Loudspeaker

1. A loudspeaker, comprising:

a box-form structure made from a single sheet of stiff lightweight sheet material having one or more folds which define a plurality of faces, with a first face and a second face of the structure respectively forming first and second panel-form bending wave acoustic radiators, the first and second faces being adjacent with a fold therebetween; and

an electro-acoustic vibration transducer coupled to the first face to apply bending wave energy to the first radiator to cause it to radiate an acoustic output when an input signal is applied to the transducer,

wherein the sheet of stiff lightweight material is sufficiently flexible at the fold(s) to be collapsible to a flat form for at least one of storage and transportation and re-erectable as a box form for use as a loudspeaker, and

wherein the second face is driven by bending wave energy which is transmitted across the fold between the adjacent first and second faces.

2. A loudspeaker according to claim 1 , wherein a fold between at least two adjacent faces comprises a parallel pair of folds.

3. A loudspeaker according to claim 1 , wherein the one or more folds are formed by grooving the sheet material.

4. A loudspeaker according to claim 3 , wherein the grooving comprises local compression of the sheet material.

5. A loudspeaker according to claim 1 , wherein the box-form structure comprises a plurality of panels made from stiff lightweight sheet material united at panel edges by connectors.

6. A loudspeaker according to claim 5 , wherein the connectors comprise hinge portions.

7. A loudspeaker according to claim 5 or claim 6 , wherein the connectors permit the transmission of bending wave energy between panels.

8. A loudspeaker according to claim 1 , wherein the one or more folds are discontinuous.

9. A loudspeaker according to claim 1 , wherein the box-form structure is a truncated pyramid.

10. A loudspeaker according to claim 9 , wherein the plane of the truncation is angled with respect to the plane of the base of the pyramid.

11. A loudspeaker according to claim 1 , wherein the radiators are resonant.

12. A loudspeaker according to claim 11 , wherein the radiator comprises a distributed mode resonator.

13. A loudspeaker according to claim 1 , wherein the box-form structure is open.

14. A loudspeaker according to claim 1 , wherein the box-form structure comprises ground engaging feet.

15. A loudspeaker according to claim 1 , further comprising:

an additional vibration transducer coupled to another of the plurality of faces.

16. A loudspeaker according to claim 1 , wherein the box-form structure comprises one of concertina and fold-out form.

17. A loudspeaker according to claim 16 , wherein at least one face of the box-form structure is formed with a fold whereby said face is foldable on itself to collapse the box-form structure.

18. A loudspeaker according to claim 17 , wherein the fold in said face is formed substantially central of said face, whereby said face can be folded in half to collapse the box-form structure.

19. A loudspeaker according to claim 17 or claim 18 , further comprising:

a tab disposed adjacent to the fold and integral with said face, the tab extending across the fold when the structure is erect to prevent folding of the said face in one direction.

20. A loudspeaker according to claim 1 , further comprising:

a support flap connected to one of the plurality of faces of the box-form structure and which can be folded to abut at least one adjacent face to hold the box-form structure erect.

21. A loudspeaker according to claim 20 , wherein the support flap is adapted to abut two adjacent faces.

22. A loudspeaker according to claim 20 or claim 21 , wherein the support flap provides a spacer between the interior surfaces of two adjacent faces when the box-form structure is collapsed, one of which interior surfaces has the vibration transducer coupled thereto, to provide a cavity for receiving the vibration transducer.

23. A loudspeaker according to claim 1 , further comprising:

a fastener to maintain the box-form structure flat when collapsed.

24. A loudspeaker according to claim 1 , wherein the stiff lightweight sheet material comprises corrugated cardboard having face skins sandwiching a corrugated core.

25. A loudspeaker according to claim 24 , wherein the box-form structure defines a front face having a base and at least one side face and wherein the corrugated core is arranged so that its corrugations extend perpendicular to the base.

26. A loudspeaker according to claim 25 , wherein the at least one side face has a base and wherein the orientation of the corrugations in the at least one side face is at an acute angle to its base.

27. A loudspeaker according to claim 1 , wherein the vibration transducer comprises an inertial electrodynamic device having a coil assembly coupled to the first radiator and a magnet assembly resiliently suspended on the first radiator.

28. A loudspeaker according to claim 27 , wherein a spider structure resiliently suspends the magnet assembly on the first radiator to prevent non-axial motion of the magnet assembly relative to the coil assembly.

29. A loudspeaker according to claim 2 , wherein the folds are formed by grooving the sheet material.

30. A loudspeaker according to claim 29 , wherein the grooving comprises local compression of the sheet material.

31. A loudspeaker according to claim 5 or claim 6 , wherein the connectors are discontinuous.

32. A loudspeaker according to claim 23 , wherein the fastener comprises at least one press stud.