US 2939970 A
Description (OCR text may contain errors)
A. I. DRANETZ ET AL SPHERICAL TRANSDUCER June 7, 1960 Filed Dec. 5. 1954 INVENTORS ABRAHAM I. DRANETZ HUGH J CULLIN 2 QTTOPNEY numeral 13 designates the inner electrode.
SPHERICAL TRANSDUCER Abraham I. Dranetz and Hugh J. Cullin, Metuchen, N.J., assignors to Gulton Industries, Inc, a corporation of New Jersey Filed Dec. 3, 1954, Ser. No. 472,966
2 Claims. ((1310-85) A still further object of our invention is to provide a transducer which may be employed in conjunction with a directional reflector or other directive device.
Other objects and advantagesof our invention will be apparent during the course of the following descrip tron.
In the accompanying drawings, forming a part of this application, and in which like numerals are employed to designate like parts throughout the same.
Figure 1 is a vertical view of an embodiment of our invention,
Figure 2 is a vertical section of the embodiment of Figure l, and
Figure 3 is employed to illustrate the theory underlying our invention. I
In the drawings, wherein for the purpose of illustration, is shown a preferred embodiment of our invention, the numeral designates the microphone assembly, the numeral 11 designates the electromechanical transducer; the numeral 12 designates the outer electrode and the The numeral 14 designates the center conductor which is electrically connected to the inner electrode 13 and the numeral 15 designates the electrical connection from the cable shield 16 to the outer electrode 12.
The numeral 17 designates the compliant insulator and the numeral 18 designates the insulating outer jacket of the cable 20. The numeral 19 designates the sealing compound and the numeral 21 designates the cable clamping surface and the numeral 22 designates the hole in the sphere through which the connections are made.
Microphone assembly 10 is spherical so that its sur face looks the same to sound which is impinged on it from any direction. In the embodiment of our invention shown in the figures, the active transducer element- 1-1 is a hollow spherical ceramic to whose surfaces have been applied electrodes 12 and 13. A small hole 22 is cut in the sphere so that cable may be inserted and the necessary electrical connections made from center condoctor 14 to inner electrode 13 and from the shield 16 to outer electrode 12. The insulating outer jacket of cable 20 is cut back where cable 20 enters hole 22 and a plastic casting compound 19 is inserted in the hole 22 alongside cable 20 and allowed to flow inside 10 and to harden and set. This insures good mechanical bonding of the cable 20 to the microphone assembly 10. Compliant insulator 17 is bonded to .10 so as to completely surround hole 22 and is compliant so that the 2,939,970 Patented June 7, 1960 2 transducer takes all the acoustic force which is applied to the microphone.
Other embodiments and methods for mounting and assembling transducers, produced in accordance with our invention, may be employed. One such method contemplates inserting a rubber or plastic-covered flexible shielded cable in hole 22 in the-transducer 11 and sealing hole 22 with hard glass or a like material.
The electrodes may be on both surfaces as shown in the figures or may be applied to either surface depending on the particular application. Or, a strain gauge wire on a metallic sphere may be employed'as the active ele- 'ment in the device. I
When microphone assembly 10 is placed in an acoustic field, the sound impinges on 10 radially and compresses and expands transducer 11 in radial mode. sonant frequency of 11 in the radial mode is very high which fact, coupled with the low mechanical Q of the sphere, permits the microphone to possess a broad frequency response beyond that attainable with diaphragm type microphones. .We have found that themicrophones and hydrophones produced in accordance with our-invention have omnidirectional response in both the yerfi cal and horizontal directions. For the purpose of this discussion the vertical planev is that which contains the axis of cable 20 and the horizontal plane is the plane which is perpendicular to the axis of cable 20.
Units produced accordingto our invention are particularly useful in high intensityfields and may be used as microphones, hydrophones, projectors or loudspeakers. This is due to the fact that these spherical sensitive elements are electro-acoustically reversible.
In the field of subaqueous sound transmisison and v reception, hydrophones have been most generally produced in the shape of hollow cylinders. The resonant frequency of a sphere in radial mode is approximately 1.7 times higher than that of a cylinder of the same'radius and in addition, there is no length mode of vibration to consider when spheres are used instead of cylinders.
In addition, we have: m=R dy R dx t 'p; i
'd R d0 where p=density t=thickness 0=time R =radius From which:
F, ma R dxdytp Likewise, the fundamental equation for the elastic force is given by:
= i E! 7 R0 The re- 3 where 2737i: i' i .i.s=str,ess f i E= Youngs modulus =Poissons ratio The force in the radial direction is given by:
F ,=F sin dy-l-F sin dx and for small values of dy and dx,
assuming.dy=dx, we obtain: Fr=2Fdy. I The force F is equal to the stress times the area or F-=S t Rode; w 3am from which, the radial force, F,, is given by;
' F,.=2E'ARtdxdy 1; 'For equilibrium, the force of inertia is equal to minus the elastic force, so that:
' AR=R-R =A (amplitude of motion) and since f, (frequency of resonance) =5 The mechanical figure of merit, Qifor a subaqueous sound receiver (hydrophone) is given by:
where LB n...
c =velocity of sound in the ceramic p =density of the ceramic t=thickness of the ceramic wall i a=mean radius c =velocity of sound in water p =density of the water '=Poissons ratio (.32 for barium titanate) For barium titanate and sea water, we obtain:
and if t is small with respect to a Q is low.
When devices made in accordance with our invention are employed for subaqueous sound reception or transmission, they are suitably treated so that the liquid in which they are'immersed cannot enter into contact with 40 the active element or the electrodes.
Our invention contemplates-the use of the units, produced in accordance therewith, in directional microphones, hydrophones, projectors and loudspeakers. The location of the unit at the focus of a parabolic reflector, for instance, results in a very good directional electroacoustical device. This is due to the fact that the inherently uniform response of the unit in free space permits sound pickup from any direction and all sound reflected from the parabolic reflector onto the sensitive element is unattenuated due to the direction of incidence to the sensitive element. The unit is equally well adapted to use in conjunction with any other type of directional device. While we have described our invention by means of specific examples and in a specific embodiment, we do not wish to be limited thereto for obvious modifications will occur to those skilled in the art without departing "from the'spiri't and scope of the invention.
' Having thus described our invention, we claim: 1. ,An electroacoustical transducer for insertion in an acoustic transmission medium comprising a hollow spherical, piezoelectric ceramic element in said medium, said ceramic element having an opening therein, electrodes aflixed to the inner and outer surfaces of said-ceramic element, a cable comprising two electrical conductors and an outer compliant sheath, said cable being aflixed to said ceramic element at said opening, means for connecting one of said electrical conductors to the inner of said electrodes and the other of said electrical conductors to the outer'of said electrodes, a compliant member surrounding said cable in contact with said ceramic element, and meansrwithin said opening for sealing said 1 opening whereby the interior of said ceramicelement is isolated from said medium; a 76 Z, The invention as described in claim 1 wherein the 5 ratio of the thickness of said ceramic element to the radius of said ceramic element is small.
References Cited in the file of this patent UNITED STATES PATENTS 1,919,796 Marrison July 25, 1933 2,399,082 Wainer Apr. 23, 1942 2,420,864 Chilowsky May 20, 1947 2,556,558 Silverman June 12, 1951 Williams Aug. 21, 1951 Langevin Oct. 16, 1956 Camp- June '11, 1957 FOREIGN PATENTS Great Britain July 2, 1910 Great Bntain Aug. 21, 1947 OTHER REFERENCES Piezotronics, Brush Electronics 00., July 1953, p. 3.