GB2256395A

GB2256395A - Noise attenuation panel

Info

Publication number: GB2256395A
Application number: GB9211223A
Authority: GB
Inventors: Robert Samual Wilson
Original assignee: Short Brothers PLC
Current assignee: Short Brothers PLC
Priority date: 1991-05-30
Filing date: 1992-05-27
Publication date: 1992-12-09
Anticipated expiration: 2012-05-27
Also published as: IL102049A0; CA2095919A1; JPH06508935A; GB2256395B; GB9211223D0; WO1992022053A1; GB9111621D0; AU1762592A; EP0586452A1; BR9205581A

Abstract

Noise attenuation panel (10) comprises a component having a front face for exposure to sound to be attenuated and is made of or includes or comprises a reinforced frontal region which includes the front face and which is made of a porous permeable composite material formed from a thermoplastic matrix material reinforced with one or more non-thermoplastic reinforcing materials. The frontal region is preferably produced by powder sintering the composite material. The reinforcing materials may be in the form of discrete particles and comprise a powder, microspheres or microballoons or in the form of fibres which may be discontinuous or continuous filament fibres. The reinforced frontal region may be formed as a facing component part with the panel including a base component part having an array of cells. Alternatively, the component has an open-celled structure which includes the frontal region.

Description

NOISE ATTENUATION PANEL The present invention relates to noise attenuation panels and is particularly, although not exclusively, concerned with noise attenuation panels for use in attenuation of noise in aero engines.

In our co-pending patent application publication GB-A2223448 there is disclosed a noise attenuation panel having a backing component part, a facing component part and a cellular component part having a multiplicity of open-ended juxtaposed cells, the backing component part extending across the ends of the cells of the cellular component part at the rear thereof and the facing component part extending across the ends of the cells of the cellular component part at the front thereof. The facing component part comprises or includes an outer facing sheet which is made of a porous permeable thermoplastics material. Preferably, the porous permeable thermoplastics material is produced by powder sintering of a thermoplastics material.

In one embodiment of the invention described in our copending application publication GB-A-2223448, the facing component part includes an inner facing sheet which is perforated and which provides structural support for the outer facing sheet.

In another embodiment of the invention described in our co-pending application GB-A-2223448 the inner facing sheet is omitted and the outer facing sheet which is made of the porous permeable thermoplastic material is adhered direct to the front face of the cellular component part using an adhesive.

In our co-pending applications nos. 9101354.0, 9101355.7 9101395.2 and 9101353.2 noise attenuation panels have been proposed which have a facing sheet or frontal region made of a porous permeable thermoplastics material and in which the porous permeable thermoplastics material is preferably produced by powder sintering the thermoplastics material.

For example, in our co-pending patent application no.

91001354.0 there is disclosed a noise attenuation panel comprising a cellular component which has a front face and a rear face and a multiplicity of cells. Each cell extends from the front face to the rear face and is formed by wall portions which extend from the front face to the rear face. A cell dividing septum element is provided in each cell in a region where the wall portions forming the cell are continuous. The septum element extends across the cell and is secured to the wall portions to divide the cell into two sub-cells.

In one embodiment of the invention disclosed in application no. 9100354.0 the noise attenuation panel comprises a backing component part, a facing component part and a cellular component as hereinbefore described.

The backing component part extends across the ends of the cells of the cellular component at the rear thereof and the facing component part extends across the ends of the cells at the front thereof. The facing component part includes an outer facing sheet which is made of a porous permeable thermoplastics material. The porous permeable thermoplastics material is produced by powder sintering the thermoplastic material.

In our co-pending patent application no. 9101355.7 there is disclosed a noise attenuation panel comprising a first cellular component part, a second cellular component part and a backing component part. The first cellular component part has a front face and a rear face and has wall portions which extend across the first cellular component part from the front face to the rear face and which provide bounding surfaces for an array of cells.

The second cellular component part has a front face and a rear face and is in the form of an open-celled structure.

The backing component part adjoins the rear face of one of the cellular component parts. The front face of that cellular component part adjoins the rear face of the other cellular component part and a facing component part adjoins the front face of said other cellular component part.

In one embodiment of the invention disclosed in application no. 9101355.7 the facing component part includes an outer facing sheet which is made of a porous permeable thermoplastics material. The porous permeable thermoplastics materials is produced by powder sintering the thermoplastics material.

In our co-pending patent application no. 9101395.3 there is provided a structural cellular component for a noise attenuation panel comprising wall portions which provide bounding surfaces for an array of cells and which terminate at opposite faces of the component, with the wall portions or predetermined ones of the wall portions forming the cells extending to at least one of the faces of the component at an angle or angles inclined to that face.

In one embodiment of the invention disclosed in application no. 9101395.3 the noise attenuation panel comprises a backing component part, a facing component part and a structural cellular component as hereinbefore described. The backing component part extends across the ends of the cells of the structural cellular component at the rear thereof and the facing component part extends across the ends of the cells at the front thereof. The facing component part includes an outer facing sheet which is made of a porous permeable thermoplastics material. The porous permeable thermoplastics material is produced by powder sintering the thermoplastics material.

In our co-pending patent application no. 9101353.2 there is provided a noise attenuation panel comprising a cellular component have an open-celled structure and a front face for exposure to sound to be attenuated. By a cellular component having an open-celled structure is meant a cellular component having a multiplicity of intercommunicating cells obtained by aggregation of particulate material or by the displacement of material by a dispersion or like technique from a body of material in liquid phase followed by a solidifying step. The cellular component has a frontal region which includes the front face and in which the cell structure provides cells of a predetermined first size and at a predetermined first density effective to offer low resistance to the passage of gaseous flow across the front face and to prevent normal ingress of liquids into the component through the front face.The component has a base region beneath the frontal region in which the cell structure provides cells of a predetermined second size and at a predetermined second density effective for sound energy absorption.

In one embodiment of the invention described in application no. 9101353.2 the base region of the cellular component is provided by a base component part formed from an open-celled rigid or flexible plastic foam providing cells of the predetermined second size and at the predetermined second density and the frontal region is provided by a facing cellular part which comprises or includes an outer facing sheet. The outer facing sheet is made from a porous permeable thermoplastics material produced by powder sintering the material.

In another embodiment of the invention disclosed in application no. 9101353.2 the component is formed from an open celled rigid or flexible plastic foam which provides cells of the predetermined first and second cell sizes and/or densities for the frontal region and base region of the component. The frontal region is produced by powder sintering the thermoplastics material.

While the panels specifically disclosed in GB2223448A, and in the above mentioned co-pending applications have been found to be successful for use in aero-engine environments, they suffer from the disadvantage that the front face for exposure to the noise to be attenuated may not be sufficiently strong to withstand the conditions found in the aero-engine environment and may require reinforcement by a reinforcing sheet which increases the cost of production of the panel.

It is one object of the present invention to provide a noise attenuation panel which has a front face or frontal region which is better adapted for use in aero-engine environments and which can be produced more readily and inexpensively than the panels of the prior proposals.

According to the first aspect of the present invention, there is provided a noise attenuation panel comprising a component having a front face for exposure to sound to be attenuated characterised in that the component is made of or includes or comprises a reinforced frontal region which includes the front face and which is made of a porous permeable composite material formed from a thermoplastics matrix material reinforced with one or more non- thermoplastic reinforcing materials.

Preferably the frontal region is produced by powder sintering the composite material.

In a first embodiment of the present invention hereinafter to be described, at least one of the reinforcing materials is in the form of discrete particles. Suitable discrete particles may be in the form of a powder, micro-spheres and/or micro-balloons.

In a second embodiment of the present invention hereinafter to be described, at least one of the reinforcing materials is in the form of fibres. Suitable fibres may be discontinuous or continuous filament fibres.

In an embodiment of the present invention the reinforced frontal region is a facing component part and the panel comprises a base component part having wall portions which provide bounding surfaces for an array of cells and which terminate at opposite faces of the base component part.

The wall portions or predetermined ones of the wall portions forming the cells may extend to at least one of the faces of the base component part at an angle or angles inclined to that face.

There may be provided in each cell in a region where the wall portions forming the cell are continuous a cell dividing septum element which extends across the cell and which is secured to the wall portion to divide the cell into two sub-cells.

In another embodiment of the present invention the component has an open-celled structure which includes the frontal region and which provides in the frontal region cells of a predetermined first size and at a predetermined first density effective to offer low resistance to the passage of gaseous flow across the front face and to prevent normal ingress of liquids into the component through the front face and the component has a base region beneath the frontal region which provides cells of a predetermined second size and at a predetermined second density effective for sound energy absorption.

In yet another embodiment of the present invention the reinforced frontal region is a facing component part and the panel further comprises a first cellular base component part which has a front face and a rear face and which has wall portions which extend across the first cellular base component part from the front face to the rear face and which provide bounding surfaces for an array of cells, and a second cellular base component part which has a front face and a rear face and which is in the form of an open-celled structure and the front face of one of the cellular base component parts is secured to or adjoins the rear face of the other cellular base component part and the facing component part is secured to or adjoins the front face of the other cellular base component part.

In still another embodiment of the present invention the component comprises a first cellular component part which includes the frontal region and a second cellular component part. The first cellular component part has an open-celled structure and the cell structure provides in the frontal region cells of a predetermined first density effective to offer low resistance to the passage of gaseous flow across the front face and to prevent normal ingress of liquids into the component through the front face and the first cellular component part has a base region beneath the frontal region in which the cell structure provides cells of a predetermined second size and at a predetermined second density effective for sound energy absorption.The second cellular component part has a front face adjoining a rear face of the first cellular component part and a rear face and wall portions which extend across the second cellular component part from the front face to the rear face and which provide bounding faces for an array of cells.

According to the second aspect of the present invention, there is provided an aero-engine including as a structural part of the engine a panel according to the first aspect of the invention.

The panels specifically disclosed in GB 2223448A and in the above mentioned pending applications when employed in an aero-engine environment become subject to wide temperature operating ranges from sub-zero to elevated temperatures giving rise to thermal expansions of the panel over the range, which can vary markedly from those of adjacent structural parts made from non-thermoplastics materials.

A further advantage of a panel according to the invention is that the inclusion of non-thermoplastic reinforcing materials in the matrix material has the effect of reducing the net coefficient of thermal expansion. As a consequence, the reinforced frontal region or skin of the panel will expand and contract more uniformly with adjacent structural parts made from non-thermoplastics materials and result in a reduction in internal stresses within the structure at different temperatures within the operating range.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: - Fig 1 is a schematic isometric view from above of a noise attenuation panel according to first and second embodiments of the invention Fig 2 is a schematic cross section of an end region of the panel shown in Fig 1, secured to a supporting channel member.

Fig 3 is a schematic isometric view from above of a noise attenuation panel according to a third embodiment of the invention Fig 4 is a schematic isometric view from above of a noise attenuation panel according to a fourth embodiment of the invention Fig 5 is a schematic isometric view from above of a noise attenuation panel according to a fifth embodiment of the invention Fig 6 is a schematic cross section of a part of a noise attenuation panel according to a sixth embodiment of the invention Fig 7 is a schematic isometric view from above of a noise attenuation panel according to a seventh embodiment of the invention Fig 8 is a schematic isometric view from above of a noise attenuation panel according to an eighth embodiment of the invention Fig 9 is a schematic cross section of part of a noise attenuation panel according to a ninth embodiment of the invention and Fig 10 is a schematic cross section of an aero engine embodying noise attenuation panels according to the invention Referring first to Figs 1 and 2, the noise attenuation panel 10 comprises a backing sheet 11, a cellular component 12 and a facing sheet 14.

The cellular component 12 is shown in generalised form in Figs 1 and 2 and may take any one of a variety of different forms, some examples of which are hereinafter to be described with reference to Figs 3 to 9.

The backing sheet 11 is, as shown in Fig 2, secured by an epoxy resin adhesive El to the lower face of the cellular component 12. The facing sheet 14 is secured to the upper face of the cellular component 12 by an epoxy resin adhesive E2. The epoxy resin adhesives El and E2 may for example be obtained from Ciba-Geigy Plastics & Additives Company Limited of Cambridge, England. The adhesives need not however be epoxy resin adhesives, but could for example be a phenolic, polyimide or thermoplastics resin.

The panel lo is of arcuate form, possibly of double curvature, and is embodied as a structural part of a duct of a nose cowl of a turbofan aero engine, the panel 10 being one of several arcuate panels disposed just upstream of the fan of the engine. The structure will usually include supporting channel members of which only one member 17 is shown in Fig 2. The panel 10 is secured to the member 17 by bonding to an outer face of a flange 18 of the channel member 17 using a carbon to carbon bond 19 and by bonding the backing sheet 11 to the outer face of a flange 20 of the channel member 17 using a carbon to carbon bond 21. The gap between the panel 10 and the base 22 of the channel member 17 may be sealed or closed by use of a mastic 23.

The facing sheet 14 comprises a sheet of porous permeable composite material produced by powder sintering the composite material. The composite material is formed from thermoplastics matrix material reinforced with one or more non-thermoplastic reinforcing materials.

Examples of suitable thermoplastics matrix materials include polyether ether ketone, polyaromatic ketone, polyphenylene sulphide, polyamide-imide, thermoplastic polyimide, polyether-imide, polyurethene and polyethylene.

In a first embodiment of the invention, the nonthermoplastics reinforcing material is in the form of discrete particles. The discrete particles may consist of any of the following materials: (1) A powder, where suitable powders include marble flour, chalk powder, dolomite (magnesium/calcium carbonate), sand, silica flour, mica flour, slate powder, vermiculite, zircon flour, aluminium powder and barium sulphate.

(2) Microspheres where suitable materials for the microspheres include carbon and silica.

(3) Microballoons where suitable materials for the microballons include phenolic materials and silica/alumina/iron oxide.

In a second embodiment of the invention, the nonthermoplastic reinforcing material consists of fibres which may be either discontinuous or continuous.

Examples of suitable reinforcing materials in fibre form include carbon/graphite, fibreglass, silcon carbide, quartz, alumina, boron carbide, alumina/boria/silica, boron, steel fibre, asbestos fibre and chopped glass strands.

The properties of the facing sheet 14 may be varied by varying the quantity and type of reinforcing material included in the composite material so as to impart strength, and/or stiffness and to reduce the net coefficient of thermal expansion or vary the flow and/or acoustic properties of the facing sheet.

The backing sheet 11 is unperforated and made from a nonporous impermeable sheet material and may be made from any of the following materials: (i) A carbon/thermoplastic composite where for example the thermoplastic is polyether ether ketone, the material being automatically tape wound or hand laid.

(ii) A carbon/epoxy resin.

(iii) An aluminium alloy.

The cellular component 12 of the noise attenuation panel 10 described with reference to Figs 1 and 2 may take any one of a variety of different forms and noise attenuation panels featuring cellular components of such different forms will now be described with reference to Figs 3 to 9.

Referring now to Fig 3, the noise attenuation panel 10 comprises a backing sheet 11, a cellular component 121 and a facing sheet 14. The facing sheet 14 is made from a porous permeable composite material and constructed in the same manner as the facing sheet 14 of the panel 10 described with reference to Figs 1 and 2.

As will be seen, the cellular component 121 comprises a multiplicity of open ended juxtaposed cells 15 of hexagonal cross section to provide a honeycomb configuration. The walls of the cells 15 extend from the front face of the cellular component 12 to the rear face.

The cells 15 are preferably provided with drainage slots 16 to allow for condensates to drain from the panel 10.

The walls of the cells 15 of the cellular component 121 are preferably made from a non-porous impermeable sheet of any of the following materials: (i) A thermoplastic such as polyether ether ketone.

(ii) A polyester fabric/phenolic resin.

(iii) A fibreglass/phenolic resin.

(iv) A NOMEX/phenolic resin (NOMEX being a registered trade mark for an aramid fibre paper impregnated with various resins to produce a structural material and by "aramid is meant an aromatic polyamide polymer).

(v) An aluminium alloy.

Referring now to Fig 4, the noise attenuation panel 10 comprises a backing sheet 11, a cellular component 122 and a facing sheet 14 of the same form and constructed in the same manner as the facing sheet 14 of the panel described with reference to Figs 1 and 2.

As will be seen from Fig 4, the cellular component 122 comprises a multiplicity of open ended juxtaposed cells 15 of hexagonal cross section to provide a honeycomb configuration. The walls of the cells 15 extend from the front face of the cellular component 12 to the rear face.

Each cell 15 is, however, divided into an upper sub-cell 151 and lower sub-cell 152 by a septum element 141. The cells 15 are preferably provided with drainage slots 16 to allow for condensates to drain from the panel 10.

The septum elements 141 are fixed in position in the cells 15 using adhesive E3 which may be an epoxy resin adhesive and as shown lie in a planar surface extending throughout the component 12. The septum elements 141 are made from a porous permeable thermoplastics material.

Examples of suitable thermoplastics materials include polyether ketone, polyether ether ketone, polyaromatic ketone, polyphenylene sulphide, polyimide-imide, thermoplastic polyimide, polyether-imide, polyurethane and polyethylene.

The septum elements 141 may also if desired be made from a porous permeable composite material in the same manner as the facing sheet 14 of the panel described with reference to Figs 1 and 2.

Referring now to Fig 5, the noise attenuation panel 10 comprises a backing sheet 11, upper and lower cellular elements 123 and 124 and a facing sheet 14. The facing sheet 14 is made from a porous permeable composite material and constructed in the same manner as the facing sheet 14 of the panel described with reference to Figs 1 and 2.

The upper cellular element 123 comprises a multiplicity of open-ended juxtaposed cells 15 of hexagonal cross section which form a honeycomb configuration and the lower cellular element 124 is made from a porous permeable thermoplastics material produced by powder sintering the thermoplastics material or is in the form of an open-celled plastics foam.

The walls of the cells of the cellular element 123 are made from a non-porous impermeable sheet of any of the materials hereinbefore prescribed for the element 121 of Fig 3.

The walls of the cellular element 123 may alternatively be made of a porous permeable composite material and in particular may be constructed in the same manner as the facing sheet 14.

The cellular element 124 is in the form of an open-celled plastics foam and may be manufactured from any suitable thermoplastics material. Examples of suitable thermoplastics materials include polyether ketone, polyether ether ketone, polyaromatic ketone, polyphenylene sulphide. polyimide-imide and thermoplastic polyimide, polyether-imide, polyurethane and polyethylene. It may if desired be made from a porous permeable composite material and constructed in the same manner as the facing sheet 14.

Referring now to Fig 6, a panel 10 comprises a backing sheet 11, cellular elements 123 and 124 and a facing sheet 14. The backing sheet 11, the two cellular elements 123 and 124 and the facing sheet 14 take the same form as the corresponding elements of the panel 10 illustrated in Fig 5 and are joined together in the same manner by an adhesive El which secures the backing sheet 11 to the cellular element 123, an adhesive E2 which secures the facing sheet 14 to the upper face of the cellular element 124 and an adhesive E3 which may be an epoxy resin adhesive which secures the upper face of the element 123 to the lower face of the element 124.

The facing sheet 14 and the cellular element 124 may be made from compatible bonding materials and the facing sheet 14 bonded direct to the upper face of the cellular element 124 during manufacture of the panel.

Referring now to Fig 7 the noise attenuation panel 10 comprises a backing sheet 11, a cellular component 12 and facing sheet 14. The facing sheet 14 comprises a sheet of a porous permeable composite material constructed in the same manner as the sheet 14 of the panel described with reference to Figs 1 and 2 and the backing sheet 11 is also of the same form as the sheet 11 of the panel described with reference to Figs 1 and 2.

The cellular component 12 comprises, as shown, a multiplicity of open-ended juxtaposed inclined cells 15 of hexagonal cross section to provide a honeycomb configuration in which the lateral edges 153 are (i) parallel to each other, (ii) lie in planes which are perpendicular to the backing sheet 11 and parallel to the edge 111 of the sheet 11 and (iii) lie on lines which are inclined to the normal to the backing sheet, whereby the length of the cells 15 between the backing sheet 11 and the facing sheet 14 is greater than the distance of separation of the two sheets 11 and 14.

The cellular component 12 in the embodiment of the invention illustrated in Fig 7 is made from a non-porous impermeable sheet as described for the component 12 in Fig 3. The cellular component 12 may however if desired be made of a porous permeable thermoplastics material or of a reinforced porous permeable composite material of the same form and constructed in the same manner as the facing sheet 14 of the panel described with reference to Figs 1 and 2.

Referring now to Fig 8, the noise attenuation panel 10 comprises a backing sheet 11, a cellular component 125 and a facing sheet 14. The facing sheet 14 is made from a porous permeable composite material and constructed in the same manner as the facing sheet 14 of the panel described with reference to Figs 1 and 2.

The backing sheet 11 is secured by an epoxy resin adhesive El to the lower face of the cellular component 125. The facing sheet 14 is secured to the upper face of the cellular component 125 by bonding the materials to each other, although an adhesive could if desired be used.

The cellular component 125 is in the form of an opencelled plastics foam which may be manufactured from any suitable thermoplastics material. Examples of suitable thermoplastics materials include polyether ketone, polyether ether ketone, polyaromatic ketone, polyphenylene sulphide, polyamide-imide, thermoplastic polyimide, polyether-imide, polyurethane and polyethylene.

The component 125 may if desired alternatively be formed from the same materials as those from which the facing sheet 14 is formed and constructed in the same manner as the facing sheet 14 of the panel described with reference to Figs 1 and 2.

The panel shown in Fig 8 has been found to have the following advantages: (1) The cellular component 125 provides an uninterrupted layer of intercommunicating cells which provide a well distributed dissipation of sound energy and dampens back cavity resonances which smooths the reactance as a function of frequency and increase high frequency absorption.

(2) At low absorption frequencies, the speed of sound in the cellular matrix produced by the component 12 is decreased, giving a greater apparent treatment depth and enhancing low frequency absorption. Furthermore, beyond these inherently point reacting arguments, axial and circumferential wave travel in the material can also be used to advantage.

(3) The use of a facing sheet 14 produced as described ensures the formation of a high strength microporous facing sheet which prevents normal ingress of liquids which would normally destroy the acoustic effectiveness of the panel and could pose a safety problem.

(4) The cellular component 125 can be used to provide structural strength while preserving the desirable characteristics which are feature of it.

Referring now to Fig 9, an alternative embodiment of the invention is illustrated in which a panel 10 comprises a backing sheet 11 and an open-celled cellular component 126. The backing sheet 11 takes the same form as the backing sheet 11 of the panel 10 illustrated in Figs 1 and 2 and is joined in the same manner by an adhesive El to the cellular component 126.

The cellular component 126 has a cellular structure obtained by aggregation of particulate material. The component 126 is manufactured in such a way as to provide a frontal region 140 which includes the front face 141 and in which the cell structure provides cells of a size and at a density effective to offer low resistance to the passage of gaseous flow across the face 141 and to prevent normal ingress of liquids and a base region 142 beneath the frontal region 140 in which the cell structure provides cells of a size and at a density effective for sound absorption.

The component 126 is made of a porous permeable composite material formed from a thermoplastics matrix material reinforced with one or more or more non-thermoplastic reinforcing materials. The reinforcing materials may be in the form of discrete particles or fibres.

The cellular componenta 125 and 126 may be in the form of an open-celled plastics foam and the component 126 reinforced in the same manner as the facing sheet 14 of the panel described with reference to Figs 1 and 2.

The cellular components 125 and 126 may be produced by any well known technique for the production of a plastics foam of open-cell structure. For example, the foam may be produced by admixing a pore-forming ingredient with a thermoplastic polymer and with a solvent and then removing the solvent and the pore-forming ingredient so as to leave a system of cells intercommunicating with one another at the points of contact between individual cells. For the component 126, the foam may be produced by admixing the non-thermoplastic reinforcing material and the pore-forming ingredient with the thermoplastic polymer.

In yet another embodiment of the invention not illustrated, the cellular component 126 in the embodiment described with reference to Fig 9 is replaced by a cellular component having a cellular structure obtained by aggregation of particulate material. The component 112 is manufactured in such a way as to provide a frontal region 140 which includes the front face 141 and in which the cell strucure provides cells of a size and at a density effective to offer low resistance to the passage of gaseous flow across the face 141 and to prevent normal ingress of liquids and a base region 142 beneath the frontal region 140 in which the cell structure provides cells of a size and at a density effective for sound absorption.The component is made from a porous permeable composite material produced by powder sintering the composite material and is in the same form and constructed in the same manner as the facing sheet 14 of the embodiment of the invention described with reference to Figs 1 and 2.

Referring now to Fig 10, an aero engine 25 is schematically illustrated and includes a turbofan power unit 26 mounted within a nacelle 27 suspended from a plyon 32. The nacelle 27 includes a nose cowl 28 having an outer wall 29 and an inner wall 30. The inner wall 30 is in part formed by noise attenuation panels P which may take the form of panels as described and illustrated with reference to Figs 1 to 9. The panels P are arranged to form part of the inner wall of the nose cowl 28 and serve to reduce noise created by the high speed flow of air passing through the duct 31 into the power unit 26, as well as to reduce noise generated by the fan blades of the unit 26.

It is to be emphasised that the panels in Fig 10 are not employed to reduce air noise by a reduction of the air speed by passage of the air through the panels, but by contrast acoustic attenuation is achieved without affecting the speed of the air which generates the noise, that is to say, the air does not pass through the noise attenuation panels P.

In the aero engine mounting arrangement illustrated in Fig 10, the power unit is carried by the wing mounted plyon 32. It will be appreciated that the noise attenuation panels according to the present invention may be equally well be employed for reducing the noise in other aero engine installations.

Where a reinforced porous permeable composite material is used as the facing sheet 14 in a noise attenuation panel as disclosed in any of the embodiments of the invention described with reference to Figs 1 to 8, there is no need for an inner facing sheet to provide structural support for the facing sheet 14. It will however be appreciated that where very high structural strength is required an inner facing sheet may be inserted between the sheet 14 and the cellular component 12.

Where an inner facing sheet is desired it may be made from an open square weave fabric formed from a carbon fibre/resin matrix composite material. The weave may be such as to provide apertures constituted by the openings between adjacent warp and weft threads of the fabric and the fabric is preferably so woven as to produce a proportion of open aperture area relative to the total surface area of the sheet of 30% or substantially 30%.

The outer facing sheet 14 is bonded or secured by an adhesive to the inner facing sheet and the inner facing sheet is then secured to the upper face of the cellular component 12 by means of an epoxy resin adhesive.

Noise attenuation panels according to the present invention and having a reinforced frontal region or facing sheet as hereinbefore described may of course be used other high level noise environments.

Claims

1. Noise attenuation panel comprising a component having a front face for exposure to sound to be attenuated characterised in that the component is made of or includes or comprises a reinforced frontal region which includes the front face and which is made of a porous permeable composite material formed from a thermoplastics matrix material reinforced with one or more non-thermoplastic reinforcing materials.

2. A panel according to claim 1 wherein the frontal region is produced by powder sintering the composite material.

3. A panel according to claim 1 or 2 wherein at least one of the reinforcing materials is in the form of discrete particles.

4. A panel according to claim 3 wherein the discrete particles are a powder.

5. A panel according to claim 3 wherein the discrete particles are micro spheres.

6. A panel according to claim 3 wherein the discrete particles are micro balloons.

7. A panel according to claim 1 or 2 wherein at least one of the reinforcing materials is in the form of fibres.

8. A panel according to claim 7 wherein the fibres are discontinuous filament fibres.

9. A panel according to claim 7 wherein the fibres are continuous filament fibres.

10. A panel according to any of claims 1 to 9 wherein the reinforced frontal region is a facing component part and wherein the panel comprises a base component part having wall portions which provide bounding surfaces for an array of cells and which terminate at opposite faces of the base component part.

11. A panel according to claim 10 wherein there is provided in each cell in a region where wall portions forming the cell are continuous a cell dividing septum element which extends across the cell and which is secured to the wall portions to divide the cell into two sub-cells.

12. A panel according to claim 10 wherein the wall portions or predetermined ones of the wall portions forming the cells extend to at least one of the faces of the base component part at an angle or angles inclined to that face.

13. A panel according to any of claims 1 to 9 wherein the reinforced frontal region is a facing component part and wherein the panel further comprises a first cellular base component part which has a front face and a rear face and which has wall portions which extend across the first cellular base component part from the front face to the rear face and which provide bounding surfaces for an array of cells, and a second cellular base component part which has a front face and a rear face and which is in the form of an open-celled structure and wherein the front face of one of the cellular base component parts is secured to or adjoins the rear face of the other cellular base component part and the facing component part is secured to or adjoins the front face of the other cellular base component part.

14. A panel according to any of claims 1 to 9 wherein the component further comprises a first cellular component part which includes the frontal region and a second cellular component part and wherein the first cellular component part has an open-celled structure and the cell structure provides in the frontal region cells of a predetermined first density effective to offer low resistance to the passage of gaseous flow across the front face and to prevent normal ingress of liquids into the component through the front face and the first cellular component part has a base region beneath the frontal region in which the cell structure provides cells of a predetermined second size and at a predetermined second density effective for sound energy absorption and the second cellular component part has a front face adjoining a rear face of the first cellular component part and a rear face and wall portions which extend across the second cellular component part from the front face to the rear face and which provide bounding faces for an array of cells.

15. A panel according to any of claims 1 to 9 wherein the component has an open-celled structure which includes the frontal region and wherein the cell structure provides in the frontal region cells of a predetermined first size and at a predetermined first density effective to offer low resistance to the passage of gaseous flow across the front face and to prevent normal ingress of liquids into the component through the front face and the component has a base region beneath the frontal region in which the cell structure provides cells of a predetermined second size and at a predetermined second density effective for sound energy absorption.

16. An aero-engine including as a structural part of the engine a panel according to any of the preceeding claims.

17. A noise attenuation panel substantially as hereinbefore described with reference to Figs 1 and 2 and any of Figs 3 to 9 of the accompanying drawings.

18. An aero-engine including as a structural part of the engine a panel substantially as hereinbefore described with reference to Figs 1 and 2 and any of Figs 3 to 9 of the accompanying drawings.