Piezoelectric microphone

July 17, 1951

P. B. WICKHAM ET AL

PIEZOELECTRIC MICROPHONE

Filed May 1, 1946

2,561,084

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32

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BY

ATTY.

Patented July 17, 1951

2,561,084

UNITED STATES PATENT OFFICE

2,561,084

PIEZOELECTRIC MICROPHONE

Parker B. Wickham, Delavan, and Samuel A.

Johnston, Walworth, Wis., assignors to The George W. Borg Corporation, Chicago, Ill., a corporation of Delaware

Application May 1,1946, Serial No. 666,284

Claims.

The present invention relates in general to microphones, and more in particular to microphones of the type in which a piezo-electric crystal is used to convert mechanical vibrations or impulses into electrical impulses. The object of the invention is a new and improved device of ' this character.

As illustrated and described herein, the invention is directed specifically to a microphone adapted for use in a watch timing apparatus for picking up the beats of a watch which is ‘ being tested. The invention may, therefore, be regarded as an improvement on the invention disclosed in United States Patent No. 2,175,021, granted October 3, 1939.

The invention will be described hereinafter ' with reference to the accompanying drawing, in which—

Fig. 1 is a top or plan view of a microphone embodying the invention;

Fig. 2 is a section on the line 2—2, Fig. 1; and

Fig. 3 is a section on the line 3—3, Fig. 2.

Referring to the drawing, the reference character i 0 indicates a case or housing, which may be a casting of zinc alloy or other suitable material. The shape of the housing 10 is clearly shown in the several figures of the drawing, from which it can be seen, that it is of circular formation, with an extension 11 at the base, from which there projects the cylindrical neck 12'. The neck 12 has an axial opening which communicates with the interior of the housing. A tubular shaft 13, only part of which is shown, is threaded into this opening, which is tapped for the purpose, and provides means for rotatably mounting the microphone on the frame or casing of a watch timing machine.

The opening in the top of the housing 18 is circular and is formed with an annular shoulder 14 on which the microphone assembly is supported. This assembly will now be described.

The reference character 15 indicates a disc of synthetic resin or similar material, which is of the proper size to fit the opening in the top of the housing 10. A relatively thick layer of soft foam rubber is indicated at 16, and a thinner layer of flexible sheet rubber at 11. These parts may be secured together by a suitable cement. The upper layer 17 should be made of a sulphur free rubber to avoid any danger of tarnishing the casing of a watch. Beneath the disc 15 there is a disc 18 of smaller diameter and composed of soft sponge rubber, which may be cemented to the disc 15.

There is a central opening extending through (Cl. 73—6) the above described parts and the metal pin 21 is located in this opening. The pin 21 is held in place by a washer 19, secured by the split ring 20. It will be noted that the parts are cdm5 pressed somewhat between the head of the pin and the washer 19, as indicated by the indentation of the head of pin 21 in the layers Π and 16.

The crystal 22 is of known construction and 10 comprises preferably a sheet or plate of Rochelle salt having thin foil electrodes on the opposite sides thereof. The terminal strips 23 and 24 are attached to these electrodes. The crystal with its electrodes is coated in known manner by a 15 layer of suitable varnish or similar compound to prevent the absorption of moisture.

The crystal 22 is mounted in a slot cut in the pin 21 near the end, where it is secured by a suitable cement. This arrangement is shown 20 clearly in Figs. 2 and 3, from which it will be seen that the crystal has no other support.

The reference character 25 indicates a resistor, having the usual terminal wires 26 and 27. This resistor is mounted on the Bakelite disc 15 by 25 means of the screw 2'8, which passes through a loop in the terminal wire 26. The terminal wires 26 and 27 are soldered to the terminal strips 23 and 24 of the crystal and since the wires are rather stiff they function effectively to support 30 the ends of the strips 23 and 24, which are thin and flexible. The pin 21 is connected to terminal wire 26 by means of a flexible lead 30 and the wire 26 is connected to the housing 10 by a wire 31, a boss 32 being provided for conven35 ience in making a soldered’ connection between wire 31 and the housing. These connections effectively ground the pin 21 and one of the crystal electrodes to the housing.

The insulated conductor 33 is connected to 40 the terminal wire 27 and extends through the tubular shaft 13 to the tick amplifier of the watch timing apparatus. This conductor affords a connection between the ungrounded electrode of the crystal and the grid of the tube at the first ampli45 fier stage. It may be stated also at this point that the resistor 25 functions as the grid leak for this tube. The end of the tubular shaft 13 is sealed around the conductor 33 by means of wax or other suitable plastic material, as in50 dicated at 35.

In the manufacture of the microphone, the so called microphone assembly, including the resistor 25, the crystal 32 and the ground wire

31, is completed outside the housing 10. The conductor 33 and the wire 31 are long enough

2,561,084 so that the former can be soldered to wire 21 and the latter to boss 32 before the microphone assembly is placed in the housing. These connections having been made, the walls of the opening in the housing may be coated with a suitable cement, after which the microphone assembly is inserted as shown in Fig. 2; with the disc 15 resting on the shoulder 14.

For the purpose of holding a watch in position on the microphone a spring 36 is provided, said spring being attached to the housing 10 by means of a screw 31, as shown in Fig. 3. The spring 36 extends upward and over the top of the microphone and at its free end carries a rubber button 38 which is in substantial alignment with the pin 21. A watch to be tested may be inserted dial up between the head of pin 21 and the button 38, the back of the case resting on the head of the pin, and is retained by the spring. In this connection it may be pointed out that the pin 21 projects only very slightly above the surface of the rubber layer 11 and since layer 16 yields readily the pin is depressed enough when a watch is inserted so that the watch not only contacts the head of the pin but the rubber layer 17 also. Thus considerable friction is developed, even if spring 36 has only a moderate amount of tension, and there is no danger of the watch slipping off when the microphone is rotated as required for testing the watch in different positions.

The microphone disclosed herein may be used with any watch timing machine which requires a means for picking up the watch beats and generating electrical impulses responsive thereto. United States Patents Nos. 2,254,649 and 2,303,540 disclose watch timing machines of this type and describe the operations involved in timing watches.

The present invention is concerned only with the microphone and it will suffice therefore to explain its operation in converting the watch beats to electrical impulses. Information as to further details of the timing operation may be obtained from the patents above referred to.

Assuming that a watch has been placed on the microphone as previously described herein, the watch case will be in contact with the head of pin 21 and the mechanical shocks produced by the beating of the watch will be transmitted to Irin 2 f and thence to the crystal 22 supported thereon. In this operation the pin 21 vibrates longitudinally, being free to do so because of the resilient support afforded by the soft rubber layers 16 and 18. Now the crystal 22 has one corner fixed to pin 21 and this corner therefore moves with the pin. Other parts of the crystal move also to a degree but due to the inertia of the crystal a certain amount of bending takes place whereby electrical potentials are developed between the crystal electrodes according to the well known piezo-electric effect. These potentials are transmitted to the tick amplifier where they are amplified to the extent necessary for proper operation of the timing apparatus. Explaining this a little more, the conductor 33, which is connected to one electrode of the crystal, extends to the grid of the tube at the first amplifier stage, while the cathode of the tube is grounded and is connected to the other electrode of the crystal by way of the ground connection to the housing 10.

The crystal may be made somewhat more responsive by loading the corner opposite the pin 21, or the adjacent corners, but such loading is ..not necessary for picking up watch beats, even the beats of the smallest wrist watches, and is not recommended because of the danger that strong shocks transmitted to pin 21 may fracture the crystal. In special cases, however, some loading may be desirable.

While designed especially for use with watch timing machines, the microphone disclosed herein is not limited to such use but is an economical and effective instrument for use in practically any situation where mechanical shocks or vibrations have to be translated into corresponding electrical potentials.