GB2145301A - An acoustic transducer and a method for making it - Google Patents

Abstract

Providing acoustic transducers with diaphragms of thickness not greater than 5 microns, preferably produced using vacuum both to hold down a plastic sheet and to tension it, a frame then being adhered to the sheet.

Description

SPECIFICATION An acoustic transducer and a method for making it This invention relates to acoustic transducers in which thin diaphragms are incorporated; for example, electret condenser microphones, and a method for making it.

It is known that lowered microphone sensitivity to vibration reduces objectionable noise (as, rubbing noise from clothing, shaking noise from walking, and feedback noise as from a nearby sound generator). It is also known that such sensitivity is lowered by lowering the mass of the moving diaphragm.

Heretofore, however, it has not been possible to reduce diaphragm thickness below 6 microns, in substance part because of difficulties in tensioning and supporting the diaphragms.

The present invention concerns a method of providing diaphragms of thickness even as little as 1 to 5 microns; and, in consequence, transducers incorporating this new diaphragms.

In accordance with this invention, a sheet of material for the diaphragm is stretched (put in tension) over a support, following which a frame is adhered thereto, following which the tension on the material is released.

In preferred embodiments, a sheet of diaphragm material is held down on a fixture by a first zone of vacuum, a central portion of the sheet is supported by a flat portion of the fixture, tension is imposed in said sheet by a second zone of vacuum around the central portion, and a multiplicity of connecting frame portions are simultaneously adhered to the central portion of the sheet.

We turn now to a description of the drawings, structure, manufacture, and operation of a preferred embodiment.

Figure 1 is a cross-sectional view of an electret condenser microphone according to the invention.

Figure 2 is an explosed view of the fixture and elements according to the method invention.

Figure 3 is a more detailed view of the connecting frame portions above referred to.

In Figure 1 is shown a transducer upper casing 10 pierced with a passage 12. Also provided is lower casing 14. Intermediate these parts are distance spacer 16, diaphragm 18, electret layer 20, affixed metallic plate 22, and carrying frame 24 carrying integrated circuit 26. This overall structure is disclosed in United States Patent No.4,331,840, "Electret Transducer with Tapered Acoustic Chamber", the contents of which are hereby incorporated herein by reference.

In Figure 2 are shown vacuum fixture 30 for use in practicing the method of the invention, a sheet of large area of the material 18a from which the diaphragm 18 is to be formed, sheet 32a comprises a multiplicity of attached support frames, and weight 34. An enlarged view of sheet 32a is shown in Figure 3.

Fixture 30 includes hold-down vacuum conduit 36 connected through branching conduits 38 to grooves 40. Fixture 30 includes also tensioning conduit 42 connecting with groove 44 surrounding central support portion 46.

The sheet 18a is polyester 1.5 microns in thickness, and metallized with 12K gold, 400 Angstroms thick. The sheet 32a is of brass, thin but of strength to support.

According to the method of the invention, sheet 18a is laid down on fixture 30. A vacuum is then drawn through conduit 36 to securely hold down sheet 18a therearound. Epoxy adhesive is spread on the lower surface of sheet 32a, which is then placed against sheet 18a cover central support portion 46. A vacuum is then drawn on tensioning conduit 42, to place the central portion of sheet 18a throughout the central portion thereof over central support portion 46, in low tension corresponding to 0.1% pre-strain or less. Fixture 30 is heated, as to 100"C., to speed adhesive cure. Weight 34 holds sheet 32a against sheet 18a during this period. Individual frames 56 supporting their respective tensioned thin films 18 are then separated.

Sheet 32a, as better shown in Figure 3, includes border strips 50, 52, intermediate strips such as 54, and frames 56 connected to the border and intermediate strips, or simply to intermediate strips, through thin connectors 58.

Tests have shown microphones made as above described to embody substantially improved lowered susceptibility to vibration.

The diaphragms 18b may be of plastic instead of metal, a choice that may be governed by desired relative changes in dimension (and therefore tension) with temperature.

Other embodiments within the following claims will occur to those in the art.

Claims (8)

1. An acoustic transducer comprises a diaphragm of thickness not greater than 5 microns.

2. A transducer according to claim 1 in which said diaphragm is of thickness in the range from 1 to 5 microns.

3. A transducer according to claim 2 which is an electret condenser microphone.

4. A transducer diaphragm comprising a thin central portion not greater than 5 microns in thickness and a frame therearound and holding said central portion in tension.

5. A method of making an acoustic transducer diaphragm comprising holding down a thin sheet therearound by means of vacuum, supporting a central portion of said thin sheet, adhering a frame to said thin sheet over said central portion, placing said thin sheet under tension by means of vacuum applied around said central portion, and curing said adhesive.

6. A method according to claim Sin which said sheet is of thickness not greater than 5 microns.

7. A method according to claim 5 in which said frame is one of a multiplicity of interconnected frames.

8. A method according claim 7 in which each said frame is integral through a connector with a supporting portion.