High efficient miniature electro-acoustic transducer with reduced dimensions

1. A magnetic circuit comprising:

an inner permanent magnet assembly and an outer permanent magnet assembly,

a magnetically permeable yoke, and

first and second air gap portions conducting first and second magnetic flux densities, respectively, the first and second air gap portions being adapted to receive first and second voice coil segments, respectively,

wherein the magnetic flux density in the first air gap portion is generated by superposition of magnetic flux generated by the inner permanent magnet assembly and magnetic flux generated by the outer permanent magnet assembly, and wherein the magnetic flux density in the second air gap portion is generated substantially exclusively by the inner permanent magnet assembly.

2. A magnetic circuit according to claim 1 , wherein the magnetic circuit further comprises third and fourth air gap portions adapted to receive third and fourth voice coil segments, respectively, wherein the magnetic flux density in the third air gap portion is generated by superposition of magnetic flux generated by the inner permanent magnet assembly and magnetic flux generated by the outer permanent magnet assembly, and wherein the magnetic flux density in the fourth air gap portion is generated substantially exclusively by the inner permanent magnet assembly.

3. A magnetic circuit according to claim 2 , wherein the first and third air gap portions are essentially linearly shaped air gap portions arranged in a substantially parallel manner.

4. A magnetic circuit according to claim 2 , wherein the second and fourth air gap portions are essentially linearly shaped air gap portions arranged in a substantially parallel manner.

5. A magnetic circuit according to claim 2 , wherein the inner and outer permanent magnet assemblies are arranged on a substantially plane portion of the magnetically permeable yoke.

6. A magnetic circuit according to claim 5 , wherein the magnetically permeable yoke comprises first and second outer pole piece portions, said first and second outer pole piece portions extending from the substantially plane portion of the magnetically permeable yoke.

7. A magnetic circuit according to claim 6 , wherein the first and second outer pole piece portions extend in a substantially perpendicular direction from the substantially plane portion of the magnetically permeable yoke.

8. A magnetic circuit according to claim 7 , wherein the magnetic circuit further comprises first and second outer pole pieces arranged on first and second permanent magnets, respectively, of the outer permanent magnet assembly.

9. A magnetic circuit according to claim 8 , wherein the first and second outer pole pieces form an integral part of a pole piece ring, said pole piece ring being arranged on the first and second pole piece portions of the magnetically permeable yoke along the second and fourth air gap portions.

10. A magnetic circuit according to claim 1 , wherein the inner permanent magnet assembly comprises an inner pole piece arranged on an inner permanent magnet.

11. A magnetic circuit comprising first and second air gap portions adapted to receive first and second voice coil segments, respectively, wherein the first air gap portion is provided between inner magnetic means and first outer magnetic means, and wherein the second air gap portion is provided between said inner magnetic means and first outer pole piece means, and wherein the first and second air gap portions are arranged essentially perpendicular to each other.

12. A magnetic circuit according to claim 11 , wherein the magnetic circuit further comprises third and fourth air gap portions adapted to receive third and fourth voice coil segments, respectively, wherein the third air gap portion is provided between said inner magnetic means and second outer magnetic means, and wherein the fourth air gap portion is provided between said inner magnetic means and second outer pole piece means, and wherein the third and fourth air gap portions are arranged essentially perpendicular to each other.

13. A magnetic circuit comprising first and second air gap portions adapted to receive first and second voice coil segments, respectively, wherein magnetic flux acting on the first voice coil segment is provided by inner magnetic means and first outer magnetic means in combination, and wherein magnetic flux acting on the second voice coil segment is essentially provided by said inner magnetic means only.

14. A magnetic circuit according to claim 13 , wherein the magnetic circuit further comprises third and fourth air gap portions adapted to receive third and fourth voice coil segments, respectively, wherein magnetic flux acting on the third voice coil segment is provided by said inner magnetic means and second outer magnetic means in combination, and wherein magnetic flux acting on the fourth voice coil segment is essentially provided by said inner magnetic means only.

15. A magnetic circuit comprising:

first and second air gap portions adapted to receive first and second voice coil segments, respectively, wherein magnetic flux acting on the first voice coil segment is provided by inner magnetic means and first outer magnetic means in combination, and wherein magnetic flux acting on the second voice coil segment is essentially provided by said inner magnetic means only,

third and fourth air gap portions adapted to receive third and fourth voice coil segments, respectively, wherein magnetic flux acting on the third voice coil segment is provided by said inner magnetic means and second outer magnetic means in combination, and wherein magnetic flux acting on the fourth voice coil segment is essentially provided by said inner magnetic means only

wherein the inner magnetic means, the first outer magnetic means and the second outer magnetic means are configured so that the magnetic flux densities in the first, second, third and fourth air gap portions are essentially equal in strength.

16. A magnetic circuit according to claim 15 , wherein the strength of the magnetic flux densities in the first, second, third and fourth air gap portions differ less than 20% from each other.