and maintain the same xed Within the housing relative to said Window.

DONALD E. ROSS.

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

UNITED STATES PATENTS Number Name Date 549,802 Lucas Nov. 12, 1895 768,573 Mundy Aug. 23, 1904 1,204,826 Schiessler Nov. 14, 1916 1,472,558 Fessenden et al. Oct. 30, 1923 1,625,245 Dorsey Api'. 19, 1927 1,973,673 Rice Sept. 11, 1934 1,991,877 Zottali Feb. 19, 1935 2,076,330 Wood et al. Apr. 6, 1937 2,121,779 Ballantine June 28, 1938 2,270,902 RubiSsoW s Jan. 27, 1942 FOREIGN PATENTS Number Country Date 844,452 France July 26, 1939 OTHER REFERENCES Product Engineering-Cork-Rubber Materials, June, 1939.

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