TECHNICAL FIELD

[0001] The disclosure relates generally to aircraft, and more particularly, but not exclusively, to noise-attenuating devices for aircraft. This application claims priority from UK Patent Application No. GB1906452.6, filed 8 ^th May 2019, the entire contents of which are expressly incorporated by reference herein.

BACKGROUND

[0002] Aircraft noise is an important consideration in designing an aircraft. This is primarily due to the increase in air traffic and the related proliferation in the number of noise events in the vicinity of airports. A significant portion of aircraft noise is generated by aircraft engines. Modern aircraft engine nacelles have acoustic liners that are intended to reduce the amount of radiated engine noise. Some acoustic liners can have a generally non-linear behaviour which can make their acoustic performance sensitive to variations in operating conditions.

SUMMARY

[0003] Aspects of the invention provide a device, a facing member, an aircraft, a nacelle and a method as claimed in the appended claims.

[0004] In an embodiment, the disclosure describes a device for attenuating noise in an environment. The device comprises:

a backing member; and

a facing sheet spaced apart from the backing member to define a cavity between the backing member and the facing sheet, the facing sheet having:

an outer side for facing the environment and an opposite inner side facing the backing member;

one or more through holes extending from the outer side to the inner side; and a coating on the outer side of the facing sheet, the coating defining an exposed surface of the facing sheet for direct exposure to the environment and a flared entrance to each of the one or more holes.

[0005] In another embodiment, the disclosure describes a noise-attenuating device. The device comprises:

a backing member; and

a facing sheet spaced apart from the backing member to define a cavity between the backing member and the facing sheet, the facing sheet having: an outer side and an opposite inner side facing the backing member;

one or more through holes extending from the outer side to the inner side; and a coating on the outer side of the facing sheet, the coating defining a flared entrance to each of the one or more holes,

wherein the noise-attenuating device is devoid of a layer of fibrous or woven material disposed over the coating.

[0006] In another embodiment, the disclosure describes a noise-attenuating device. The device comprises:

a backing member; and

a facing sheet spaced apart from the backing member to define a cavity between the backing member and the facing sheet, the facing sheet having:

an outer side and an opposite inner side facing the backing member;

one or more through holes extending from the outer side to the inner side; and a non-adhesive coating on the outer side of the facing sheet, the non-adhesive coating defining a flared entrance to each of the one or more holes.

[0007] The coating may include paint. The coating may include a primer and paint.

[0008] The coating may have a thickness that is at least 10% of a diameter of the one or more holes. The coating may have a thickness that is between 10% and 15% of a diameter of the one or more holes. The coating may have a thickness that is between 5% and 20% of the diameter of the one or more holes. The coating may have a thickness between 0.1 mm and 0.4 mm.

[0009] Each of the one or more holes may have a diameter between 0.5 mm and 2 mm. Each of the one or more holes may have a diameter between 1 mm and 2 mm.

[0010] The one or more flared entrances defined by the coating may extend inside the one or more respective holes.

[0011] The coating may define a bell mouth entrance to each of the one or more holes.

[0012] The facing sheet may be made from a fibre-reinforced composite material.

[0013] The device may have a sole degree of freedom.

[0014] Embodiments may include combinations of the above features. [0015] In another aspect, the disclosure describes an aircraft comprising a device as disclosed herein.

[0016] In another aspect, the disclosure describes a nacelle for an aircraft engine, the nacelle comprising a device as disclosed herein.

[0017] In another aspect, the disclosure describes a facing member of a device for attenuating noise in an environment. The facing member comprises:

an outer side for facing the environment and an opposite inner side;

one or more through holes extending from the outer side to the inner side; and a coating on the outer side of the facing member, the coating defining an exposed surface of the facing sheet for direct exposure to the environment and a flared entrance to each of the one or more holes.

[0018] The coating may include paint. The coating may include a primer and paint.

[0019] The coating may have a thickness that is at least 10% of the diameter of the one or more holes. The coating may have a thickness that is between 10% and 15% of the diameter of the one or more holes. The coating may have a thickness that is between 5% and 20% of the diameter of the one or more holes. The coating may have a thickness between 0.1 mm and 0.4 mm.

[0020] Each of the one or more holes may have a diameter between 0.5 mm and 2 mm. Each of the one or more holes may have a diameter between 1 mm and 2 mm.

[0021] The one or more flared entrances defined by the coating may extend inside the one or more respective holes.

[0022] The coating may define a bell mouth entrance to each of the one or more holes.

[0023] The facing sheet may be made from a fibre-reinforced composite material.

[0024] Embodiments may include combinations of the above features.

[0025] In a further embodiment, the disclosure describes a method for manufacturing a facing sheet of a device for attenuating noise in an environment. The method comprises:

receiving a precursor to the facing sheet, the precursor having: an outer side and an opposite inner side; and

one or more through holes extending from the outer side to the inner side; and applying a coating to the outer side of the precursor, the coating defining an exposed surface of the facing sheet for direct exposure to the environment and a flared entrance to each of the one or more holes.

[0026] The coating may include paint. The coating may include a primer and paint.

[0027] The coating may have a thickness that is at least 10% of the diameter of the one or more holes. The coating may have a thickness that is between 10% and 15% of the diameter of the one or more holes. The coating may have a thickness that is between 5% and 20% of the diameter of the one or more holes.

[0028] The one or more holes may have a diameter between 0.5 mm and 2 mm.

[0029] The coating may have a thickness between 0.1 mm and 0.4 mm.

[0030] The one or more flared entrances defined by the coating may respectively extend inside the one or more holes.

[0031] The outer sheet may be made from a fibre-reinforced composite material.

[0032] The one or more holes in the facing sheet may have a substantially uniform diameter.

[0033] Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

[0034] Further details of these and other aspects of the subject matter of this application will be apparent from the detailed description included below and the drawings. DESCRIPTION OF THE DRAWINGS

[0035] Embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0036] FIG. 1 is a top plan view of an example aircraft including one or more noise-attenuating devices as disclosed herein;

[0037] FIG. 2 is an axial cross-sectional view of an example aircraft engine including one or more noise-attenuating devices as disclosed herein;

[0038] FIG. 3 is a partial perspective view including a cutaway portion of an example noise attenuating device;

[0039] FIG. 4 is a flowchart of an example method for manufacturing a facing sheet of the noise attenuating device of FIG. 3;

[0040] FIGS. 5A-5D show a schematic cross-sectional portion of the facing sheet of the noise attenuating device of FIG. 3 and sequentially illustrate steps for manufacturing the facing sheet;

[0041] FIG. 6 shows an enlarged schematic cross-sectional portion of the facing sheet of FIG. 5D;

[0042] FIG. 7 shows a magnified photograph of a cross-sectional portion of a sample facing sheet;

[0043] FIG. 8 shows plots indicative of the acoustic resistance of sample noise-attenuating devices having coated and uncoated facing sheets with an open area of about 7%, at different operating conditions; and

[0044] FIG. 9 shows plots indicative of the acoustic resistance of sample noise-attenuating devices having coated and uncoated facing sheets with an open area of about 4.5%, at different operating conditions.

DETAILED DESCRIPTION

[0045] This disclosure relates to the design and manufacturing of noise-attenuating devices sometimes referred to as“acoustic liners”,“acoustic panels” or“acoustic treatment” for aircraft or other applications. In some embodiments, the noise-attenuating devices described herein may be suitable for use in nacelles of aircraft engines, passenger cabins of aircraft, trains, trucks or other vehicles, structural framework/bodies of aircraft and other vehicles, and in industrial/civil or other applications requiring noise attenuation.

[0046] In some embodiments, the noise-attenuation devices described herein may have an improved (e.g., more linear) acoustic response without the need for a layer of fibrous or woven material (e.g., metallic wire mesh) bonded to the outer side of the facing sheet. In some embodiments, the methods disclosed herein may permit the manufacturing of single-layer (i.e., single/sole degree of freedom) acoustic liners that exhibit a more linear acoustic response, that are less costly to manufacture, that are of lower weight, and that are more robust compared to some existing acoustic liners. The methods disclosed herein may also be used to manufacture double-layer (i.e., double degree of freedom) acoustic liners.

[0047] Aspects of various embodiments are described through reference to the drawings.

[0048] FIG. 1 is a top plan view of an example aircraft 10 including a noise-attenuating device 12 as disclosed herein. Aircraft 10 may be a corporate, private, commercial or any other type of aircraft. For example, aircraft 10 may be a fixed-wing or rotary-wing aircraft. In some embodiments, aircraft 10 may be a narrow-body, twin engine jet airliner or an (e.g., ultra-long-range) business jet. Aircraft 10 may be a drone controlled remotely. Aircraft 10 may have fuselage 14, wings 16, empennage 18 and one or more engines 20. Noise-attenuating device(s) 12 may be integrated in engine(s) 20 or other part of aircraft 10.

[0049] FIG. 2 is an axial cross-sectional view of an example turbofan aircraft engine 20 including one or more noise-attenuating devices 12 as disclosed herein. Noise-attenuating device(s) 12 may be integrated into nacelle 22 of engine 20. Engine 20 may include core engine 24 carrying fan blades 26 and surrounded by nacelle 22, which provides annular bypass duct 28 for conducting a high-speed gaseous fan stream to an annular outlet nozzle 30. Nacelle 22 may include thrust reverser 32 shown in the upper portion of FIG. 2 in a stowed position and in the lower portion of FIG. 2 in a deployed position. In order to reduce noise emanating in the fan duct in the region of fan 12 and/or of thrust reverser 32, part(s) of the walls of nacelle 22 may be lined with noise-attenuating devices 12. Noise-attenuating device(s) 12 may be disposed at one or more locations within flow ducts of engine 20. Such flow duct(s) may include inlet duct 34, bypass duct 28 and/or outlet nozzle 30 of nacelle 22. For example, noise-attenuating device 12 may be used as a lip acoustic liner/panel or an inlet acoustic liner/panel.

[0050] Noise-attenuating device 12 may be part of a single-piece or multi-piece acoustic liner/panel having a generally linear (e.g., flat or planar) or curved configuration. For example, noise attenuating device 12 may be part of a single-piece or a multi-piece annular-shaped acoustic liner/panel for installation into nacelle 22 and defining a noise-attenuating region that extends substantially 360 degrees about (e.g., central) axis A. The term “substantially” as used herein may be applied to modify any quantitative representation which could permissibly vary without resulting in a change in the basic function to which it is related.

[0051] FIG. 3 is a partial perspective view including a cutaway portion of an example noise attenuating device 12. In some embodiments, noise attenuating device 12 may be manufactured according to the methods disclosed herein and may have a more linear behaviour than some existing single-layer acoustic liners. Noise attenuating device 12 may include a perforated facing member such as facing sheet 36, honeycomb or other cellular member (e.g., core) 38 and one or more sound-reflecting backing member (e.g., plate) 40. Alternatively, facing sheet 36 may be secured in a spaced-apart relationship from backing member 40 without cellular structure 38 disposed therebetween. Accordingly, noise-attenuating device 12 may include one or more cavities defined between facing sheet 36 and backing member 40 and may be constructed with or without cellular structure 38.

[0052] Cellular structure 38 may be of any type suitable for use in noise attenuation device 12 and its selection may be dependent on the specific application for noise attenuating device 12. In some embodiments, cellular structure 38 may have a honeycomb configuration. For example, in some embodiments, cellular structure 38 may include an aramid-fiber reinforced honeycomb structure sold under the trade name NOMEX® by FI EXCEL COMPOSITES. Cellular structure 38 may be bonded between facing sheet 36 and backing member 40. For example, cellular structure 38 may be securely joined with facing sheet 36 and with backing member 40 with a suitable adhesive and/or with other suitable means (e.g., fastener(s)). In some embodiments, such joining of cellular structure 38 with facing sheet 36 and with backing member 40 may hinder noise transmission leakage between adjacent cells 42 of cellular structure 38 during use of noise-attenuating device 12.

[0053] Cellular structure 38 may partition the space between facing sheet 36 and backing member

40 into a plurality of cavities or cells 42. In some embodiments, cells 42 may be hollow (e.g., air filled). Depending on the shape and configuration of noise-attenuating device 12, the walls of cellular structure 38 defining cells 42 may be generally perpendicular to facing sheet 36. Facing sheet 36 may have one or more holes 44 extending therethrough. The number, size, shape and spacing of through holes 44 may be selected to establish desired acoustic performance of noise-attenuating device 12.

[0054] In some embodiments, an arrangement of holes 44 may be selected to define a percentage open area that is between 5% and 12% in one or more selected areas of facing sheet 36. In some embodiments, an arrangement of holes 44 may be selected to define a percentage open area that is between 4% and 10% in one or more selected areas of facing sheet 36. In some embodiments, an arrangement of holes 44 may be selected to define a percentage open area that is between 5% and 20% in one or more selected areas of facing sheet 36. In some embodiments, an arrangement of holes 44 may be selected to define a percentage open area that is up to about 33% in one or more selected areas of facing sheet 36. In some embodiments, an arrangement of holes 44 may be selected to define a percentage open area that is between 3% and 35% in one or more selected areas of facing sheet 36. In some embodiments, backing member 40 may also be perforated to allow the passage of heated air for anti-icing purposes.

[0055] During use, facing sheet 36 may be exposed to an environment in which noise attenuation is desired. For example, facing sheet 36 may face a source of noise and attenuation of the sound waves that impinge facing sheet 36 may occur according to a number of mechanisms such as: energy loss due to friction when the sound waves penetrate facing sheet 36 through holes 44; pressure loss when the sound waves expand into cells 42; and reactive cancellation of a sound wave entering and travelling in cell 42 by a previous sound wave that has been reflected and is returning from backing member 40. In some embodiments, the depth of cells 42 (i.e., thickness of cellular structure 38) may be selected (i.e., tuned) to attenuate a desired frequency or range of acoustic frequencies. In some embodiments, one or more holes 44 may be in communication with each cell 42 and each cell 42 may function as a Helmholtz resonant cavity.

[0056] Noise-attenuating device 12 shown in FIG. 3 may be a single (i.e., sole) degree of freedom

(i.e., single-layer) acoustic liner/panel but aspects of the present disclosure are also applicable to double degree of freedom acoustic liners/panels not specifically shown herein. For example, in a double degree of freedom acoustic liner/panel, perforated facing sheet 36 may be backed by two layers of cellular structure 38 that are separated from each other by a perforated septum sheet and/or a woven or mesh-like septum. Double-layer (e.g., double degree of freedom) acoustic liners can be configured to couple two Helmholtz resonators in series.

[0057] The acoustic performance of noise-attenuating device 12 may be dependent on its construction including the specification of holes 44 and also on the depth of cells 42 in cellular structure 38. Generally, noise-attenuating devices 12 of the types referenced herein may be designed to reduce the effective perceived noise level. In some embodiments, the thickness of the cellular structure 38 (i.e., the space between facing sheet 36 and backing member 40) may be between 12-50 mm.

[0058] FIG. 4 is a flowchart of a method 100 for manufacturing facing sheet 36 of noise-attenuating device 12. The use of method 100 may increase the linearity in the acoustic behavior of noise-attenuating device 12 without the need for a porous layer such as a (e.g., metallic, stainless steel, fiberglass, plastic and/or carbon) woven wire cloth or mesh adhesively bonded to or otherwise disposed on the exterior of facing sheet 36. Accordingly, noise attenuating device 12 may be devoid of a layer of fibrous or woven material disposed on the exterior of facing sheet 36. [0059] FIGS. 5A-5D sequentially illustrate steps of an example method 100 for manufacturing facing sheet 36. Method 100 is described below in reference to FIGS 4 and 5A-5D. Method 100 may include receiving precursor 46 to facing sheet 36 (see block 102 in FIG. 4). In various embodiments of method 100, precursor 46 may be received in a perforated or imperforated state. When precursor 46 is received in a perforated state (see block 104), method 100 may proceed to block 106 where coating 48 is applied to outer side 46A of precursor 46. When precursor 46 is not perforated at block 104, method 100 may include forming one or more holes 44 through precursor (see block 108) before applying coating 48 at block 106.

[0060] The forming of holes 44 through precursor 46 may be carried out by abrasive blasting (also referred to as “grit blasting” or“abrasive micromachining”), conventional (e.g., multi-spindle or single spindle) mechanical drilling, laser drilling or other suitable means. Floles 44 may have a circular cross- sectional profile. Floles 44 may have a diameter that is substantially uniform (i.e., constant) along each of their respective lengths. In some embodiments, all holes 44 formed in precursor 46 may have substantially the same diameter. Alternatively, one or more holes 44 may have a different diameter than one or more other holes 44 formed in the same precursor 46.

[0061] The term“precursor” as used herein is intended to encompass a component that is used in the manufacturing of a part such as facing sheet 36 but that is at a (e.g., pre-final) stage in the manufacturing process that precedes the final part. In some embodiments, precursor 46 may be a composite laminate including an assembly of layers of fibrous material(s) (e.g., carbon fibre, glass fibre and/or natural fibre) joined together in a (e.g., polymeric) matrix material to provide the desired engineering properties. In other words, precursor 46 may be made from a fibre-reinforced composite material and may be in sheet form. In some embodiments, precursor 46 may be made of a metallic material such as an aluminum alloy for example.

[0062] In reference to FIGS. 5A-5D, precursor 46 may have outer side 46A and an opposite inner side 46B. Prior to applying coating 48, precursor 46 may have one or more through holes 44 extending from outer side 46A to inner side 46B (see FIG. 5B). Coating 48 may be applied to outer side 46A of precursor 46 so that coating 48 may define an exposed surface 36A of facing sheet 36 for direct exposure to environment 50 in which noise attenuation is desired. Coating 44 may also define flared entrance 52 to each of the one or more holes 44. FIG. 5A shows an imperforated precursor 46 being received. FIG. 5B shows precursor 46 with hole 44 formed therethrough. FIG. 5C shows the perforated precursor 46 with primer 48A applied on outer surface 46A of precursor 46. FIG. 5D shows facing sheet 36 including paint 48B having been applied over primer 48A. [0063] In some embodiments, coating 48 may be bonded to outer surface 46A of precursor 46 and selected to provide some environmental protection for precursor 46 since exposed surface 36A defined by coating 48 may be directly exposed to environment 50 outside of noise attenuating device 12 without an intermediate substance (e.g., layer) applied to exposed surface 36A. For example, noise-attenuating device 12 may be devoid of a layer of fibrous or woven material disposed over coating 48 of facing sheet 36. For example, noise-attenuating device 12 may be devoid of a layer such as a (e.g., metallic, stainless steel, fiberglass, plastic and/or carbon) woven wire cloth or mesh adhesively bonded to or otherwise disposed on the exterior of facing sheet 36. Coating 48 may be non-adhesive. In some embodiments, coating 48 does not define an adhesive bonding system intended to adhesively bond an outer (e.g., fibrous or woven) layer to precursor 46. In some embodiments, coating 48 is not part of such adhesive bonding system.

[0064] In some embodiments, coating 48 may be selected to provide erosion/wear protection, weathering protection, thermal protection and/or protection against ultraviolet exposure. In various embodiments, coating 48 may be a single-layer coating or may include multiple layers applied sequentially to collectively form coating 48. In some embodiments, coating 48 may include a suitable paint 48B. For example, coating 48 may include a suitable aircraft-qualified paint system such as those sold under the trade name SFIERWIN-WILLIAMS for example. In some embodiments, coating 48 may include a suitable primer 48A that is first applied to outer side 46A of precursor 46 (see FIG. 5C), and, a suitable paint 48B that is subsequently applied over primer 48A (see FIG. 5D). Primer 48A may facilitate adherence of paint 48B to precursor 46. Coating 48 may be applied in liquid form onto outer surface 46A and permitted to cure. Coating 48 may be applied using any suitable applicator (e.g., brush and/or sprayer). For example, coating 48 may be applied by spraying material of coating 48 in liquid form onto outer surface 46A of precursor 46. It is understood that suitable surface preparation of outer side 46A of precursor 46 may be required depending on the type of coating 48 selected.

[0065] In some embodiments, coating 48 can be only primer 48A (i.e., be devoid of any other coating other than primer 48A). In some embodiments, coating 48 can include primer 48A and another type(s) of top coat(s) such as paint 48B disposed over primer 48A. In some embodiments, coating 48 can include a substantially clear lacquer (e.g., varnish type coating). In various embodiments, different types of coatings 48 may be used depending on whether precursors 46 is made from metallic or composite material.

[0066] FIG. 6 shows an enlarged schematic cross-sectional portion of facing sheet 36. During operation of noise-attenuating device 12, exposed surface 36A of facing sheet 36 may be directly exposed to environment 50 in which noise attenuation is desired. In some situations, exposed surface 36A may be exposed to a flow of fluid (e.g., air) illustrated by arrow F. In some applications, the conditions to which noise-attenuating device 12 is exposed may vary. Inside nacelle 22 (see FIG. 2) for example, the velocity of the flowing air in bypass duct 28 may vary and the frequency, amplitude and pressure field of the sound may also vary with changes in operating conditions (e.g., output power level) of engine 20. Accordingly, it may be desirable for noise-attenuating device 12 to exhibit a more linear acoustic response to facilitate tailoring of the performance of noise attenuating device 12 over a range of operating conditions. It was discovered that coating 48 applied to the outer side of facing sheet 36 and defining flared entrances 52 to individual holes 44 may be used to increase the linearity of the acoustic response compared to an otherwise non-linear single-layer acoustic liner that does not have such coating 48 and also does not have an outer layer of fibrous or woven material.

[0067] The application of coating 48 after the forming of holes 44 in precursor 46 may modify the entrances to individual holes 44 in precursor 46. As shown in FIG 5B, the intersection of each hole 44 with outer side 46A of precursor 46 may be a relatively sharp edge after forming hole 44. However, the application of coating 48 may result in a flared entrance 52 being formed for each hole 44. Flared entrance 52 may be a product of the application of coating 48 without any subsequent modification of the geometry of flared entrance 52. For example, method 100 of FIG. 4 may, in some embodiments, be devoid of further steps that alter the geometry of flared entrance 52 after block 106. Flared entrance 52 defined by coating 48 may, in some embodiments, increase the linearity in the acoustic response of noise-attenuating device 12. The geometry (e.g., cross-section profile and radius R shown in FIG. 6) of flared entrance 52 may depend on the thickness T1 of coating 48 and also the viscosity of the coating material when coating 48 is applied in liquid form to precursor 46. For example, flared entrance 52 may have a geometry that spreads gradually outward from hole 44 and have a shape that resembles the end of a trumpet. Flared entrance 52 may be rounded. The shape of flared entrance 52 may also be described as a rounded funnel shape, a bell mouth or a horn. Flared entrance 52 may define a rounded edge of holes 44.

[0068] In some embodiments, coating 48 may be selected to have thickness T1 that is at least

10% of diameter D1 of the one or more holes 44. Thickness T1 is illustrated in FIG 6 as representing a total thickness that is the sum of thickness T2 of primer 48A and thickness T3 of paint 48B. In some embodiments, coating 48 may have thickness T1 that is between 10% and 15% of diameter D1 of the one or more holes 44. In some embodiments, coating 48 may have a thickness that is between 5% and 20% of diameter D1 of the one or more holes 44. In some embodiments, the one or more holes 44 may have a diameter that is between 0.5 mm and 2 mm. In some embodiments, coating 48 may have a thickness between 0.1 mm and 0.4 mm. In some embodiments, thickness T2 of primer 48A may be between 0.02 mm and 0.08 mm. In some embodiments, thickness T3 of paint 48B may be between 0.04 mm and 0.4 mm.

[0069] In some embodiments, flared entrance 52 or part thereof may have a radius R of curvature as shown in FIG. 6 that depends on the thickness T1 of coating 48 and also on the viscosity of the coating material when applied. It is understood that the cross-sectional shape of flared entrance 52 as shown in FIG. 6 may not be perfectly arcuate. In some embodiments, radius R may be substantially equal to or greater than thickness T1 of coating 48. In some embodiments, radius R may have a value that is significantly greater than thickness T 1 of coating 48. In some embodiments where coating 48 is primer 48A only, radius R may be equal to or greater than 0.02 mm. In some embodiments, where coating 48 includes primer 48A and paint 48B, radius R may be equal to or greater than 0.06 mm. In various embodiments, radius R may be between 0.02 mm and 1 mm. In some embodiments, radius R may be greater than 0.1 mm. In some embodiments, radius R may be greater than 0.2 mm. In some embodiments, radius R may be greater than 0.3 mm. In some embodiments, radius R may be greater than 0.4 mm. In some embodiments, radius R may have a value between 0.5 mm and 0.9 mm. In some embodiments, radius R may have a value of about 0.7 mm.

[0070] Flared entrance 52 defined by coating 48 may have an enlarged diameter D3 that is larger than diameter D1 of hole 44. Flared entrance 52 may serve to guide sound waves into hole 44 and the enlarged diameter D3 may have an effect of increasing the effective perforated area of facing sheet 36. In some embodiments, some material of coating 48 may also be applied to the interior of hole 44 during application by spraying for example. During the application of coating 48 in liquid form, some coating material may droop along the wall of hole 44 and cure to form bulge 54 on the interior of hole 44. This may cause flared entrance 52 defined by coating 48 to extend inside hole 44 below outer side 46A of precursor 46. Bulge 54 may partially occlude hole 44 and reduce the size of the opening to hole 44 as illustrated by reduced diameter D2. Reduced diameter D2 may be smaller than diameter D1 of hole 44. Thickness T1 of coating 48 and the viscosity of the coating material during application may affect the geometry of the resulting flared entrance 52 and consequently affect the acoustic response of noise-attenuating device 12. It is understood that parameters of coating 48 should be selected so that coating 48 does not completely occlude hole 44. Precursor 46 may have a thickness FI that represents the height of holes 44 through precursor 46. As shown in FIG. 6, hole(s) 44 may each have respective diameters D1 that are uniform along their respective heights FI.

[0071] FIG. 7 shows a magnified photograph obtained via optical microscope of a cross-sectional portion of an actual sample facing sheet 36 manufactured according to the present disclosure, at the location of hole 44. Stippled lines have been added to the photograph for emphasis to demarcate the boundaries of precursor 46 and of coating 48.

[0072] FIG. 8 shows graph 56 including curves indicative of sound-attenuation performance measured at different operating conditions during tests performed on actual noise-attenuating devices having facing sheets with an open area of about 7%. The horizontal axis indicates frequencies of the sound waves impinged on the facing sheet and the vertical axis represents the normalized acoustic resistance which is a component of the overall acoustic impedance. The curves A1 , B1 , C1 and D1 shown in stippled lines are associated with a noise-attenuating device with an uncoated facing sheet and the curves A2, B2, C2 and D2 shown in solid lines are associated with noise-attenuating device 12 with facing sheet 36 having an example coating 48. The curves represent tests performed to illustrate the effect of air velocity (see arrow F in FIG. 5) in the form of a grazing flow, to which the exterior of noise-attenuating device 12 is exposed, and sound frequency on the acoustic resistance of the coated and uncoated noise-attenuating devices. The noise-attenuating devices were inserted into a test section where flight-representative noise and air flow conditions in front of the noise-attenuating devices were induced, and the acoustic performance of the noise-attenuating devices was evaluated. The air flow velocity was incrementally increased and the acoustic resistance of the noise-attenuating devices was measured at different grazing flow velocities and at different sound frequencies. The same sound pressure level exposed to the test noise-attenuating devices was maintained at each measured point. Noise sources disposed both upstream and downstream of the noise attenuating devices (in relation to the airflow direction) were used.

[0073] Curve A1 is associated with a noise-attenuating device with an uncoated facing sheet at a condition where there is no grazing flow of air across the facing sheet (i.e., Mach 0.0). Curve A2 is associated with noise-attenuating device 12 with coated facing sheet 36 at the condition where there is no grazing flow of air across exposed surface 36A of facing sheet 36 (i.e., Mach 0.0).

[0074] Curve B1 is associated with the noise-attenuating device with the uncoated facing sheet at a condition where a grazing flow of air across the facing sheet has a speed of Mach 0.3. Curve B2 is associated with noise-attenuating device 12 with coated facing sheet 36 at the condition where the grazing flow of air across exposed surface 36A of facing sheet 36 has a speed of Mach 0.3.

[0075] Curve C1 is associated with the noise-attenuating device with the uncoated facing sheet at a condition where a grazing flow of air across the facing sheet has a speed of Mach 0.5. Curve C2 is associated with noise-attenuating device 12 with coated facing sheet 36 at the condition where the grazing flow of air across exposed surface 36A of facing sheet 36 has a speed of Mach 0.5.

[0076] Curve D1 is associated with the noise-attenuating device with the uncoated facing sheet

36 at a condition where a grazing flow of air across the facing sheet has a speed of Mach 0.7. Curve C2 is associated with noise-attenuating device 12 with coated facing sheet 36 at the condition where the grazing flow of air across exposed surface 36A of facing sheet 36 has a speed of Mach 0.7.

[0077] Graph 56 shows that the solid-line curves A2, B2, C2 and D2 associated with the coated noise-attenuating device 12 have a smaller spread of acoustic resistance values compared to the stippled- line curves A1 , B1 , C1 and D1 associated with the uncoated noise-attenuating device. This may indicate that the acoustic resistance values of the coated noise-attenuating devices 12 are less sensitive to the variation in the velocity of the grazing flow than the uncoated noise-attenuating device. Both the coated and uncoated noise-attenuating devices represented in graph 56 had holes having a diameter of about 1 mm. The coated noise-attenuating device 12 had a coating thickness T1 of about 0.3 mm.

[0078] FIG. 9 shows graph 58 including curves indicative of sound-attenuation performance measured at different operating conditions during tests performed on actual noise-attenuating devices having facing sheets with an open area of about 4.5%. The horizontal axis indicates frequencies of the sound waves impinged on the facing sheet and the vertical axis represents the normalized acoustic resistance. The curves E1 , F1 , G1 and H 1 shown in stippled lines are associated with a noise-attenuating device with an uncoated facing sheet and the curves E2, F2, G2 and H2 shown in solid lines are associated with noise-attenuating device 12 with facing sheet 36 having an example coating 48. The curves represent tests performed to illustrate the effect of air velocity (see arrow F in FIG. 5) in the form of a grazing flow, to which the exterior of noise-attenuating device 12 is exposed, and sound frequency on the acoustic resistance of the coated and uncoated noise-attenuating devices. The noise-attenuating devices were tested in the same manner as described above in relation to FIG. 8.

[0079] Curve E1 is associated with a noise-attenuating device with an uncoated facing sheet at a condition where there is no grazing flow of air across the facing sheet (i.e., Mach 0.0). Curve E2 is associated with noise-attenuating device 12 with a coated facing sheet 36 at the condition where there is no grazing flow of air across exposed surface 36A of facing sheet 36 (i.e., Mach 0.0).

[0080] Curve F1 is associated with the noise-attenuating device with the uncoated facing sheet at a condition where the grazing flow of air across the facing sheet has a speed of Mach 0.3. Curve F2 is associated with noise-attenuating device 12 with the coated facing sheet 36 at the condition where the grazing flow of air across exposed surface 36A of facing sheet 36 has a speed of Mach 0.3.

[0081] Curve G1 is associated with the noise-attenuating device with the uncoated facing sheet at a condition where the grazing flow of air across the facing sheet has a speed of Mach 0.5. Curve G2 is associated with noise-attenuating device 12 with coated facing sheet 36 at the condition where the grazing flow of air across exposed surface 36A of facing sheet 36 has a speed of Mach 0.5.

[0082] Curve H1 is associated with the noise-attenuating device with the uncoated facing sheet

36 at a condition where a grazing flow of air across the facing sheet has a speed of Mach 0.7. Curve H2 is associated with noise-attenuating device 12 with the coated facing sheet 36 at the condition where the grazing flow of air across exposed surface 36A of facing sheet 36 has a speed of Mach 0.7. [0083] Graph 58 also shows that the solid-line curves E2, F2, G2 and H2 associated with the coated noise-attenuating device 12 have a smaller spread of acoustic resistance values compared to the stippled-line curves E1 , F1 , G1 and H1 associated with the uncoated noise-attenuating device. This may indicate that the acoustic resistance values of the coated noise-attenuating devices 12 are less sensitive to the variation in the velocity of the grazing flow than the uncoated noise-attenuating device. Both the coated and uncoated noise-attenuating devices represented in graph 58 had holes 44 having a diameter of about 1 mm. The coated noise-attenuating device 12 had a coating thickness T1 of about 0.3 mm.

[0084] Aspects and embodiments of the invention may be further understood with reference to the following non-limiting numbered clauses:

1 . A device for attenuating noise in an environment, the device comprising:

a backing member; and

a facing sheet spaced apart from the backing member to define a cavity between the backing member and the facing sheet, the facing sheet having:

an outer side for facing the environment and an opposite inner side facing the backing member;

one or more through holes extending from the outer side to the inner side; and a coating on the outer side of the facing sheet, the coating defining an exposed surface of the facing sheet for direct exposure to the environment and a flared entrance to each of the one or more holes.

2. The device as defined in clause 1 , wherein the coating includes paint.

3. The device as defined in clause 1 , wherein the coating includes a primer and paint.

4. The device as defined in any one of clauses 1 to 3, wherein the coating has a thickness that is at least 10% of a diameter of the one or more holes.

5. The device as defined in any one of clauses 1 to 3, wherein the coating has a thickness that is between 10% and 15% of a diameter of the one or more holes.

6. The device as defined in any one of clauses 1 to 3, wherein the coating has a thickness that is between 5% and 20% of a diameter of the one or more holes.

7. The device as defined in any one of clauses 1 to 6, wherein each of the one or more holes has a diameter between 0.5 mm and 2 mm. 8. The device as defined in any one of clauses 1 to 6, wherein each of the one or more holes has a diameter between 1 mm and 2 mm.

9. The device as defined in any one of clauses 1 to 8, wherein the coating has a thickness between 0.1 mm and 0.4 mm.

10. The device as defined in any one of clauses 1 to 9, wherein the one or more flared entrances defined by the coating extend inside the one or more respective holes.

1 1 . The device as defined in any one of clauses 1 to 10, wherein the coating defines a bell mouth entrance to each of the one or more holes.

12. The device as defined in any one of clauses 1 to 1 1 , wherein the facing sheet is made from a fibre- reinforced composite material.

13. The device as defined in any one of clauses 1 to 12, wherein the device has a sole degree of freedom.

14. A noise-attenuating device, the device comprising:

a backing member; and

a facing sheet spaced apart from the backing member to define a cavity between the backing member and the facing sheet, the facing sheet having:

an outer side and an opposite inner side facing the backing member;

one or more through holes extending from the outer side to the inner side; and a coating on the outer side of the facing sheet, the coating defining a flared entrance to each of the one or more holes,

wherein the noise-attenuating device is devoid of a layer of fibrous or woven material disposed over the coating.

15. A noise-attenuating device, the device comprising:

a backing member; and

a facing sheet spaced apart from the backing member to define a cavity between the backing member and the facing sheet, the facing sheet having:

an outer side and an opposite inner side facing the backing member;

one or more through holes extending from the outer side to the inner side; and a non-adhesive coating on the outer side of the facing sheet, the non-adhesive coating defining a flared entrance to each of the one or more holes. 16. An aircraft comprising the device as defined in any one of clauses 1 to 15.

17. A nacelle for an aircraft engine, the nacelle comprising the device as defined in any one of clauses 1 to 15.

18. A facing member of a device for attenuating noise in an environment, the facing member comprising:

an outer side for facing the environment and an opposite inner side;

one or more through holes extending from the outer side to the inner side; and

a coating on the outer side of the facing member, the coating defining an exposed surface of the facing sheet for direct exposure to the environment and a flared entrance to each of the one or more holes.

19. The facing member as defined in clause 18, wherein the coating includes paint.

20. The facing member as defined in clause 18, wherein the coating includes a primer and paint.

21. The facing member as defined in any one of clauses 18 to 20, wherein the coating has a thickness that is at least 10% of a diameter of the one or more holes.

22. The facing member as defined in any one of clauses 18 to 20, wherein the coating has a thickness that is between 10% and 15% of a diameter of the one or more holes.

23. The facing member as defined in any one of clauses 18 to 20, wherein the coating has a thickness that is between 5% and 20% of a diameter of the one or more holes.

24. The facing member as defined in any one of clauses 18 to 23, wherein each of the one or more holes has a diameter between 0.5 mm and 2 mm.

25. The facing member as defined in any one of clauses 18 to 23, wherein each of the one or more holes has a diameter between 1 mm and 2 mm.

26. The facing member as defined in any one of clauses 18 to 25, wherein the coating has a thickness between 0.1 mm and 0.4 mm.

27. The facing member as defined in any one of clauses 18 to 26, wherein the one or more flared entrances defined by the coating extend inside the one or more respective holes. 28. The facing member as defined in any one of clauses 18 to 27, wherein the coating defines a bell mouth entrance to each of the one or more holes.

29. The facing member as defined in any one of clauses 18 to 28, wherein the facing sheet is made from a fibre-reinforced composite material.

30. A method for manufacturing a facing sheet of a device for attenuating noise in an environment, the method comprising:

receiving a precursor to the facing sheet, the precursor having:

an outer side and an opposite inner side; and

one or more through holes extending from the outer side to the inner side; and applying a coating to the outer side of the precursor, the coating defining an exposed surface of the facing sheet for direct exposure to the environment and a flared entrance to each of the one or more holes.

31 . The method as defined in clause 30, wherein the coating includes paint.

32. The method as defined in clause 30, wherein the coating includes a primer and paint.

33. The method as defined in any one of clauses 30 to 32, wherein the coating has a thickness that is at least 10% of a diameter of the one or more holes.

34. The method as defined in any one of clauses 30 to 32, wherein the coating has a thickness that is between 10% and 15% of a diameter of the one or more holes.

35. The method as defined in any one of clauses 30 to 32, wherein the coating has a thickness that is between 5% and 20% of a diameter of the one or more holes.

36. The method as defined in any one of clauses 30 to 35, wherein the one or more holes have a diameter between 0.5 mm and 2 mm.

37. The method as defined in any one of clauses 30 to 36, wherein the coating has a thickness between 0.1 mm and 0.4 mm.

38. The method as defined in any one of clauses 30 to 37, wherein the one or more flared entrances defined by the coating respectively extend inside the one or more holes. 39. The method as defined in any one of clauses 30 to 38, wherein the outer sheet is made from a fibre-reinforced composite material.

40. The method as defined in any one of clauses 30 to 39, wherein the one or more holes in the facing sheet have a substantially uniform diameter.

[0085] The above description is meant to be example only, and one skilled in the relevant arts will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. The present disclosure may be embodied in other specific forms without departing from the subject matter of the claims. The present disclosure is intended to cover and embrace all suitable changes in technology. Modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims. Also, the scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.