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The invention relates to a method of manufacturing a composite acoustic panel employed in an inlet passage of a gas turbine engine (1). The acoustic panel comprises a permeable face-layer (8), an impermeable backing sheet (9) and a sound absorbing layer (10) disposed therebetween. The method compris

The invention relates to a method of manufacturing a composite acoustic panel employed in an inlet passage of a gas turbine engine (1). The acoustic panel comprises a permeable face-layer (8), an impermeable backing sheet (9) and a sound absorbing layer (10) disposed therebetween. The method comprises a double polymerisation process for the face-layer and the remainder of the acoustic panel and finally a perforation step to perforate the face-layer according to a pre-determined perforation distribution (11.1, 11.2, 11.3, 11.4, 11.5).

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1. A method of manufacturing a composite acoustic panel comprising a permeable face-layer, an impermeable backing sheet and a sound absorbing layer disposed therebetween, the method comprising the steps of: laying-up a first composite material laminate to form a face-layer;subjecting said first lami

1. A method of manufacturing a composite acoustic panel comprising a permeable face-layer, an impermeable backing sheet and a sound absorbing layer disposed therebetween, the method comprising the steps of: laying-up a first composite material laminate to form a face-layer;subjecting said first laminate to a first polymerisation cycle to form a substantially continuous face-layer at a first polymerisation pressure of approximately atmospheric pressure or at a pressure greater than atmospheric pressure;laying-up of a sound absorbing layer onto the polymerised face-layer, the sound absorbing layer comprising a plurality of discrete cells defined by cell walls;laying-up a second composite material laminate onto the sound absorbing layer to form a backing sheet;subjecting the entire composite acoustic panel to a second polymerisation cycle at a second polymerisation pressure; andperforating the face-layer at a plurality of positions and to a pre-determined depth, wherein the perforations range in diameter from 50 microns to 1.6 mm and are arranged to coincide with and penetrate at least one of the cell walls of the sound absorbing layer thereby connecting two adjacent cells. 2. A method as described in claim 1, wherein the first polymerisation cycle is performed in an autoclave and the second polymerisation cycle is performed out-of-autoclave. 3. A method as described in claim 1, wherein the first step of polymersiation cycle is performed out-of-autoclave and a second stage of polymerisation cycle is also performed out-of-autoclave. 4. A method according to claim 1 wherein a second polymerisation pressure is atmospheric pressure or between 1 and 1.5 bar. 5. A method according to claim 1 wherein a first polymerisation pressure is between 1 and 1.5 bar. 6. A method as described in claim 1 wherein the face-layer, impermeable backing sheet and sound absorbing layer are each layed-up to define a generally cylindrical or barrel shape corresponding in shape to a predetermined engine inlet. 7. A method as claimed in claim 6, wherein the face-layer is layed-up around a collapsible mandrel. 8. A method as claimed in claim 6 wherein the face-layer is layed-up around a tapered mandrel allowing the polymerised part to be extracted axially therefrom. 9. A method as claimed in claim 1, wherein the polymerised face-layer is additionally provided with an adhesive layer before the sound absorbing layer is layed-up on to said polymerised face-layer. 10. A method as claimed in claim 1, wherein the sound absorbing layer is additionally provided with an adhesive layer before the backing sheet is layed-up on to said sound absorbing layer. 11. A method as claimed in claim 1, wherein the sound absorbing layer is layed-up on to the polymerised face-layer such that ends of the layer extending around the face-layer are coupled together to define a continuous/seamless sound absorbing layer. 12. A method as claimed in claim 1, wherein the discrete cells are in a honeycomb arrangement. 13. A method as claimed in claim 12, wherein the cells each comprise a septum. 14. A method as claimed in claim 13, wherein the septum comprises a rim or tail extending towards the face-layer. 15. A method as claimed in claim 1, wherein the septum comprises a rim or tail extending towards the backing sheet. 16. A method as claimed in claim 1 wherein an electrically insulating layer is arranged between the face-layer and the sound absorbing layer and/or between the sound absorbing layer and the backing sheet. 17. A method as claimed in claim 1 wherein the perforations are formed by means of a drill head comprising a plurality of drill spindles. 18. A method as claimed in claim 1 wherein the perforations are formed by means of a laser adapted to ablate the material used to form the face-layer. 19. A method as claimed in claim 18, wherein the laser further comprises a beam splitter adapted to allow multiple perforations to be formed simultaneously. 20. A method as claimed in claim 1, wherein the face-layer is moved with respect to the perforation apparatus by means of a rotating member arranged to receive and secure the acoustic panel thereto. 21. A method as claimed in claim 1 wherein the face-layer comprises a percentage of open area of between 1 and 35%. 22. A method as claimed in claim 1, wherein the perforations are arranged to coincide with at least a portion of the wall of each cell forming the sound absorbing layer. 23. A method as claimed in claim 1 where the perforation step and a pre-determined distribution is applied to the face layer irrespective of the start position of the perforation step. 24. A method of preforating a face-layer of a composite acoustic panel with a predetermined perforation distribution pattern, said panel comprising a seamless face-layer, an impermeable backing sheet and a sound absorbing layer disposed therebetween, wherein the sound absorbing layer is formed of a plurality of discrete cells in a honeycomb arrangement, and the cells each comprise a septum, the method comprising steps of:A—aligning a perforating apparatus relative to the seamless face-layer at a first preforating position;B—activating the perforating apparatus to form at least one perforation through the face-layer;C—moving the acoustic panel and the perforating appartus relative to one another by a predetermined amount to align the preforating appartus with a subsequent perforation position; andD—repeating steps B and C until the desired perforation distribution is achieved, and wherein the perforations extend into the sound absorbing layer to a maximum depth corresponding to the position of a septum where a septum tail extends toward the backing sheet or to a maximum depth corresponding to the position of the septum tail where the septum extends towards the face-layer, andthe perforations range in diameter from 50 microns to 1.6 mm and are arranged to coincide with and penetrate at least one wall of the cells of the sound absorbing layer defining a boundary between adjacent cells thereby connecting two adjacent cells. 25. A method of perforating an acoustic panel, said panel comprising a substantially continuous face-layer, an impermeable backing sheet and a honeycomb sound absorbing layer disposed therebetween, said layer comprising a plurality of discrete cells, the method comprising the steps of: A—pre-determining a perforation depth h according to: h=t+∈ whereh is the total perforation deptht is the thickness of the face-layer∈ is the perforation depth into the sound absorbing layerB—pre-determining a perforation spacing and a distribution pattern for a face-layer surface;C—aligning a perforating apparatus with the face-layer;D—perforating the face-layer to the predetermined perforation depth h with perforations ranging in diameter from 50 microns to 1.6 mm and with at least one perforation coinciding with and penetrating at least one wall of the cells of the sound absorbing layer thereby connecting two adjacent cells;E—moving the acoustic panel relative to the perforating appartus to align the perforating apparatus with a subsequent perforation position; andF—repeating steps D and E until a desired portion of the face-layer has been perforated. 26. A method as claimed in claim 25, wherein the sound absorbing layer is a single layer cavity without a septum or a metallic foam and the perforation depth into the sound absorbing layer c is between 0.5 and 1 mm. 27. A method as claimed in claim 25, wherein the sound absorbing layer is a double-layer septum bonded onto honeycomb walls and the perforation depth into the sound absorbing layer ε is between 0.5 and 5 mm.