US2003908A - Acoustic device - Google Patents

Description

June 4, 1935. T. SMITH EIAL ACOUSTIC DEVICE Filed April 25, 1934 L. T. SMITH MEMO/Q5 A. L. THU/PAS ATTORNEY Patented June 4/, 1935 UNITED STATES PATENT OFFICE Lee T. Smith, Madison, N. 1., and Albert L. Thuras, New York, N. Y., assignors to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application April 25, 1934, Serial No. 722,286

'7 Claims.

This invention relates to acoustic devices, and more particularly, to diaphragms for transmitters or microphones.

In a telephone transmitter or microphone, it

5 is desirable to have a diaphragm that is substantially non-hygroscopic, that will not be substantially affected by variations in atmospheric temperatures, that is low in density and in mechanical stiffness, that may be shaped to and retain a desired contour, and that is flame-resistant.

An object of this invention is to satisfy the need for such a diaphragm.

A feature of this invention comprises a diaphragm of polymerized styrol.

A more complete understanding of this invention will be obtained from the detailed description which follows, read with reference to the appended drawing, wherein:

Fig. 1 is a cross-sectional view of a fragment of an acoustic device embodying this invention; and

Fig. 2 is a perspectiveview of a diaphragm embodying this invention, partly broken away and partly in section.

A diaphragm of polymerized styrol may be made by pressing the solid material into shape; or, by first dissolving the solid material in a suitable solvent, for instance, benzene, toluene,

, ethylene dichloride, trichlorethylene, or the like, and the spraying, brushing, dipping, or spinning it onto a plane surface, allowing the solvent to evaporate to leave the material in sheet form.. Polymerized styrol may be ob-,- tained on the market under the names of Meta- Styrene", Victron and Resoglaz.

When the spinning method is employed, the

' solution is poured on a revolving plane. surface,

, preferably glass, on which it spreads to a thickness dependent upon the viscosity of the solution, the rate of rotation of the surface, and surrounding atmospheric conditions. A continuous process similar to that used in the manufacture of motion picture films from solutions of cellulose nitrate or cellulose acetate may be used, also.

A diaphragm of a particular configuration, for instance, one having a central stiffened portion, an outer peripheral portion, and an intermediate portion connecting the outer and the central portions and permitting the central portion to vibrate substantially as a whole in response to impulses of sound wave frequency, may be obtained by operating on the polymerized styrol sheet with suitably shaped dies.

The drawing shows a fragment of an electrodynamic type transmitter or microphone I0, comprising a casing II providing an enclosure for a magnet structure I2, which supports a diaphragm I3 embodying this invention.

The magnet structure comprises a center pole I4, for instance, of cobalt iron, having a domeshaped or cap member I5, for'instance, of magnetic iron, secured thereto by any suitable means, for instance, bolt I 6, and an outer pole II, for instance, of cobalt steel. A plate pole I8, preferably annular, of magnetic iron and forming an annular air gap I9 with the member I5, is supported on the outer pole, and has a pcripheral, upwardly extending flange or ledge portion 20.

The diaphragm I3 is formed, preferably, from a sheet of polymerized styrol of substantially uniform thickness. A diaphragm as thin as .001 to .0011" and of outside diameter of approximately 1" has been found to give satisfactory results. Such a diaphragm is substantially nonhygroscopic and unaffected by atmospheric temperature variations, is low in densityand in mechanical stiffness, may be readily and permanently shaped to a desired contour, and is flame-resistant. The diaphragm comprises a central, stiffened, dome-shaped portion 2| of shape similar to that of the upper surface 22 of the member I5; an outer peripheral and annular portion 23, adapted to be mounted on and secured by a suitable adhesive to the ledge portion surface 24; and an intermediate curved, outwardly concave portion 25, connecting the outer and central portions and being adapted to permit the portion 2I to vibrate substantially as a unit or piston, in response to impulses of sound wave frequency. A coil 26 is attached to the underside of the diaphragm at the narrow, annular, planar'portion 21 connecting the central and intermediate portions, and is disposed in the air gap l9. Member 28 is a damping plate attached to the cap member by screws 29 (one only being shown), and providing a narrow restricted passage 30 connecting the air spaces or chambers 3|, 32 through the air gap IS with the air space or chamber 33 in the magnet structure. Sound wave energy has access to the diaphragm through the apertures 34 in the casing.

Although this invention has been described with reference to a specific embodiment thereof, it will be understood that it is capable of other forms and sizes without departing from the spirit of the invention, which is to be considered as limited in scope, therefore, by the appended ciaims, only.

What claimed is:

1. An coustic device comprising a diaphragm of polymerized styrol.

2. An acoustic device comprising a diaphragm response to impulses of sound wave frequency. 5. An acoustic device comprising a diaphragm of polymerized styrol of a thickness of about .001 to .0011".

6. An acoustic device comprising a diaphragm o1 polymerized styrol substantially 1 mil in thickness throughout, having a central stifiened portion, a peripheral mounting portion, and an intermediate portion connecting the peripheral and central portions.

7. An acoustic device comprising a diaphragm of polymerized styrol having a central stiflened portion and being substantially 1 mil in thickness throughout.

LEE T. SMITH.

ALBERT L. THURAS.

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