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An acoustic panel includes a first cellular layer with cells having a first axis, a second cellular layer with cells having a second axis, and a perforated septum layer separating the first layer and the second layer. The cells of both layers have an end distal to the septum layer and an end which m

An acoustic panel includes a first cellular layer with cells having a first axis, a second cellular layer with cells having a second axis, and a perforated septum layer separating the first layer and the second layer. The cells of both layers have an end distal to the septum layer and an end which meets the septum layer, and the maximum diameter of the cells of the first layer is less than the maximum diameter of the cells of the second layer.

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1. An acoustic panel comprising a first cellular layer with cells having a first axis,a second cellular layer with cells having a second axis,and a perforated septum layer separating the first layer and the second layer, whereinthe cells of both layers have an end distal to the septum layer and an e

1. An acoustic panel comprising a first cellular layer with cells having a first axis,a second cellular layer with cells having a second axis,and a perforated septum layer separating the first layer and the second layer, whereinthe cells of both layers have an end distal to the septum layer and an end that meets the septum layer,the maximum diameter of the cells of the first layer is less than the maximum diameter of the cells of the second layer, andthe septum layer is at least 0.5 mm thick, but no more than 1.5 mm thick, the septum layer has an open area of at least 5% and no more than 20%, and the perforations in the septum layer are provided as holes having a diameter of at least 1 mm and no more than 5 mm. 2. The acoustic panel as claimed in claim 1 wherein the cells of the first cellular layer and second cellular layer have a substantially constant cross sectional area along their respective axes. 3. The acoustic panel as claimed in claim 1 wherein the distal end of the cells of the first layer is covered by a porous sheet. 4. The acoustic panel as claimed in claim 1, wherein the distal end of the cells of the second layer is closed by an imperforate sheet. 5. The acoustic panel as claimed in claim 1 wherein the ratio of first layer cell maximum diameter to second layer cell maximum diameter is substantially the same for all regions of the panel. 6. The acoustic panel as claimed in claim 1 wherein the first layer cell height is substantially different to the second layer cell height. 7. The acoustic panel as claimed in claim 6 wherein the first layer cell height is less than the second layer cell height. 8. The acoustic panel as claimed in claim 1 wherein at least some of the first layer cells have different heights to one another. 9. The acoustic panel as claimed in claim 1 wherein at least some of the second layer cells have different heights to one another. 10. The acoustic panel as claimed in claim 1 wherein the first and second axes are substantially aligned with one another. 11. The acoustic panel as claimed in claim 1 wherein the second axis is at an angle to the first axis. 12. The acoustic panel as claimed in claim 1 wherein at least some of the second cells comprise a substantially right angle bend such that they are “L” shaped in cross section along their central axis. 13. The acoustic panel as claimed in claim 1 wherein each of the cells is substantially polygonal. 14. The acoustic panel as claimed in claim 1 wherein the maximum diameter of the first layer cells is no less than 5 mm and no more than 18 mm. 15. The acoustic panel as claimed in claim 1 wherein the first layer cells height is no less than 13 mm and no greater than 38 mm. 16. The acoustic panel as claimed in claim 1 wherein the first layer cells height is no less than 5 mm and no greater than 26 mm. 17. The acoustic panel as claimed in claim 1 wherein the maximum diameter of the second layer cells is no less than 40 mm and no more than 500 mm. 18. The acoustic panel as claimed in claim 17 wherein the maximum diameter of the second layer cells is no less than 160 mm and no more than 300 mm. 19. The acoustic panel as claimed in claim 17 wherein the maximum diameter of the second layer cells is no less than 40 mm and no more than 150 mm. 20. The acoustic panel as claimed in claim 1 wherein the second layer cells height is no less than 25 mm and no greater than 500 mm. 21. The acoustic panel as claimed in claim 20 wherein the second layer cells height is no less than 38 mm and no greater than 368 mm. 22. The acoustic panel as claimed in claim 3 wherein the porous sheet is a perforated sheet, the perforations provided as holes having a diameter of at least 0.1 mm and no more than 2 mm. 23. The acoustic panel as claimed in claim 22 wherein the porous sheet is at least 0.5 mm thick, but no more than 1 mm thick. 24. The acoustic panel as claimed in claim 22 wherein the porous sheet has an open area of at least 8% and no more than 12%. 25. The acoustic panel as claimed in claim 22, wherein the porous sheet has an open area of at least 8% and no more than 10%. 26. The acoustic panel as claimed in claim 1 wherein the perforations in the septum layer are provided as holes having a diameter of at least 2 mm and no more than 4 mm. 27. The acoustic panel as claimed in claim 1 wherein the septum layer prevents propagation therethrough of acoustic waves having a frequency of greater than approximately 2 kHz. 28. A casing which defines a gas flow path, the casing comprising: an acoustic panel as claimed in claim 1,wherein the porous sheet defines, at least in part, the gas flow path, andthe first axis of the first layer cells being substantially at an angle to the direction of the gas flow path. 29. The casing as claimed in claim 28 wherein the first axis of the first layer cells are substantially perpendicular to the direction of the gas flow path. 30. The casing as claimed in claim 28 wherein the casing comprises at least part of a compressor section for a turbomachine. 31. The casing as claimed in claim 28 wherein the casing comprises at least part of a combustor section for a turbomachine. 32. The casing as claimed in claim 28 wherein the casing comprises at least part of a turbine section for a turbomachine.