Loudspeaker

1. A loudspeaker comprising:

a voice coil; and

a diaphragm attached to the voice coil, the diaphragm having a generally rectangular outer shape and comprising:

an outer rim area having an outer edge and an inner edge, the diaphragm being fixed in position at the outer edge;

and an inner section surrounded by the outer rim area, the inner section comprising:

an outermost area adjacent to the outer rim area and coupled to the voice coil, the outermost area having the same generally rectangular outer shape as the diaphragm; and

an inner area surrounded by the outermost area, the inner area comprising;

a central rib area having a plurality of ribs in a periodic pattern, the ribs being oriented parallel to the shorter sides of the outermost area; and

two lateral transition areas between the edges of the central rib area and the shorter side edges of outermost area.

2. A loudspeaker as claimed in claim 1 , wherein the lateral transition areas are mirror symmetric about a line parallel to the shorter edges of the rectangular outer shape.

3. A loudspeaker as claimed in claim 1 , wherein the lateral transition areas each comprise a dome.

4. A loudspeaker as claimed in claim 3 , wherein each dome comprises a first portion which provides a height increase from a lowest height at the boundary with the periodic rib structure to a maximum height, and a second portion which provides a more sharply curved height decrease from the maximum height to the lowest height where the diaphragm is attached to the voice coil.

5. A loudspeaker as claimed in claim 4 , wherein the second portion has a smooth transition to the first portion and a discontinuous transition to the lowest height where the diaphragm is attached to the voice coil.

6. A loudspeaker as claimed in claim 1 , wherein the plurality of ribs in the central rib area extend fully between the longer sides of the outermost area.

7. A loudspeaker as claimed in claim 1 , wherein the height of the plurality of ribs in the central rib area does not exceed the height of the lateral transition areas.

8. A loudspeaker as claimed in claim 1 , wherein the longer sides of the generally rectangular diaphragm are less than 100 times the diaphragm thickness.

9. A loudspeaker as claimed in claim 1 , wherein the generally rectangular outer shape of the diaphragm comprises a rectangle with rounded corners having substantially the same radius.

10. A loudspeaker as claimed in claim 9 , wherein the width of the lateral transition areas in a direction parallel to the longer sides of the generally rectangular shape of the diaphragm corresponds to the radius of the corner curves of an inner edge of the outermost area.

11. A portable electronic device comprising a loudspeaker, the loudspeaker comprising:

a voice coil; and

a diaphragm attached to the voice coil,

wherein the diaphragm has a generally rectangular outer shape, and comprises an outer rim having an outer edge at which the diaphragm is fixed in position and an inner section within the outer rim, wherein the inner section comprises:

an outermost area which is coupled to the voice coil and having the same generally rectangular outer shape;

an inner area comprising a periodic rib structure, with the ribs running parallel to the shorter side of the rectangular outer shape, and two lateral transition areas between the edges of the periodic rib structure and the shorter side edges of outermost area.

12. A portable electronic device as claimed in claim 11 , comprising a mobile telephone.