US2518331A - Piezoelectric crystal mounting - Google Patents

Description

Aug. 8, 1950 w. KALlN 2,518,331

PIEZOELECTRIC CRYSTAL MOUNTING Filed May 6, 1948 INVENTOR n4 KAL/N BV AGENT Patented Aug. 8, 1950 PIEZOELECTRIC CRYSTAL MOUNTING Walter Kalin, West Millington, N. J assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application May 6, 1948, Serial No. 25,345

'4 Claims.

This invention relates to an improved form of enclosure for piezoelectric crystals, particularly useful in the construction of a crystal microphone.

It is important in such instruments to protect the crystal from moisture and from variations of atmospheric pressure which may produce mechanical damage and change in sensitivity. The invention provides a novel form of hermetically sealed crystal enclosure which excludes moisture and renders the crystal substantially independent of pressure changes. That is the principal object of the invention.

A-further object of the invention is to provide a simple and easily assembled hermetically sealed housing enclosing a piezoelectric crystal.

[Aswill appear, the opposite faces of the enclosure serveas electrodes for the enclosed crystaland to provide such a feature is a further object of the invention.

The following description will make clear the nature of the invention, referring to the accompanying drawing, in which:

Fig. 1 is a plan View of a completed microphone using the invention;

' i Fig. 2 is a diametral section of the microphone taken along the line 2-2 of Fig. 1 and at right angles to the plane of that figure;

Fig; 3 is a lengthwise vertical section on an enlarged scale of the crystal and its housing,- and i Fig. 4 is a plan view on the same scale of the assembly shown in Fig. 3.

In all figures, like elements are designated by like numerals.

Referring to Fig. 1, microphone diaphragm l0, loosely fitted in case II, is partly cut away to show the crystal enclosure 12 in relation to case II and to terminal block I3. Block 13 is of methyl methacrylate and electrodes 23 and 24 are secured to it by screw 25 with suitable insulation, as shown in Fig. 2. The outside diameter of case II, in a particular microphone, is approximately 1.6 inches.

In Fig. 2 the configurations of diaphragm Ill and of case II are seen to be as follows:

Case ll, of depth about inch, is a metal cup, preferably of aluminum, spun to have a flat bottom I4 and a cylindrically upturned wall which is turned outward and again upward to provide an annular shelf l5, and outer cylindrical Wall 16 within which diaphragm I is fitted with a small clearance. Shelf I supports rings I! of impregnated paper on which rests diaphragm l I), crimped around its edge as indicated at l8.

. 2 Rings I! are suitably constructed and used as taught by H. F, Fruth in United States Patent 1,833,642,-I\Tovember 24, 1931.

Diaphragm It! may be of Duralumin formed in the shape of a shallow cone, crimped as at 18 around the edge, and flattened at the apex to a circular area 19 in a plane parallel to the bottom of case H. In the small clearance around the edge of diaphragm in there is applied a small quantity of a sealing compound to seal off the space below the diaphragm without restraint of the diaphragm movement.

Cemented to the bottom I4 internally of case H are two similar Pyralin blocks 20 and 2|, of shape evident from Figs. 1 and 2. By these blocks crystal 22 in enclosure 12 is supported at each end. The lower face of enclosure 12 is cemented to blocks 28 and 2| while the upper face ofthe enclosure is cemented to the circular fiat portion I9 of diaphragm Hi; this portion preferably has a central hole through which cement reaches it upper surface.

Crystal 22 is a Rochelle salt bimorph crystal of known construction, consisting of two oppositely oriented crystal elements conductively cemented together at their opposing faces, whereby the voltages generated in the two elements by mechanical stresses are in series. The crystal dimensions are approximately /64 inch by 4 inch by 1 inch.

Referring now to Figs. 3 and 4, crystal 221 is installed within a plastic frame 30 composed of a methyl methacrylate block internally cut out as shown in Fig. 4 to clear the crystal ends by about /64 inch, the side clearance increasing from this amount to about /32 inch on each side at the transverse center line of the'crystal. Conveniently, frame 30 has straight sides and rounded ends permitting its location with suitable clearance within microphone case I l as evident in Figs. 1 and 2. The thickness of frame 30 is'that'of the crystal itself.

The hermetic sealing of crystal 22 in frame 30 is provided as follows:

A thin sheet of aluminum foil 3 I, plastic coated on one side to insure good moisture protection, and some .0015 inch thick, including coating, is laid fiat on the table with the conducting surface face up. Frame 30 is placed on this face and cemented to it with a suitable waterproof cement such as vistac. Crystal 22 is lowered into position within frame 30 and is cemented to foil 3| at the two ends of the crystal face with electrically conducting cement. These points of contact between crystal 22 and the foil 3| are TENT. OFFICE f] also the points of support between the outer surface of the crystal assembly and the blocks and 2|. The foil 3| is then trimmed to the outline of frame except for a narrow transverse strip 32 at one end which is later turned back on the foil sheet of which it is a p Frame 30 and crystal 22 are then turned over and a similar foil sheet 33 is cemented into place and similarly trimmed with strip 34 like strip 32. However, there is this exception. This upper crystal face is cemented to the foil 33 with conducting cement at its center so as to establish a firm bond between crystal 22, the composite foil- 33 and the circular flat portion I!) of the diaphragm 10. As the exposed surface of enclosure I2 is non-conductive, strips 32 and 34 are turned back on sheets 3! and 33 to expose conducting surfaces to which electrical connections can be made. In Fig. 4, foil sheet 33 is partially cut away. 7 e

To blocks 24 and 2|, Fig. 2', cemented at the proper locations to the bottom l4 of microphone caseit, is now cemented side-3i of the moistureproof assembly just described whereby crystal- 22- and frame 3e are supported at each end by the blocks 20, 2!. At one end, the right in Fig. 2,- pressurecontacts 23- and 2A- seize andmake electrical contact with the turned over strips 32' and 34.

Microphone diaphragm lfl'is: then installed and its central flat portion i9 is cemented to'upper foil sheet 33 covering frame so and" crystal 22. Sound waves incident on diaphragm lfi cause crystal 22 to vibrate correspondingly with a loop at its center and a node'at each end.

The purpose of providing increased clearance at. the mid-line betweencrystal and surrounding plastic frame isnow obvious. The vibrationof the crystal: is. sharedby the-portions ofthe foil sheets between neighboring edges of crystal and frame and the stiffness of these portionsis-greatly reduced atthe'line of maximum-crystal displacement.

The hermetically sealed enclosure consistingof plastic frame 3ii and foil sheets 3 land33 withthe crystal therein enclosedis wholly moisture-proof. The eifect on crystal. sensitivity of changes in atmospheric pressure is substantially eliminated, inasmuch as the volume of air entrapped betweencrystal and frame is so minute that staticpressure difierentials built up between the inside of this frame assembly l2 and the inside of the microphone case H are ineffectual-towards interference with microphone performance. It will be seen that the invention thus renders the performance of a crystal microphone independent or conditions of weather or altitude above the earths'surface, a result extremely'desirable when the instrumentis used on' board an aircraft.

It will be understood that'th'e type of crystal and thedimensions given in the foregoing. description of the invention'a'resolely illustrative.

What is claimed is:

1. A mounting for a piezoelectric crystal, comprising a non-metallic rigid frame of thickness that of the crystal and having upper and lower surfaces coplanar with the corresponding surfaces of the crystal, said frame being internally cutout to clear the crystal by a gap narrow at each end thereof and increasing in width from each such end toward the transverse center line of the crystal on each side, and individual sheets of conducting material conductively cemented to corresponding surfaces of the crystal and hermetically sealed to" the frame to cover said surfaces and bridge the gap. whereby the crystal is held between the conducting sheets clear of the inner wall of the frame.

2'. A mounting" as in claim 1 wherein each sheet is externally coated with a plastic film and is at one of its ends returned upon itself to expose a conductingsurface adapted toreceive an electrical connection.

3. For a piezoelectric crystal in the formof a slab, a moisture-proof housing comprising a nonmetallic fiat-surfaced rigid frame having the same thickness asthe crystal and cut out in ternally toreceive the crystalwith circumscribing clearance and a pair of sheets of conducting material each coated over one surface with: aplasticfilm, the u-ncoated surface of one of said sheets being hermetically sealed to one-side'of the'frame while theuncoatedsurface of the other of said sheets is hermetically sealed tothe opposite side ofthe frame, the crystal being located within the frame freely thereof and conductive lycemented on each side-to the: sheet adjacent-thermos 4. A mounting for apiezoelectriccrystal comprising. arigicl plastic frame. of the same thick nessas the crystal and surrounding the crystal with clearance therefrom at all points on the perimeter thereof, the frame having upper and lower surfaces each inthe same plane es -the corresponding surface of the crystal and sheets of electrically conducting. material individually connected to the surfaces oftthe crystal and hermetically sealed to thecQrreSponding-surfaces of the frame, each sheet-being coated with a-plastic film on its outer surface andextending beyond the frame at one end thereof,,the extended portion of: the: sheet beingreturned upon itself to expose a portion of its inner surface.

WALTER-KALINZ REFERENCES cram) The following. references are of I record in th file of this patent;

UNIT-ED STATES'A A'IENTS Number Name" Date 2,266,768 K rnel"- i -neci 23". 1941' 2,386,279 Tibbetts Oct: 9; 1945 2,414,489" Shomerl Jan. 21', 1'94?

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