WO2008003583A1

WO2008003583A1 - Miniature electro-acoustic transducer with integrated coupling coil

Info

Publication number: WO2008003583A1
Application number: PCT/EP2007/056065
Authority: WO
Inventors: Morten Kjeldsen Andersen; Kurt Sørensen
Original assignee: Sonion Horsens A/S
Priority date: 2006-07-07
Filing date: 2007-06-19
Publication date: 2008-01-10
Also published as: KR20090027749A; CN101491112A; EP2041995A1

Abstract

The present invention relates to a miniature electro-acoustic transducer assembly comprising a magnet circuit comprising a permanent magnet assembly arranged to generate a magnetic flux in an air gap, a voice coil positioned at least partly in t he air gap, the voice coil being operatively connected to a displaceable diaphragm, and a coupling coil formed by an electrically conducting path, the coupling coil being adapted to couple to a T-coil of an associated hearing aid by generating, in relation to the miniature electro-acoustic transducer, an external electro- magnetic field having a spatial extension overlapping said coil of the associated hearing aid, wherein diaphragm displacements are essentially unaffected by current flowing in the electrically conducting path forming the coupling coil.

Description

MINIATURE ELECTRO- ACOUSTI C TRANSDUCER WITH INTEGRATED COUPLING COIL

FIELD OFTHE INVENTION

The present invention relates to a miniature electro-acoustic transducer comprising an integrated coupling coil for electro-magnetic coupling to a T-coil of an associated hearing aid. In particular, the present invention relates to a miniature electro-acoustic transducer where the coupling coil is a separate electro-magnetic device that does not form an integral part of any voice coil.

BACKGROUND OF THE I NVENTI ON

US 2005/0244022 discloses a removable bezel for use with for example a cellular phone to enhance operation with a hearing aid. The removable bezel has an integrated electromagnetic coil that is coupled to an audio output device of the cellular phone. The electromagnetic coil is either inductively coupled to an acoustic transducer within the cellular phone or coupled thereto directly by direct electrical connections.

The electro-magnetic coil suggested in US 2005/0244022 constitutes an integrated part of a removable bezel and provides an enhanced magnetic field to a T-coil magnetic pick up within most conventional hearing aids.

It is a disadvantage of the arrangement suggested in US 2005/0244022 that the acoustic transducer of the cellular phone and the coil integrated in the bezel of the cellular phone are separate and discrete components. In case of a cellular phone not having an electro- magnetic coil integrated in its bezel, the bezel needs to be replaced before effective electromagnetic coupling to a T-coil of an associated hearing aid can be achieved.

WO 03/063545 discloses a displaceable diaphragm for a miniature electro-acoustic transducer. The diaphragm comprises a flexible sheet having first and second sheet sections. Each sheet section has one or more electrically conductive paths, forming one or more coils, integrated therewith. Each of the coils has an active part which is positioned in a magnetic field so that forces acting on the active part, in response to currents passing through the active part, are transformed to displacements of the diaphragm. Thus, the coils of WO 03/063545 are all optimised for causing maximum displacements of the diaphragm in response to currents passed through the coils.

A consequence of the coils being optimised for diaphragm displacements is that essentially no stray electro-magnetic fields from the coils are detectable on the exterior of the miniature electro-acoustic transducer disclosed in WO 03/063545. Thus, the coils of WO 03/063545 are not suitable for providing an effective electro-magnetic coupling to a T-coil of an associated hearing aid.

It is an object of the present invention to provide a miniature electro-acoustic transducer offering enhanced electro-magnetic coupling to a T-coil of an associated hearing aid.

SUMMARY OF THE I NVENTI ON

The present invention relates, in a first aspect, to a miniature electro-acoustic transducer assembly comprising a magnet circuit, a first coil, and a second coil. The magnetic circuit may comprise a permanent magnet assembly arranged to generate a magnetic flux in an air gap. The first coil may be positioned at least partly in the air gap. The first coil may be regarded as a moving voice coil operatively connected to a displaceable diaphragm. The second coil may be regarded as a coupling coil formed by an electrically conducting path. The second coil (coupling coil) may be adapted to couple to a T-coil of an associated hearing aid by generating, in relation to the miniature electro-acoustic transducer, an external electro- magnetic field having a spatial extension overlapping the coil of the associated hearing aid. The diaphragm displacements may be essentially unaffected by current flowing in the electrically conducting path forming the second coil.

The term "essentially unaffected" means that current flowing in the second coil does not introduce diaphragm displacements that are sufficient to generate detectable air pressure variations. Thus, electrical current flowing in the second coil does not contribute to diaphragm displacements. On the contrary, the first coil may be optimised to produce maximum diaphragm displacements in response to current passed through it. This may be achieved by positioning the first coil in the air gap where it is exposed to a magnetic flux generated across the air gap. Thus, by ensuring maximum spatial overlap between the magnetic flux and the first coil, effective diaphragm displacements may be obtained when a drive current is passed through the first coil.

In order to ensure that diaphragm displacements are essentially unaffected by current passed through the second coil, the second coil may be positioned outside the air gap and is therefore not exposed to the magnetic flux generated in the air gap. Thus, since the second coil is positioned outside the air gap and, consequently, is not exposed to the magnetic flux, current passing through the second coil does not introduce diaphragm displacements. Contrary to the first coil, the second coil is adapted to generate an externally detectable electro-magnetic field that may be picked-up by a T-coil of an associated hearing aid. The first and second coils of the miniature electro-acoustic transducer may be connected in series. The series connection may be established by first passing a drive current through the first coil and then passing the same driving current through the second coil. Alternatively, the drive current may be passed through the first and second coils in the opposite order. In an alternative configuration, the drive current may firstly be passed through a first part of the second coil, then the first coil and finally through a second part of the second coil.

The series-connected first and second coils may have a total electrical impedance within the range 20 to 50 ohms, such as within the range 30 to 40 ohms, such as approximately 35 ohms. The impedance of the first coil may be approximately 32 ohms whereas the impedance of the second coil may be approximately 3 ohms. Generally speaking, if the first and second coils are connected in series, the impedance of the first coil may be approximately ten times larger than the impedance of the second coil.

In an alternative embodiment, the first and second coils may be connected in parallel. According to this embodiment the impedance of the second coil may be approximately ten times larger than the impedance of the first coil. Thus, in a parallel configuration of the first and second coils the first coil may have an impedance of approximately 32 ohms whereas the impedance of the second coil may be approximately 320 ohms or even higher.

According to one embodiment of the present invention, the diaphragm may comprise a flex- print portion having the second coil arranged on an upper surface portion thereof whereas the first coil may be operatively connected to a lower surface portion of the flex-print portion. The lower surface portion may be substantially parallel with the upper surface portion.

In a second embodiment, the second coil may be arranged on a foldable carrier comprising first and second coil portions arranged on respective first and second portions of the foldable carrier. The foldable carrier is arranged on an exterior surface portion of the transducer. The foldable carrier may comprise a foldable flex-print having the first and second coil portions arranged on respective first and second flex-print portions. The foldable flex-print is, in a folded state, arranged on an exterior surface portion of a cover of the electro-acoustic transducer. The first and second flex-print portions may be separated by a folding line along which the foldable flex-print is adapted to be folded so that the first and second coil portions may be arranged on opposing flex-print portions when the foldable flex-print is in a folded state. Each of the first and second flex-print portions may comprise one or more sound openings. The one or more sound openings are adapted to be aligned with one or more sound outlets in the cover of the electro-acoustic transducer. The dimensions of the one or more sound openings in the flex-print portions may be dimensioned to match the corresponding dimensions of the one or more sound outlets in the cover. The shape of the one or more sound openings in the flex-print portions may in principle be arbitrary, such as circular, elliptical, rectangular, and quadratic or a combination thereof. The sound outlets in the cover may be arranged as a through-hole or opening, or optionally, as multiple through- holes or openings with an acoustical damper having specific acoustical properties arranged therein.

In a third embodiment, the second coil may be arranged in an indentation or groove formed in a cover covering the diaphragm of the electro-acoustic transducer. The indentation or groove formed in the cover covering the diaphragm may extend primarily along the circumference of the cover. Thus, the shape of the second coil according to this embodiment will be given by the circumference of the cover. The indentation or groove formed in the cover may be formed in combination with an exterior housing portion edge of the transducer. The orientation of the exterior housing portion edge and coil supporting portion of the cover may be arranged substantially perpendicular to each other. The exterior housing portion edge, the coil supporting portion of the cover, and a bridging portion of the cover encapsulates three surfaces of the second coil, which effectively protects the second coil against for example accidental drops.

In a fourth embodiment, the second coil may be arranged as an integral part of a thermoformed cover covering the diaphragm of the electro-acoustic transducer. The thermoformed cover may comprise a flex-print having an average thickness within the range 100-500 μm, such as within the range 200-300 μm. The second coil may comprise electrically connected outer and inner portions, such as an outer and an inner half, wherein the outer portion of the second coil may be arranged on an exterior surface portion of the thermoformed cover. The inner portion of the second coil may be arranged on an interior surface portion of the thermoformed cover.

The miniature electro-acoustic transducer according to the present invention may further comprise a magnetically permeable outer housing portion forming part of the magnet circuit in that at least part of the magnetically permeable outer housing portion may form an outer pole piece of the magnet circuit. The magnetically permeable outer housing portion may comprise an essentially circular through-going opening, wherein an inner surface or edge of said through-hole or opening defines the air gap. By this is meant that the inner surface of the through-hole or opening may define the outer boundary of the air gap. The inner boundary of the air gap may be defined by an inner pole piece, or alternatively, by a bobbin, as discussed below.

In one embodiment, the magnet circuit may comprise an annular permanent magnet having a first surface attached to the magnetically permeable outer housing portion. The magnet circuit may further comprise a magnetically permeable yoke attached to a second surface of the annular permanent magnet. The second surface may be substantially parallel to the first surface. The magnetically permeable yoke may comprise a magnetically permeable bobbin substantially concentrically arranged with the annular magnet.

Instead of the bobbin, the magnet circuit may comprise a centre magnet substantially concentrically arranged with the annular magnet. The centre magnet may be arranged with a first surface attached to the magnetically permeable yoke. The magnet circuit may further comprise a magnetically permeable centre pole piece substantially concentrically arranged with the annular magnet. The centre pole piece may be attached to a second surface of the centre magnet. The second surface of the centre magnet may be substantially parallel to the first surface of the centre magnet.

One or more air flow passages may be provided in the magnetically permeable outer housing portion. The one or more air flow passages allow air trapped below the diaphragm to escape to the exterior of the transducer. Furthermore, a first and a second contact arrangement for providing a first electrical connection and a second electrical connection from the exterior of the transducer to the interior of the transducer may be included. Each of the first and second contact arrangements may comprise an electrically conducting resilient member and a U- shaped clamp. The electrically conducting resilient member of each contact arrangement may comprise a spring element comprising a helically-shaped spring member. Each of the U- shaped clamps may form an exterior contact pad arranged to receive a spring element. In addition, each of the U-shaped clamps may form an interior contact pad arranged to receive a wire end from the first coil. Each of the U-shaped clamps may comprise a flex-print, which is folded along a folding line. Each of the U-shaped clamps therefore comprises an inner and an outer flex-print portion, which are essentially parallel to each other.

In a second aspect, the present invention relates to a portable communication unit comprising a miniature electro-acoustic transducer according to the first aspect of the present invention. The portable communication unit may be selected from the group consisting of: cellular phones, PDAs, game consoles and portable computers.

The above summary of the present invention is not intended to represent each embodiment, or every aspect, of the present invention.

BRI EF DESCRI PTI ON OF THE DRAWI NGS

The present invention will now be explained in further details with reference to the accompanying drawings, wherein Fig.1 shows a coupling coil arranged on a foldable flex-print.

Fig.2 shows the flex-print of Fig. 1 in a folded state.

Figs.3a and 3b show a miniature electro-acoustic transducer having a folded flex-print arranged thereon.

Figs.4a and 4b show a miniature electro-acoustic transducer having a coupling coil arranged along the circumference of a transducer cover.

Figs.5a and 5b show top and bottom views of a flex-print diaphragm having a coupling coil integrated therewith.

Figs.6a and 6b show top and bottom views of a cover for a miniature electro-acoustic transducer, the cover having an integrated coupling coil.

While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OFTHE INVENTION

In its most general aspect the present invention relates to a miniature electro-acoustic transducer having a coupling coil integrated wherewith. The term "coupling coil" is to be understood herein as a separate component capable of generating an, in relation to the miniature electro-acoustic transducer, electro-magnetic field which can be picked up by a T- coil of an associated hearing aid. Thus, the coupling coil is a separate electro-magnetic device that does not form an integral part of any voice coil.

Fig. 1 shows a first embodiment of the present invention. A carrier 1 in the form of a foldable flex-print has electrically conductive paths 2, 3 arranged thereon. Each of the electrically conductive paths 2, 3 of Fig. 1 constitutes a half coil and they may be fabricated by any suitable material, such as copper. The electrically conducting paths 2, 3 may be manufactured using conventional techniques applied to process conventional PCBs. The pitch of the electrically conducting paths is approximately 0.4 mm in that the width of the electrically conducting paths is approximately 0.2 mm. Similarly, the distance between the electrically conducting paths is 0.2 mm.

The foldable flex-print is adapted to be folded along the dashed line so that the electrically conducting paths 2, 3 are located on opposing inner surface portions of the folded flex-print 1. The electrically conductive paths 2, 3 are arranged to be connected in series when the flex-print 1 is in its folded state, as shown in Fig.2. This series connection is established by connecting contact pad 4 to contact pad 5, and connecting contact pad 6 to contact pad 7. The resulting electrically conducting path, now forming the complete coupling coil, is electrically accessible via contact terminals 8 and 9.

The foldable flex- print 1 comprises a plurality of sound openings 10, 11, 12, 13. In a folded state of the flex-print 1 opening 10 is arranged to be aligned with opening 12, and opening 11 is arranged to be aligned with opening 13 as depicted in Fig.2.

In Fig.3a, a folded flex-print has been positioned on a cover 14 of a miniature electromagnetic transducer 15. The folded flex-print is positioned on the cover 14 in such a way that sound openings 10, 11, 12, 13 are aligned with corresponding sound outlets in the cover 14 of the miniature electro- magnetic transducer 15 to allow audible signals generated by the miniature electro- magnetic transducer 15 to propagate freely to the surroundings.

The contact terminals 8, 9 are connected in series with the contact terminals 16, 17 of a voice coil (see Fig.4b) positioned within the m iniature electro- magnetic transducer 15. The voice coil, which is positioned in an air gap of a magnet circuit, is operatively connected to a displaceable diaphragm. When an electrical current is passed through the voice coil, a force acts on the diaphragm. This applied force causes the diaphragm to be displaced from its rest position in accordance with the electrical current passed through the voice coil. In this way, audible signals may be generated.

If the voice coil and coupling coil are driven simultaneously, they are, preferably, driven in phase. The reason for this is that in case the coils are driven out of phase (for example, 180 degrees out of phase), the electro-magnetic fields generated by the two coils counteract each other. Such counteracting behaviour will lead to a reduced coupling efficiency with a T-coil of an associated hearing aid. Thus, the voice coil and the coupling coil must, preferably, be driven in phase, or alternatively, the coupling coil must be driven separately, i.e. without being operatively connected to the voice coil.

The impedance of the voice coil itself is around 32 ohms whereas the impedance of the coil formed by the electrical path on the flex-print is significantly smaller, typically around 3 ohms. Thus, the impedance of the voice coil is approximately ten times larger that the impedance of the coil formed by the electrical path on the flex-print.

Fig.3b shows the miniature electro-magnetic transducer of Fig.3a from a bottom perspective. The contact term inal 8 is electrically connected to contact term inal 16 via connector 18 which, typically, is a copper path on a flex-print. Thus, if both the voice coil and the coupling coil formed by the electrical path on the foldable flex-print are to be driven in series, a drive signal from an associated drive circuit should be applied to the terminals 9 and 17. If only an audible signal is required, only the voice coil needs to receive the drive signal from the drive circuit. In this situation, the drive signal should only be applied to terminals 16 and 17. The magnet circuit of the m iniature electro- magnetic transducer will be described in further details in connection with Fig.4b.

A second embodiment of the present invention is depicted in Fig.4a and 4b. Fig.4a shows a m iniature electro- magnetic transducer 15 having a coupling coil 21 arranged near the circumference of the cover 14. The cover 14 has sound outlets 20 in the form of through- holes or openings arranged therein. The coupling coil 21 , according to this second embodiment of the present invention, is implemented as a wounded wire, such as an enamelled copper wire, dimensioned to fit into the groove formed by the cover 14 and a magnetically permeable outer housing portion 19. The coupling coil 21 shown in Fig.4a and 4b has an essentially rectangular cross-sectional profile, but other cross-sectional profiles, such as essentially circular, elliptical etc. may also be applicable.

Fig.4b shows a cross-sectional profile of a miniature electro-acoustic transducer comprising a coupling coil according to the second embodiment. Also, Fig.4b shows an example of how the magnet circuit can be implemented. The magnet circuit of Fig.4b is constituted by an annular magnet 22, a centre magnet 23, a centre pole piece 24 and magnetically permeable yoke 25. The annular magnet and the centre magnet are permanent magnets. Part of the magnetically permeable outer housing portion 19 forms an outer pole piece which, in combination with the centre pole piece 24, defines an air gap adapted to receive a voice coil 26. The voice coil 26 is operatively connected to a displaceable diaphragm 27. When a time varying electrical drive signal is passed through the voice coil 26 the diaphragm 27 is displaced accordingly.

The centre magnet and/or the annular magnet may comprise NdFeB compounds having a remanence flux density of at least 1.2 T, a coercive force of at least 1000 kA/m and an energy product of at least 300 kJ/m³. As an example, an NdFeB N44H may be applied. The air gap may have awidth in the range 0.5 - 0.8 mm, such as around 0.6 mm. The average magnetic flux density in the air gap may be in the range 0.3 - 1.5 T, such as in the range 0.5 - 1 T. Preferably, the voice coil is made of a wound copper wire or a wound Copper-Clad Alum inium (CCA) wire. I n the case of a CCA wire, the copper content may be around 15%.

Suitable pole piece materials are low carbon content steel materials, such as materials similar to Werkstoff-No. 1.0330 (St 2), 1.0333 (St 3), 1.0338 (St 4), all in accordance to DIN EN 10130. The magnetically permeable outer housing portion may also be manufactured in one of these materials.

The diaphragm may be constituted by a polymer film having a thickness in the range 5 -25 μm. The diaphragm may be a single-layer diaphragm, or it may be a multi-layer diaphragm where a second polymer film is attached to at least part of a bigger polymer film. By laminating a diaphragm with another diaphragm the stiffness of specific regions of the diaphragm may be significantly increased. The types of polymer films may be polyarylate (PAR), polyetherimide (PEI), polyrtheretherketone (PEEK), polyphenylene sulphide (PPS), polyethylenenapthalate (PEN), terephtalate (PET) or polycarbonate (PC).

In a third embodiment, a flex-print based diaphragm can be used as a carrier for a coupling coil as set forth in Figs.5a and 5b. As depicted in Fig.5a, the coupling coil portions 28 according to this third embodiment is arranged on a substantially plane and stiff flex-print portion 29 which is attached to a flexible surround 30. The flex-print portion 29 can be attached to the surround 30 by means such as for example gluing, heating or ultrasound- based welding.

The edge 31 along the circumference of the surround 30 is adapted to be attached to the housing of the miniature electro-acoustic transducer or to the magnet circuit of the miniature electro-acoustic transducer. Electrical wires 32, 33 are integrated in the surround 30 so that a drive signal from an associated drive circuit can be provided to the coupling coil portions 28 provided on the flex-print 29 via contact pads 34, 35, which are electrically connected to contact pads 36, 37, respectively. End contact pads 38, 39 are electrically connected to respective ones of contact pads 40, 41 which are interconnected to form an electrical path between the wires 32, 33. Alternatively, the contact pads 40, 41 can be connected to wire ends of a voice coil whereby a series connection of the coupling coil portions 28 is established.

The surround can be a moulded surround which is made of a soft material, such as silicone, rubber or a similar soft material. The surround material can be chosen independently of the flex-print material. It should be mentioned that the flex-print material can be replaced by another material, such as Kapton® (polyimide), aluminium, nylon, etc.

As mentioned in connection with the first embodiment of the present invention, the impedance of an associated voice coil is around 32 ohms, whereas the impedance of the coupling coil formed by the electrical path on the flex-print portion 29 is significantly smaller, typically around 3 ohms. Thus, the series connection of the voice coil and the coupling coil is slightly exceeding 32 ohms.

Fig. 6 shows top and bottom views of another embodiment of the present invention. Fig. 6a shows a top view of a cover 42 for a miniature electro-acoustic transducer. The cover is made of a relatively stiff material, such as a 200-300 μm thick thermoformed flex-print. By applying such a relative stiff flex-print, the coupling coil 43 can be integrated with the cover. As seen in Fig. 6a, half of the coupling coil 43 is arranged on the top side of the cover whereas, as seen in Fig. 6b, the other half of the coupling coil is arranged on the bottom side of the cover. The two halves are electrically connected via centre point 44 whereby an electrical connection is established between coil ends 45, 46. The cover 42 is also equipped with sound outlet openings 47 so that sound generated by the transducer is allowed to escape to the surroundings. The coupling coil can be coupled in series or in parallel with a voice coil of miniature electro-acoustic transducer.

It should be noted that the layout of the coupling coil shown in Fig. 6 can be different. For example, a layout similar to that depicted in Fig. 5 is also applicable.

While the present invention has been described with reference to one or more particular embodiments, those skilled in the art will recognize that many changes may be made thereto without departing from the spirit and scope of the present invention. Each of these embodiments and obvious variations thereof is contemplated as falling within the spirit and scope of the claimed invention, which is set forth in the following claims.

Claims

CLAI MS

1. A miniature electro- acoustic transducer assembly comprising

- a magnet circuit comprising a permanent magnet assembly arranged to generate a magnetic flux in an air gap;

- a first coil positioned at least partly in the air gap, the first coil being operatively connected to a displaceable diaphragm; and

- a second coil formed by an electrically conducting path, the second coil being adapted to couple to a T-coil of an associated hearing aid by generating, in relation to the miniature electro-acoustic transducer, an external electro-magnetic field having a spatial extension overlapping said coil of the associated hearing aid;

wherein diaphragm displacements are essentially unaffected by current flowing in the electrically conducting path forming the second coil.

2. A miniature electro-acoustic transducer according to claim 1 , wherein the first and second coils of the miniature electro-acoustic transducer are connected in series or in parallel.

3. A miniature electro- acoustic transducer according to claim 2, wherein the series or parallel-connected first and second coils have a total electrical impedance within the range 20 to 50 ohms, such as within the range 30 to 40 ohms, such as approximately 35 ohms.

4. A miniature electro- acoustic transducer according to any of claims 1-3, wherein the diaphragm comprises a flex-print portion having the second coil arranged on an upper surface portion thereof.

5. A miniature electro-magnetic transducer according to claim 4, wherein the first coil is operatively connected to a lower surface portion of the flex-print portion, the lower surface portion being substantially parallel with the upper surface portion.

6. A miniature electro- acoustic transducer according to any of claims 1-3, wherein the second coil is arranged on a foldable carrier comprising first and second coil portions arranged on respective first and second portions of the foldable carrier, the foldable carrier being arranged on an exterior surface portion of the transducer.

7. A miniature electro-acoustic transducer according to claim 6, wherein the foldable carrier comprises a foldable flex-print having the first and second coil portions arranged on respective first and second flex-print portions, the foldable flex-print being, in a folded state, arranged on an exterior surface portion of a cover of the electro-acoustic transducer.

8. A miniature electro-acoustic transducer according to claim 7, wherein the first and second flex-print portions are separated by a folding line along which the foldable flex-print is adapted to be folded.

9. A miniature electro-acoustic transducer according to claim 8, wherein the first and second coil portions are arranged on opposing flex-print portions when the foldable flex-print is in a folded state.

10. A miniature electro- acoustic transducer according to any of claims 7-9, wherein each of the first and second flex-print portions comprises one or more sound openings, said one or more sound openings being adapted to be aligned with one or more sound outlets in the cover of the electro-acoustic transducer.

11. A miniature electro- acoustic transducer according to any of claims 1-3, wherein the second coil is arranged in an indentation or groove formed in a cover covering the diaphragm of the electro-acoustic transducer.

12. A miniature electro- acoustic transducer according to claim 11, wherein the indentation or groove formed in the cover covering the diaphragm extends primarily along the circumference of said cover.

13. A miniature electro- acoustic transducer according to any of claims 1-3, wherein the second coil is arranged as an integral part of a thermoformed cover covering the diaphragm of the electro-acoustic transducer.

14. A miniature electro-acoustic transducer according to claim 13, wherein the thermoformed cover comprises a flex- print having an average thickness within the range of 100-500 μm, such as within the range of 200-300 μm.

15. A miniature electro- acoustic transducer according to claim 14, wherein the second coil comprises electrically connected outer and inner portions, wherein the outer portion of the second coil is arranged on an exterior surface portion of the thermoformed cover, wherein the inner portion of the second coil is arranged on an interior surface portion of the thermoformed cover.

16. A miniature electro-acoustic transducer according to any of the preceding claims, further comprising a magnetically permeable outer housing portion forming part of the magnet circuit.

17. A miniature electro- acoustic transducer according to claim 16, wherein at least part of the magnetically permeable outer housing portion forms an outer pole piece of the magnet circuit.

18. A miniature electro- acoustic transducer according to claim 17, wherein the magnetically permeable outer housing portion comprises an essentially circular through-going opening, wherein an inner surface of said through-going opening defines the air gap.

19. A miniature electro- acoustic transducer according to any of claims 16-18, wherein the magnet circuit comprises an annular permanent magnet having a first surface attached to the magnetically permeable outer housing portion.

20. A miniature electro- acoustic transducer according to claim 19, wherein the magnet circuit comprises a magnetically permeable yoke attached to a second surface of the annular permanent magnet, the second surface being substantially parallel to the first surface.

21. A miniature electro- acoustic transducer according to claim 20, wherein the magnetically permeable yoke comprises a magnetically permeable bobbin substantially concentrically arranged with the annular magnet.

22. A miniature electro-acoustic transducer according to claim 20, wherein the magnet circuit comprises a centre magnet substantially concentrically arranged with the annular magnet, the centre magnet being arranged with a first surface attached to the magnetically permeable yoke.

23. A miniature electro-acoustic transducer according to claim 22, wherein the magnet circuit comprises a magnetically permeable centre pole piece substantially concentrically arranged with the annular magnet, the centre pole piece being attached to a second surface of the centre magnet, the second surface of the centre magnet being substantially parallel to the first surface of the centre magnet.

24. A miniature electro- acoustic transducer according to any of claims 16-23, wherein one or more air flow passages are provided in the magnetically permeable outer housing portion, the one or more air flow passages allowing air trapped below the diaphragm to escape to the exterior of the transducer.

25. A miniature electro-acoustic transducer according to any of the preceding claims, further comprising a first and a second contact arrangement for providing a first and a second electrical connection from the exterior of the transducer to the interior of the transducer, each of the first and second contact arrangements comprising an electrically conducting resilient member and a U-shaped clamp.

26. A miniature electro-acoustic transducer according to claim 25, wherein the electrically conducting resilient member of each contact arrangement comprises a spring element.

27. A miniature electro-acoustic transducer according to claim 26, wherein each of the U- shaped clamps forms an exterior contact pad arranged to receive a spring element, and wherein each of the U-shaped clamps forms an interior contact pad arranged to receive a wire end from the first coil.

28. A miniature electro-acoustic transducer according to any of claims 25-27, wherein each of the U-shaped clamps comprises a flex-print.

29. A portable com munication unit comprising a miniature electro- acoustic transducer according to any of the preceding claims.

30. A portable communication unit according to claim 29, wherein the portable communication unit is selected from the group consisting of: cellular phones, PDAs, game consoles and portable computers.