ELECTROSTATIC TRANSDUCER AND DIAPHRAGM

Abstract

An electrostatic transducer, a diaphragm (2) therefor, and corresponding methods of manufacture are disclosed. The electrostatic FIG. 1 transducer is preferably for use in a motor vehicle. A composite laminated diaphragm (2) is manufactured by providing a first insulating layer (4), providing a conductive layer (6) on a surface of the first insulating layer (4), and bonding a second insulating layer (10) to the conductive layer (6) such that the second insulating layer (10) extends over the conductive layer (6). The first and second insulating layers (4, 10) each comprise a sheet of uncharged insulating material. The thickness of the composite laminated diaphragm (2) is less than 20 μm. Manufacturing the electrostatic transducer comprises securing a first conductive stator, a first insulating spacer and the diaphragm (2) in a stack with the first insulating spacer between the first conductive stator and the diaphragm (2) to provide a spacing of less than 1 mm between the first conductive stator and the diaphragm (2).

Claims

1. A method of manufacturing an electrostatic transducer suitable for use in a motor vehicle, the method comprising manufacturing a composite laminated diaphragm and assembling the electrostatic transducer; wherein manufacturing the composite laminated diaphragm comprises: providing a first insulating layer, wherein the first insulating layer comprises a sheet of uncharged insulating material; providing a conductive layer on a surface of the first insulating layer; providing a second insulating layer, wherein the second insulating layer comprises a sheet of uncharged insulating material; bonding the second insulating layer to the conductive layer such that the second insulating layer extends over the conductive layer; wherein the composite laminated diaphragm has a thickness that is less than 20 μm; and wherein assembling the electrostatic transducer comprises: providing a first conductive stator and a first insulating spacer; securing the first conductive stator, the first insulating spacer and the diaphragm in a stack with the first insulating spacer between the first conductive stator and the diaphragm to provide a spacing of less than 1 mm between the first conductive stator and the diaphragm.

2. The method of claim 1, further comprising installing or using the electrostatic transducer in a motor vehicle.

3. The method of claim further comprising: providing a second conductive stator and a second insulating spacer; securing the second conductive stator and the second insulating spacer in the stack with the second insulating spacer between the second conductive stator and the diaphragm to provide a spacing of less than 1 mm between the second conductive stator and the diaphragm.

4. (canceled)

5. The method of claim 1, wherein bonding the second insulating layer to the conductive layer comprising applying an adhesive layer to the conductive layer and overlaying the second insulating layer on the adhesive layer or applying an adhesive layer to the second insulating layer and overlaying the second insulating layer on the conductive layer.

6. The method of claim 1, wherein the adhesive layer comprises an acrylic-based adhesive.

7. The method of claim 1, wherein the adhesive layer has a thickness of 1 μm to 10 μm.

8. The method of claim 1, wherein the conductive layer has a thickness that is less than 1% of a thickness of the composite laminated diaphragm.

9. The method of claim 1, wherein the conductive layer has a thickness of 5 nm to 50 nm.

10. The method of claim 1, wherein the first insulating layer has a thickness of 5 μm to 15 μm.

11. The method of claim 1, wherein the second insulating layer has a thickness of 5 μm to 15 μm.

12. The method of claim 1, wherein the composite laminated diaphragm has at least one of a length that is greater than 1 cm and a width that is greater than 1 cm.

13. The method of claim 1, wherein at least one of the first insulating layer and the second insulating layer is formed from a polymer material.

14. The method of claim 1, wherein at least one of the first insulating layer and the second insulating layer is formed from a material with a dielectric breakdown strength greater than 500 V/μm.

15. The method of claim 1, wherein at least one of the first insulating layer and the second insulating layer is formed from a material with a dielectric constant less than 2.5.

16. The method of claim 1, wherein the at least one of the first insulating layer and the second insulating layer is formed from a capacitor film.

17. (canceled)

18. The method of claim 1, further comprising manufacturing the diaphragm from a composite material or film comprising the first and second insulating layers and the conductive layer, wherein the composite material or film is substantially isotropic in respect of at least one of: a Young's Modulus of the composite material or film, a Coefficient of Thermal Expansion of the composite material or film, and a yield strength or tensile strength of the composite material or film.

19. The method of claim 1, further comprising manufacturing the diaphragm from a composite material or film comprising the first and second insulating layers and the conductive layer, wherein the composite material or film has at least one parameter for which respective measured values thereof are matched between two or more layers of the composite material or film, wherein the at least one parameter is selected from the group consisting of a Coefficient of Thermal Expansion, a Young's modulus, a yield strength and a tensile strength.

20. The method of claim 1, further comprising manufacturing the diaphragm from a composite material or film comprising the first and second insulating layers and the conductive layer, wherein at least one parameter measured for the composite material or film has a value or values which match(es) a corresponding value or corresponding values of the same parameter(s) measured for at least one the first stator and the first spacer, wherein the at least one parameter includes one or more parameters selected from the group consisting of a Coefficient of Thermal Expansion, a Young's modulus, a yield strength and a tensile strength.

21. The method of claim 1, further comprising manufacturing the diaphragm from a composite material or film comprising the first and second insulating layers and the conductive layer, wherein the composite material or film has the following properties: i) a glass transition temperature of at least 120° C.; ii) at least one parameter for which respective measured values thereof are matched between two or more layers of the composite material or film, wherein the at least one parameter is selected from the group consisting of a Coefficient of Thermal Expansion, a Young's modulus, a yield strength and a tensile strength; and iii) a Surface Energy in the range of from 30 to 60 dynes/cm and/or a Polar Surface Energy greater than 12 dynes/cm.

22. The method of claim 1, wherein the first and second insulating layers are both formed from a material which comprises, or which consists essentially of, a polyaryletheretherketone (PEEK), a polyetherimide (PEI), or a polyethylene-naphthalate (PEN).

23. An electrostatic transducer suitable for use in a motor vehicle comprising: a first conductive stator; a composite laminated diaphragm; and a first insulating spacer disposed between the first conductive stator and the diaphragm to provide a spacing of less than 1 mm between the first conductive stator and the diaphragm; wherein the composite laminated diaphragm comprises: a first insulating layer formed from a sheet of uncharged insulating material; a conductive layer on a surface of the first insulating layer; a second insulating layer extending over and bonded to the conductive layer, wherein the second insulating layer is formed from a sheet of uncharged insulating material; wherein the composite laminated diaphragm has a thickness that is less than 20 μm.

24-48. (canceled)

Description

[0153] Certain preferred embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0154] FIG. 1 shows a cross-section of a composite laminated diaphragm in accordance a first embodiment of the present invention;

[0155] FIG. 2 shows a cross-section of a composite laminated diaphragm in accordance with a second embodiment of the present invention;

[0156] FIG. 3 shows an exploded view of an electrostatic transducer incorporating the diaphragm of the embodiment of FIG. 1; and

[0157] FIG. 4 shows a cross-section of a composite laminated diaphragm in accordance a fourth embodiment of the present invention.

[0158] FIG. 1 shows a cross-sectional view of a composite laminated diaphragm 2 in accordance with a first embodiment of a present invention. The diaphragm 2 comprises a first insulating layer 4 which serves as a substrate. The first insulating laying 4 is made from biaxially-oriented polypropylene (BOPP) and is 7 μm thick.

[0159] A conductive layer 6 is deposited on a surface of the first insulating layer 4. The conductive layer 6 is an 8 nm thick layer of gold. In this embodiment, the conductive layer 6 is deposited on the first insulating layer 4 using vapour deposition, although any other suitable method known to the skilled person may be used.

[0160] Overlaid on the conductive layer 6 is an adhesive layer 8. In this example, the adhesive layer is applied as a coating to the second insulating layer 10. The second insulating layer is then overlaid on the conductive layer 6 and pressure is applied to cause the layers to adhere together. However, any other suitable method known to the skilled person may be used, e.g. the adhesive layer 8 may be applied to the conductive layer 6 as a coating (e.g. in a liquid form by spraying) and then the second insulating layer 10 overlaid on the adhesive. The second insulating layer 10 is also 7 μm thick and made from biaxially oriented polypropylene (BOPP).

[0161] After the second insulating layer 10 has been overlaid on the adhesive, the adhesive is cured in order to set it. The layers may be pressed together during the curing process, depending (for example) on the specific adhesive used. In this example, the adhesive is a viscoelastic acrylic-based adhesive. The adhesive layer is 5 μm thick.

[0162] It will be appreciated that due to the difference in order of magnitude between the thickness of the gold conductive layer 6 and the insulating and adhesive layers 4, 8, 10, the layer thicknesses in FIG. 1 are not shown to scale.

[0163] The electrical and mechanical properties of the layers 4, 6, 8, 10 are shown below in Tables 1 and 2. The properties shown include the Young's modulus, which affects the stiffness of the diaphragm, and thus its acoustic properties. The dissipation factor affects the diaphragm's energy dissipation, and thus the Q (quality) factor of its modes.

TABLE-US-00001 TABLE 1 Volume Density Young's Layer Material Name Thickness (kg m.sup.−3) Modulus 4 BOPP Film 7 μm 910 3.0 GPa (length), 5.3 GPa (width) 6 Acrylic Adhesive 5 μm 1050 600 MPa 8 Gold coating 8 nm 19,400 78 GPa on BOPP Film 10 BOPP Film 7 μm 910 3.0 GPa (length), 5.3 GPa (width)

TABLE-US-00002 TABLE 2 Dielectric Dielectric Breakdown Damping or Layer Constant Strength (V/μm) dissipation factor 4 2.2 550 0.0004 6 2 — 0.03 8 2.0 — 0.0003 10 2.2 550 0.0004

[0164] Tables 3 and 4 show the electrical and mechanical properties of some example materials that may be used for the first and/or second insulating layers. Table 3 shows example layer thickness ranges that may be used for each material.

TABLE-US-00003 TABLE 3 Material Thickness Volume Density Young's Name (μm) (kg m.sup.−3) Modulus BOPP 3-18 910 E.sub.1 = 4.6 GPa E.sub.2 = 2.6 GPa BOPET 3-18 1395 E.sub.1 = 4.5 GPa E.sub.2 = 4.3 GPa PPS 12-25  1350 E.sub.1 = 4 GPa   E.sub.2 = 3.8 GPa PEI 5-20 1270 E.sub.1 = 2.9 GPa E.sub.2 = 2.9 GPa PEN 5-20 1360 E.sub.1 = 6.1 GPa E.sub.2 = 6.1 GPa PI 7.5-25.sup.  1540 E.sub.1 = 3.2 GPa E.sub.2 = 3.2 GPa PEEK 6-25 1260 E.sub.1 = 2.6 GPa E.sub.2 = 2.8 GPa

TABLE-US-00004 TABLE 4 Material Dielectric Dielectric Breakdown Damping or Name Constant Strength (V/μm) dissipation factor BOPP 2.2 600 0.0002 BOPET 3.3 330 0.0005 PPS 3 470 0.03 PEI 3.2 490 0.004 PEN 3 300 0.003 PI 3.9 280 0.003 PEEK 3.5 270 0.002

[0165] Table 5 shows the environmental properties of some materials which may be used for the first and/or second insulating layers.

TABLE-US-00005 TABLE 5 Glass Coefficient of Surface Continuous Transition Thermal Expansion Energy use Material Temperature Tg CTE (°/C.) (dynes/ temperature Name (° C.) (2 directions) cm) (° C.) BOPP 105 49 × 10.sup.−5 (MD) 29 85 62 × 10.sup.−5 (CD) BOPET 110 15 × 10.sup.−6 (MD) 42 105 11 × 10.sup.−6 (CD) PPS 140 40 × 10.sup.−6 (MD) 38 160 40 × 10.sup.−6 (CD) PEI 214 49 × 10.sup.−5 (MD) 40-45 185 62 × 10.sup.−5 (CD) PEN 150 13 × 10.sup.−6 (MD) 40 160 13 × 10.sup.−6 (CD) PI 216 20 × 10.sup.−6 (MD) 40-50 200 18 × 10.sup.−6 (CD) PEEK 143 60 × 10.sup.−6 (MD) 34-38 150 60 × 10.sup.−6 (CD)

[0166] As discussed below with reference to FIG. 3, when the diaphragm is installed in a push-pull type electrostatic transducer, a DC bias voltage is applied to the conductive gold layer 6 and a varying drive signal voltage is applied to the stators of the electrostatic transducer to cause the diaphragm 2 to be deflected in response to the drive signal. Small regions of the adhesive layer 8 and the second insulating layer 10 may be omitted during manufacture (or subsequently removed) to expose a part of the conductive layer for providing electrical contacts (not shown).

[0167] FIG. 2 shows a composite laminated diaphragm 12 in accordance with a second embodiment of the present invention. The diaphragm 12 comprises a first insulating layer 14 which serves as a substrate. The first insulating layer 14 is 7 μm thick and is made from biaxially-oriented polypropylene (BOPP). Similarly to the embodiment of FIG. 1, a conductive layer 16 is deposited on one surface of the first insulating layer 14. The conductive layer 16 is an 8 nm thick layer of gold deposited by vapour deposition.

[0168] In the contrast with the embodiment of FIG. 1, no adhesive layer is provided in this embodiment. Instead, a second insulating layer 18 is overlaid on the conductive layer 16, and the layers 16, 18 are bonded together using ultrasonic welding. The second insulating layer 18 is also 7 μm thick and made from biaxially-oriented polypropylene (BOPP). Electrical contacts (not shown) are provided in the same way as discussed above with reference to FIG. 1.

[0169] As mentioned previously, in embodiments with an adhesive layer the additional mass from the adhesive can provide internal damping which dampens resonant behaviour, e.g. at lower frequencies. In embodiments without the adhesive layer, the internal damping may therefore be less. However, the mass of the diaphragm is less compared with an equivalent diaphragm having an adhesive layer. The relatively lower mass results in resonant phenomena being higher in frequency, where they may either be sufficiently damped by the insulating layers, or they may be high enough in frequency that they are above the audio range of interest, e.g. above 20 kHz for audio applications (20 kHz being the typical upper range of human hearing).

[0170] As mentioned above, in embodiments having an adhesive layer, the adhesive may be selected such that the adhesive layer is air- and moisture-tight. In embodiments without an adhesive, this air- and moisture-tightness may instead be provided by bonding the insulating layers together with the conductive layer in an air- and moisture-tight way over the entire diaphragm (e.g. by ensuring the bonding is air- and moisture-tight around the entire perimeter of the diaphragm).

[0171] It will be appreciated that in the above two embodiments, specific materials and thicknesses are given, but in other embodiments, different thicknesses and/or different materials may be used. In addition, other variations (such as deposition methods, etc.) may be used. It is to be appreciated that the individual manufacturing steps (e.g. deposition/application of the conductive layer, application of the adhesive layer, overlaying of the second insulating layer, etc.) may be carried out in accordance with manufacturing techniques known per se in the art.

[0172] FIG. 3 shows an exploded view of an electrostatic transducer 20 in accordance with an embodiment of the invention. The electrostatic transducer 20 comprises a composite laminated diaphragm 2 manufactured and having a structure as described above with reference to FIG. 1. The electrostatic transducer 20 further comprises a first stator 24 and a second stator 26. Each stator 24, 26 comprises a planar conductive plate with an array of holes provided therein.

[0173] The electrostatic transducer 20 also comprises a first spacer 28 which is positioned between the first stator 24 and diaphragm 2. A second spacer 30 is positioned between the second stator 26 and the diaphragm 2. Each spacer 28, 30 is provided with large apertures 32. The electrostatic transducer also comprises a first supporting frame 34 and a second supporting frame 36, each having large apertures 38, which correspond to and are aligned with the apertures 32 in the spacers.

[0174] When the electrostatic transducer is assembled, the diaphragm 2, spacers 28, 30 and stators 24, 26 are overlaid on each other and clamped together by the frames 34, 36, which are held together using screws 40. The spacers 28, 30 hold the stators 24, 26 in a spaced relation with the diaphragm 2 between them. Each spacer 28, 30 is 0.8 mm thick, so that the spacing between the diaphragm 2 and each of the stators 24, 26 is 0.8 mm.

[0175] In use, a DC bias of 1800V is applied to the conductive layer of the diaphragm 2. As discussed above, electrical contacts are provided on the conductive layer by removal or omission of a portion of the second insulating layer and adhesive layer from a region selected for applying the contact. The electrical contacts and voltage sources of the transducer are omitted from FIG. 3 for clarity.

[0176] To drive the movement of the diaphragm 2, a varying drive signal voltage corresponding to the desired audio signal is applied to the first stator 24, and a corresponding inverted signal applied to the second stator 26. The DC bias supplied to the diaphragm 2 creates an electrostatic field between the diaphragm and the stators, and the varying voltages applied to the stators results in a force on the diaphragm that causes it to vibrate, producing an acoustic wave corresponding to the drive signal voltage applied to the stators. The desired audio signal is thus reproduced.

[0177] FIG. 4 shows a cross-sectional view of a composite laminated diaphragm 42 in accordance with a fourth embodiment of a present invention. The diaphragm 42 comprises a first insulating layer 44 which serves as a substrate. The first insulating layer 44 is made from ULTEM® UTF120. In this example, the first insulating layer 44 is 5 μm thick, although other thickness are possible depending on acoustic performance requirements, e.g. 7 μm, 10 μm or other thicknesses.

[0178] A conductive layer 46 is deposited on a surface of the first insulating layer 44. The conductive layer 46 is a 25 nm thick layer of aluminium which is deposited on the first insulating layer 44 by sputtering or metal vapour deposition.

[0179] Overlaid on the conductive layer 46 is an epoxy-based adhesive layer 48 which is applied as a coating to the conductive layer 46 following plasma treatment of the conductive layer 46. The second insulating layer 50 is then rolled onto the adhesive layer 48, subjected to further plasma treatment, and pressure is applied using heated rollers to cause the layers to adhere together. The adhesive is cured at a temperature of 130° C. The second insulating layer 50 is also 5 μm thick and made from ULTEM® UTF120 (although similarly to the first insulating layer 44, other thickness are possible depending on acoustic performance requirements, e.g. 7 μm, 10 μm or other thicknesses). The adhesive layer is approximately 4 μm thick.

[0180] It will be appreciated that the layer thicknesses in FIG. 4 are not shown to scale.

[0181] It will be appreciated that only four embodiments of the invention have been described above, and that other embodiments and variations on the above embodiments are possible within the scope of the invention.