Submarine signal microphone

Aug. 12, 1947. w. j. brown 2,425,594

SUBMARINE SIGNAL MICROPHONE Filed March 4, 1943

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HG.S

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INVENTOR

MJ BROMN

BY

ATTORI^Y

Patented Aug. 12, 1947

2,425,594

UNITED STATES PATENT OFFICE

2,425,594

SUBMARINE SIGNAL MICROPHONE

William J. Brown, West Englewood, N. J., assignor to Western Electric Company, Incorporated, New York, N. Y., a corporation of New York

Application March 4,1943, Serial No. 477,922

Claims.

This invention relates to submarine signaling devices and more particularly to microphones of the type adapted to be mounted on or in the hull of a ship and responsive to submarine signals.

Microphones mounted on or in the hull of a ship are subjected to actuating forces due to two sources, namely, the signal sounds transmitted thereto through the sea and the noises within the ship itself. The noises, of course, result in interference with and distortion of the signals translated by the microphones and the magnitude of the effect thereof may become sufficient to render the output of the microphone unintelligible, having poor correspondence to the signal intended to be translated by the microphone.'

One object of this invention is to improve the fidelity of translation of signals by a submarine signaling device. More specifically one object of this invention is to obtain a high signal to noise ratio for a microphone adapted for use in submarine signaling, whereby signals received by the microphone are translated with substantial fidelity.

In accordance with one feature of this invention, a microphone adapted for mounting on or In the hull of a ship is constructed of two electromechanical units both of which are responsive to noises originating on the ship and only one of which is responsive to the submarine signals the microphone is intended to translate and the outputs of the two units are combined in such relation that the responses of the two units to noises substantially cancel each other.

The invention and the above noted and other features thereof will be understood more clearly and fully from the following detailed description with reference to the accompanying drawing in which:

Fig. 1 is a side view partly in section illustrating one manner in which a microphone constructed in accordance with this invention may be mounted on the hull of a ship;

Fig. 2 is a side view in section of a microphone illustrative of one embodiment of this invention;

Fig. 3 is an enlarged perspective view of one of the electromechanical translating units included in the microphone shown in Fig. 2; and

Figs. 4 and 5 are circuit diagrams illustrating ways in which the microphone shown in Fig. 2 may be operated.

Referring now to the drawing, the microphone illustrated in Fig. 2 comprises a base or foundation plate 10 of substantial mass, for example of metal, and two circular caps ί I, also for exam pip of metal, secured to the base of foundation plate (Cl. 177—386) by a plurality of screws 12. The base or foundation plate 10 may be secured in water-tight relation to the hull of a ship, a portion of which is shown at 13 in Fig. 1, one of the caps 11 fitting 5 in an aperture in the hull and having its outer surface in direct contact with the sea.

Each of the caps 11 is provided with a central, circular diaphragm portion 14 connected to the body of the cap by a relatively thin annulus 15 1.0 so that the diaphragm portions may move substantially bodily normal to their faces. Each cap is provided also with an annular groove 16 and the base or foundation member 10 is provided with similar grooves each opposite one of the 15 grooves 16. Seated in the juxtaposed grooves are resilient rings 17, for example of soft rubber, which are compressed by screws 18 bearing against metallic annuli 19 to form a water-tight seal between each cap 11 and the base of founda20 tion plate 10.

The microphone comprises further a pair of substantially identical electromechanical translating units 20, each of which is held between the base or foundation plate 10 and one of the 25 diaphragms 14. Each of the translating units 20, as shown more clearly in Fig. 3, includes a plurality of piezoelectric crystals 21, for example Rochelle salt slabs or blocks, having electrodes 22, for example conductive coatings, on the ma³⁰ jor faces thereof, to which suitable leading-in conductors 23 are connected. The several crystals 21 of each unit are secured together in faceto-face relation, have their similar electrical and mechanical axis parallel and are connected elec35 trically in parallel. Each crystal assembly has secured to opposite ends thereof a thin insulating, for example ceramic, plate 24, the inner plate 24 being cemented to the base or foundation plate 10 and each of the outer plates 24 bearing 40 against the corresponding diaphragm 14.

In the assembly of the microphone, the screws 12 which secure the caps 11 to the base or foundation plate 10 are tightened so that the two units 20 are placed under equal compression.

⁴⁵ As illustrated in Fig. 4, in one manner of operation of the microphone, the two translating units 20 are connected to similar preamplifiers 25 and the outputs of these preamplifiers are connected in opposition to amplifier 26, the out50 put of which is supplied to an indicating instrument 27, such, for example, as a telephone receiver.

Noises originating on or within the ship or at the propeller thereof are transmitted, in the 55 form of vibrations, through the hull 13 to the

2,425,604 base or foundation plate 10 and thence to both of the translating units 20. The noise energy is translated by the units into two electrical variations or signals which are substantially of equal magnitude and opposite phase. Hence, these variations as impressed upon the input of the amplifier 26 substantially cancel one another. Signals propagated through the sea and picked up by the microphone, however, are incident upon and actuate only one of the diaphragms 14 and are translated into corresponding electrical variations by only one of the units 20.

Hence, the input to the amplifier 26 is composed of two signals, due to noise, which substantially cancel each other and a third signal corresponding to that picked up by the microphone. Thus, as will be apparent, a high signal to noise ratio is obtained and the output of the amplifier 26 corresponds to the signal picked up by the microphone and is substantially free of distortion due to noise.

Because of dissimilarities in the two units 20, in the initial compression thereof, in the transmission of energy to the two units 20 by the plate 10 and other factors, the outputs of the two units in response to noise may not be of exactly equal magnitudes and may not be exactly 180 degrees out of phase. Departure from amplitude equality of these two outputs may be corrected by adjusting one or both of the preamplifiers 25 or otherwise controlling the output of one or both units. Departures from phase opposition may be compensated for, as shown’ in Fig. 5, by providing an adjustable phase shifting device 28 in association with one of the units 20. This device may be connected, advantageously, between the amplifier 29 associated with the one unit and the input of a transformer 30 connected to the indicating device.

In compensating for amplitude inequality and departure from phase opposition, the preamplifiers and phase shifting device may be adjusted under normal noise conditions and in the absence of a submarine signal so that .the indication obtained at the indicating device is a minimum.

In a properly constructed microphone of the design shown in Fig. 2, noise cancellations of the order of 15 decibels over octave bands within the frequency range of approximately 500 to 5000 cycles per second and cancellations of single frequencies at the center of the bands of the order of 25 decibels have been obtained without either phase or amplitude compensation.

Although specific embodiments of this invention have been shown and described, it will be understood that they are but illustrative and that various modifications may be made therein without departing from the scope and spirit of this Invention as defined in the appended claims